OR                              SERIES

 Satellite Images for Land Cover Monitoring
 Navigating Through the Maze
Introduction                                                                            Land Use Mapping and
Policy makers, managers, scientists                                                     Economic Planning
and the public can view the changing                                                    • Map and monitor land cover changes.
environment using satellite images.                                                     • Monitor urban growth.
More than 60 Earth observing                                                            • Map of land-water boundaries.
satellites are collecting images of the                                                 • Site power plants and
Earth’s surface. Remote sensing                                                           other industries.
satellite systems for land cover
                                                                                        • Site for transportation and
assessment are operated by a grow-
                                                                                          transmission routes.
ing number of countries including
India, the United States, Japan,                                                        Geology and Geomorphology
France, Canada and Russia.                                                              • Recognize different rock types.
     The focus of this publication is                                        1972       • Map major geologic units.
satellite systems for land cover mon-                                                   • Monitor surface mining
                                             management, urban growth, sustain-
itoring. On the reverse is a table that                                                   and reclamation.
                                             able development and much more.
compares a selection of these sys-                                                      • Site solid waste storage.
tems, whose data are globally avail-             Even though many satellite sensors
                                             monitor the earth, which sensor to use     • Monitor volcanic activity.
able in a form suitable for land cover
analysis. We hope the information            depends on the type of environmental
presented will help you assess the           information needed.
utility of remotely sensed images to         What Can Remote Sensing Do For You?
meet your needs.
                                             Remote sensing plays an integral role
What is Remote Sensing?                      in environmental assessment. Remote
Remote sensing is the discipline of          sensing will never replace fieldwork
observing the Earth’s surface without        but it offers great support in:
being directly in contact with it. It        • Remote and difficult to access areas
allows us to obtain information about          like dense forests, glaciated areas,
our planet and humans activities from a        swamps, high elevations, etc.
distance, which can reveal interesting       • Areas undergoing rapid changes.
features that may not be possible or         • Countries with poor infrastructure                                       1989
affordable from ground level. This             and limited transportation.
gives a global perspective on changes        • Areas of natural hazards: flooded        Water and Coastal Resources
and the interaction of our complex             areas, active volcanic regions, forest   • Determine surface water areas.
biosphere components.                          fire areas, etc.                         • Monitor environmental effects of
    The tools for remote sensing are         • Constructing a broad overview or a         humans activities.
sensors installed on planes or satellites.     detailed map of a large area.            • Map floods and flood plains
Airborne sensors are typically photo-             Remote sensing techniques can         • Determine the extent of snow and ice.
graphic cameras. Although an impor-          increase the speed in which one can        • Measure glacial features.
tant source of environmental informa-        analyze a landscape and therefore help
tion, airborne image collections often                                                  • Map shoreline changes.
                                             make quick and focused decisions.
are poorly documented and for small                                                     • Trace oil spills and pollutants.
areas. In many countries the collection           Some applications include:
                                                                                            Attention should also be paid to the
of airborne photographs is restricted.       Agriculture and Forestry                   fact that remote sensing allows multi-
    Satellite sensors acquire images of      • Discriminate vegetation, crop, and       temporal analysis. This means that an
the Earth and transmit the data to             timber types.                            area of interest can be monitored over
ground receiving stations located            • Measure crop and timber acreage.         time so that changes can be detected.
throughout the world. Once these raw                                                    This allows us to analyze phenomena
                                             • Estimate crop yields.
images are processed and analyzed,                                                      like vegetation growth during different
                                             • Measure change in forest cover.          seasons, the extent of annual floods,
they can document changing environ-
mental conditions like pollution, glob-      • Assess drought impact.                   the retreat of glaciers or the spread of
al climate change, natural resource          • Determine soil conditions.               forest fires or oil spills.
Issues that Affect Selection of Images           Most Earth observing satellites          optimize the selection of the bands so
Satellites and their sensors can differ in   orbit from pole to pole and are sun-         that they can best distinguish between
many ways. Most satellite sensors            synchronous, that is they always pass        different types of land cover with the
image the Earth with several bands           overhead at the same time of the day.        fewest number of bands. Typical bands
where a band is sensitive to a specific      Overlap between paths is least near the      are: blue, green, red, near infrared, mid
range of wavelengths within the elec-        equator and most at upper latitudes.         infrared and thermal infrared.
tromagnetic spectrum. The sensors            Areas close to the pole may not be                The blue band has the best penetra-
discussed here are those sensitive to        covered at all. In contrast, some weather    tion of water, so is useful for coral reef
visible and infrared wavelengths.            satellites are geostationary. They           and sea grass monitoring. Conversely,
Beyond these wavelengths the interpre-       monitor only one hemisphere, but at all      the blue band is very sensitive to
tation of the data can become very           times of the day.                            moisture in the atmosphere, so in humid
specialized. However for some applica-       What is spatial resolution?                  climates is very noisy. The green band is
tions, wavelengths such as those used in                                                  relatively high and the red band is rela-
                                             Spatial resolution of digital satellite      tively low for vegetation. The near-
radar sensors, are very important.           sensors is the distance along the ground
     The defining characteristics of a                                                    infrared bands are high for vigorously
                                             between samples. As the satellite moves      growing vegetation. Mid or short wave
sensor are the size of an image, the         across the Earth surface it records the
region of the Earth that can seen, the                                                    infrared bands are sensitive to moisture.
                                             brightness of the surface at regular         Thermal infrared bands are sensitive to
smallest feature that can be distin-         intervals. The spatial resolution of a
guished, the bands used, how often                                                        emitted thermal radiation, such as hot
                                             digital sensor is distinct from that of a    spots in a city or the temperature distri-
images are collected, and when was the       photographic camera. The former is
sensor in operation. Important non-tech-                                                  bution of a lake.
                                             based on sampling rates, while the latter
nical issues include acquisition costs       is based on the grain size in the film. If   What are repeat cycles and dates
and data sharing limitations. Licensing      you zoom into a photograph, the image        of operation?
agreements vary over time, by product,       will degenerate into random noise. A         The orbital paths of most operational
by sensor and by organization.               satellite image will decompose into tiny     satellites are fixed. The repeat cycle of a
How much area does an image cover?           black and white or colored squares or        sensor is the number of days before an
The area covered by a single satellite       picture elements called pixels.              area can be seen again. The repeat cycle
image is defined by the path width and            If a satellite image has a spatial      varies from every day to weeks. Some
the distance along the path. The path        resolution of 30 meters this means that      sensors have overlapping paths and
width is limited by how far to each side     one pixel in the image represents a          some sensors are pointable which effec-
the sensor can see. The path width can       square of 30 x 30 meters on the Earth’s      tively reduces the period between repeat
vary from as little as 8 kilometers to       surface. In an image of this resolution      coverage.
over 2000 kilometers. The distance           one cannot see small buildings, but can           The repeat cycle is critical for two
along the path is more arbitrary. For        definitely see a football field. Some        reasons. One, some types of change or
high-to-medium resolution sensors the        sensors image the Earth with a very low      monitoring require frequent observa-
tendency is to create nominally square       resolution, more than 1 km, suitable for     tions. Two, cloud cover may obscure the
images. The image length for low reso-       general land cover mapping of large          ground. Sensors with long repeat cycles,
lution images may be an entire path.         features or monitoring vegetation vigor.     which tend to be high spatial resolution
                                             Others, with a resolution of 1 meter, can    sensors, may only acquire a cloud free
                                             resolve individual trees and are suitable    image once every several years in some
                                             for validation of coarser images.            climates.
                                             What is spectral resolution?                 How do these characteristics interact ?
                                             As spatial resolution is a sampling of       Wide paths tend to be associated with
                                             space, spectral resolution is a sampling     low spatial resolution. High spatial reso-
                                             of the electromagnetic spectrum.             lution means large data volumes. Wide
                                             Conceptually many narrow bands               paths are linked to short repeat cycles.
                                             would allow different types of land               Tradeoffs exist between spatial and
                                             cover to be more easily distinguished. In    spectral resolution. To achieve high
                                             reality, much of the information             spatial resolutions some sensors have a
                                             contained in narrow bands may be             panchromatic band, which is essentially
                                             redundant. Sensor designers attempt to       a very wide band.
             Satellite Remote Sensing for Environmental Assessment and Monitoring
Satellite sensors are available with resolutions from 1 m to 1 km. These resolutions are suitable for both large area and
local applications.
In remote areas, satellite images may provide the only practical source of information.

For some applications, such as land cover analysis over large areas, no cost effective alternative exists.

Satellite images can be effectively analyzed either by image interpretation or by computer analysis.

Many satellite sensors collect images for the entire world.

Satellite image archives exist that document the Earth’s surface back to the early 1960s.

Many of the satellite images can be acquired at no cost or at the cost of reproduction with no restrictions on
image sharing.

In all cases, expert and local knowledge should be used to validate or guide the analysis of satellite images.

The views expressed in this text do not necessarily reflect those of the agencies cooperating in this project. Satellite
sensor specifications may be approximated or summarized. The designations used and material presented above do not
imply the expression of any opinion whatsoever on the part of the cooperating agencies concerning the legal status of any
country, territory, city, or area or of its authorities, or of the delineation of its frontiers or boundaries.
This publication was prepared by Claudia Künzer and Gene Fosnight, UNEP/GRID–Sioux Falls. Our sincere appreciation
to Bhaskar Ramachandran, Kim Giese, Rebecca Johnson, Saud Amer and Larry Tieszen, Raytheon Company, EROS Data
Center for their valuable contributions to this publicaton.

                                                              For further information on remote sensing view:
   Satellite Systems for Land Cover Monitoring      
             Navigating Through the Maze
                                                              or contact:
                                                              Dr. Ashbindu Singh
                                                              Regional Coordinator, North America
                                                              UNEP Division of Early Warning and Assessment,
                                                              EROS Data Center, Sioux Falls, SD 57198
                                                              Phone: 605-594-6107 or 6117
                                                              Fax: 605-594-6119
                                         Spatial Extent: The total area covered by an image.              Spatial Resolution or Pixel Size: The spatial resolu-               Spectral Resolution: The sensors collect light in differ-                Repeat Cycle and Dates in Operation: Frequent cov-
                                         The width of the image is the path width. The length of          tion of a sensor is a function of how frequently the sen-           ent parts of the spectrum. Grey line shows typical                       erage increases the chances of cloud-free images.
                                         an image is not always constant, although a tendency             sor samples the ground. The pixel size is usually close             vegetation curve.                                                        Multiple images within a growing season allows knowl-
                                         exists for approximately square images. The nominal              to the sampling rate of the sensor. Objects on the                  Blue:           penetrates shallow waters                                edge of plant phenology to be used in land cover map-
                                         number of scenes needed to map Africa (30,264,000                ground need to be considerably larger than the spatial              Green:          high for green vegetation                                ping. Images collected through many years allows the
                                         km2) and Costa Rica (50,000 km2) are provided for                resolution to be detected. Example images are 375-by-               Red:            low for green vegetation                                 detection of land-use and land-cover change. In some
                                         each sensor. The actual number of scenes may be con-             375 pixels. The estimated number of land pixels for                 Near IR:        high for vigorously growing vegetation                   cases more than one satellite is operational, decreasing
                                                                                                                                                                                                                                                       the period between coverages.
                                         siderably higher depending on overlap and the shape of           Africa and Costa Rica are provided for each sensor.                 Mid IR:         sensitive to moisture variation
                                         the country.                                                     Estimates are in millions of pixels.                                Thermal IR: emitted thermal radiation

                                                                                                                                                                                                           Near IR
                                         185 km wide                                                      80 m                                                                                                                                           18 day repeat cycle for Landsats 1-3
                                                                                                                                                                                                                                                                                                                  Questions That Need to be Asked

                                                                                                                                                                                                                       Mid IR
                Multi-Spectral Scanner


                                         170 km long                                                                                                                                                                                                     16 day repeat cycle for Landsats 4 & 5

                                                                                                                                                                                                                                                         Operational since 1972 on Landsats 1-5.
                                                                                                                                                                                                                                                                                                                  Before Using Satellite Images
                                         Africa: 960                                                      Africa: 5,000
                                         Costa Rica: 2                                                    Costa Rica:8                                                                                                                                                                                            Often simple answers do not exist for selecting
                                                                                                          (with 30 m pxiels)                                                                                                                                                                                      and using satellite images in projects.
                                                                                                                                                                                                                                                                                                                    • Compromises may be required even in the most
                                         Phnom Penh, Cambodia                                             Phnom Penh, Cambodia                                                                                                                                                                                        ideal scenario. In the simplest terms a satellite
                                                                                                                                                                                                                                                                                                                      image must be available.

                                                                                                                                                                                           0.5                1.0                     5.0   10.0
                                                                                 Image courtesy of NASA                                             Image courtesy of NASA
                                                                                                                                                                                                                                                                                                                    • No simple method exists for calculating cost

                                                                                                                                                                                                           Near IR
                                         185 km wide                                                      30 m bands 1-5,7                                                                                                                               16 day repeat cycle

                                                                                                                                                                                                                      Mid IR
                                                                                                                                                                                                                                                                                                                      of data acquisition. Many issues need


                                         170 km long                                                      60 m thermal

                                                                                                                                                                                                                                                         TM available since 1982 on Landsats 4 and 5.
                Thematic Mapper

                                                                                                          15 m panchromatic                                                                                                                                                                                           be considered.
                                                                                                                                                                                                                                                         ETM+ available since 1999 on Landsat 7.                    • Satellite images are pictures of the Earth. Cost
                                         Africa: 960
                                         Costa Rica:2                                                     Africa: 34,000                                                                                                                                 The panchromatic band is only available on                   and expertise is needed to extract information for
                                                                                                          Costa Rica: 56
                                                                                                          (with 20 m pixels)
                                                                                                                                                                                                                                                         Landsat 7 ETM+                                               the satellite images.
                                                                                                                                                                                                                                                                                                                    • Data providers fall into three broad and
                                         Phnom Penh, Cambodia                                             Phnom Penh, Cambodia                                                                                                                                                                                        overlapping types: government agencies, quasi-
                                                                                                                                                                                           0.5                1.0                     5.0   10.0
                                                                                 Image courtesy of NASA                                             Image courtesy of NASA
                                                                                                                                                                                                                                                                                                                      public organizations and private companies.
                                                                                                                                                                                                                                                                                                                      Services available and copyright issues need to

                                                                                                                                                                                                           Near IR
                                         60 km wide                                                       20 m multispectral                                                                                                                             26 day repeat cycle

                                                                                                                                                                                                                       Mid IR
                                                                                                                                                                                                                                                                                                                      be considered.


                                         60 km long                                                       10 m panchromatic

          Multi-spectral and

                                                                                                                                                                                                                                                         5 day repeat viewing with pointable sensor

                                         Africa: 8500                                                     Africa: 76,000                                                                                                                                 Satellite operational since 1986.                        Criteria for sensor selection:
                                         Costa Rica: 14                                                   Costa Rica: 125
                                                                                                                                                                                                                                                                                                                   • Coverage: Are images available for your area?
                                                                                                                                                                                                                                                                                                                   • Spatial resolution: What size are the features you
                                         Sevilla, Spain                                                   Near Sevilla, Spain                                                                                                                                                                                        need to map?
                                                                                                                                                                                           0.5                1.0                     5.0   10.0
                                                                                                                                                                                                                                                                                                                   • Spectral resolution: What types of features need
                                                                          Image courtesy of SPOT Image

                                                                                                                                              Image courtesy of SPOT Image
                                                                                                                                                                                                                         Wavelength                                                                                  to be distinguished?

                                                                                                                                                                                                           Near IR
                                         2200 km wide                                                     1 km                                                                                                                                           4 out of 5 day repeat cycle                               • Dates of operation: During what years are

                                                                                                                                                                                                                      Mid IR

                                         variable length                                                                                                                                                                                                                                                             images available?

                                                                                                                                                                                                                                                         Satellite operational since 1986.
                                                                                                                                                                                                                                                                                                                   • Repeat cycle: What types of variation through

                                         Africa: 6                                                        Africa: 31                                                                                                                                                                                                 time are important: within or between years?
                                         Costa Rica: 1                                                    Costa Rica: 0.05
                                                                                                                                                                                                                                                                                                                  Criteria for image selection:
                                         Nile River and Red Sea                                           Nile River and Delta                                                                                                                                                                                     • How much cloud cover is acceptable?
                                                                                                                                                                                           0.5                1.0                     5.0   10.0
                                                                          Image courtesy of SPOT Image                                        Image courtesy of SPOT Image
                                                                                                                                                                                                                                                                                                                   • How critical is the time of year?
                                                                                                                                                                                                                                                                                                                   • How important is the year?

                                         142 km wide                                                      23.5 m                                                                                                                                         24 day repeat cycle
                                                                                                                                                                                                           Near IR

                                                                                                                                                                                                                      Mid IR

                                         142 km long                                                      70 m mid IR

                                                                                                                                                                                                                                                                                                                   • If multiple images are needed, do they need to

                                                                                                                                                                                                                                                         Satellite operational since 1996.
                                                                                                                                                                                                                                                                                                                     be in specific seasons of one year, the same
                                                                                                                                                                                                                                                                                                                     season in multiple years, and so on?

                                         Africa: 1500                                                     Africa: 55,000
                                         Costa Rica: 3                                                    Costa Rica: 91
                                                                                                          (with 23.5 m pixels)                                                                                                                                                                                    Cost of data acquisition:
                                         Iran                                                             Iran                                                                                                                                                                                                     • Who you work for and where you live may
                                                                                                                                                                                           0.5                1.0                     5.0   10.0
                                                                                                                                                                                                                                                                                                                     affect cost of data.
                                                                        Image courtesy of Space Imaging                                     Image courtesy of Space Imaging
                                                                                                                                                                                                                        Wavelength                                                                                 • Redistribution policies may affect with whom

                                                                                                                                                                                                                                                                                                                     you can share your data.
                                                                                                                                                                                                            Near IR

                                         70 km wide                                                       5.8 m                                                                                                                                          24 day repeat cycle
                                                                                                                                                                                                                       Mid IR


                                         70 km long                                                                                                                                                                                                                                                                • Data may be available at little or no cost.

                                                                                                                                                                                                                                                         5 day repeat viewing with pointable sensor

                                                                                                                                                                                                                                                         Satellite operational since 1996.                         • If no-cost data are available, do they fit your
                                         Africa: 6200                                                     Africa: 900,000                                                                                                                                                                                            needs: resolution, data of acquisition, and so on?

                                         Costa Rica: 11                                                   Costa Rica: 1500

                                                                                                                                                                                                                                                                                                                  Cost of data analysis:
                                         St. Petersburg, Russia                                           St. Petersburg, Russia                                                                                                                                                                                   • Do you have staff to do the analysis or will the
                                                                        Image courtesy of Space Imaging                                     Image courtesy of Space Imaging                0.5                 1.0                    5.0    10.0                                                                    analysis be contracted?
                                                                                                                                                                                                                                                                                                                   • Satellite images can be either manually interpret-
                                                                                                                                                                                                            Near IR

                                         810 km wide                                                      188 m                                                                                                                                          5 day repeat cycle
                                                                                                                                                                                                                       Mid IR

                                                                                                                                                                                                                                                                                                                     ed or they can be classified with a computer.


                                         810 km long

                                                                                                                                                                                                                                                         Satellite operational since 1996.                           Different types of expertise are needed and dif-
                                                                                                                                                                                                                                                                                                                     ferent types of costs are incurred.

                                         Africa: 50                                                       Africa: 860
                                         Costa Rica: 1                                                    Costa Rica: 1.5
                                                                                                                                                                                                                                                                                                                   • Is training needed? If so, what type of training?
                                                                                                                                                                                                                                                                                                                   • The cost of validating the quality of the analysis
                                                                                                                                                                                                                                                                                                                     is often a forgotten and hidden cost. Without val-
                                         Tunisia and Italy                courtesy of Euromap             Sicily
                                                                                                                                                                                                                                                                                                                     idation the value of the analysis is limited.
                                                                                                                                                                                           0.5                 1.0                    5.0    10.0
                                                                              Image courtesy of Euromap                                          Image courtesy of Euromap
                                                                                                                                                                                                           Near IR

                                         2330 km wide                                                     250-1000 m                                                                                                                                     1 day repeat cycle
                                                                                                                                                                                                                      Mid IR

                                                                                                          Band dependent

                                         variable length

                                                                                                                                                                                                                                                         Satellite operational since 2000.
                                                                                                                                                                                                                                                                                                                  Other More Specialized Sensors
                                                                                                                                                                                                                                                                                                                  Important for Land Cover Monitoring

                                         Africa: 6                                                        Africa: 500
                                         Costa Rica: 1                                                    Costa Rica: 0.8
                                                                                                          (with 250 m pixels)
                                                                                                                                                                                                                                                                                                                   • Radar sensors have all-weather capability for
                                         Nile River and Red Sea                                           Nile River and Delta                                                                                                                                                                                       quantifying variation in surface roughness that is
                                                                                                                                                                                           0.5                1.0                     5.0   10.0                                                                     crucial for monitoring floods, fires, oil spills,

                                                                               Image courtesy of NASDA                                             Image courtesy of NASDA
                                                                                                                                                                                                                        Wavelength                                                                                   wind speed and wind direction. Examples of
                                                                                                                                                                                                           Near IR

                                                                                                                                                                                                                                                                                                                     radar sensors include Radarsat, ERS, JERS,
                                                                                                                                                                                                                      Mid IR

                                         60 km wide                                                       15 m vis and near IR.                                                                                                                          16 day repeat cycle


                                                                                                          30 m mid IR

                                         60 km long                                                                                                                                                                                                      4 day repeat cycle with pointable sensor                    SIR-C, Seawifs, and Seawinds
                                                                                                          90 m thermal IR
                                                                                                                                                                                                                                                         Satellite operational since 2000.                         • Cameras were carried on many older satellites

                                         Africa: 8500                                                     Africa: 135,000                                                                                                                                                                                            and continue to be used on many manned space
                                         Costa Rica: 14                                                   Costa Rica: 225                                                                                                                                                                                            missions. The older photographic images extend
                                                                                                          (with 15 m pixels).
                                                                                                                                                                                                                                                                                                                     the environmental record back to 1962 and many
                                                                                                                                                                                                                                                                                                                     are of high spatial resolution. Examples of space
                                         Phnom Penh, Cambodia                                             Phnom Penh, Cambodia
                                                                                                                                                   Image courtesy of NASDA
                                                                                                                                                                                           0.5                1.0                     5.0   10.0                                                                     photography include Corona, Argon, Shuttle,
                                                                               Image courtesy of NASDA                                                                                                                   Wavelength
                                                                                                                                                                                                                                                                                                                     Gemini and Apollo handheld, and TK-350,
                                                                                                                                                                                                           Near IR

                                                                                                                                                                                                                                                                                                                     KVR-1000, MK-4, and KFA-1000.
                                                                                                                                                                                                                      Mid IR

                                         3000 km wide                                                     1.1 km                                                                                                                                          1 day repeat cycle


                                         variable length

                                                                                                                                                                                                                                                          Satellite operational since 1978.                        • Sensors on geostationary satellites have the
                                                                                                                                                                                                                                                                                                                     unique ability to provide continuous coverage of

                                         Africa: 6                                                        Africa: 26                                                                                                                                                                                                 a region. These satellites are at much higher alti-
                                         Costa Rica: 1                                                    Costa Rica: 0.042                                                                                                                                                                                          tudes than are most Earth observing satellites and
                                                                                                                                                                                                                                                                                                                     are designed for weather monitoring. A critical
                                                                                                                                                                                                                                                                                                                     ability of sensors on these satellites is to monitor
                                         Nile River and Red Sea                                           Nile River and Red Sea               courtesy of NOAA
                                                                                                                                                                                           0.5                1.0                     5.0   10.0                                                                     cloud cover and to provide rainfall estimates.
                                                                                Image courtesy of NOAA                                              Image courtesy of NOAA                                              Wavelength                                                                                   Examples of these satellites include GMS,
                                                                                                                                                                                                                                                                                                                     GOES-E, GOES-W, GOMS, Meteosat and Insat.
                                                                                                                                                                                                           Near IR

                                         11 km wide                                                       4m multispectral                                                                                                                             5 day repeat cycle
                                                                                                                                                                                                                      Mid IR

                                                                                                                                                                                                                                                                                                                   • Further information on new sensors as they are


                                         11 km long                                                       1m panchromatic

                                                                                                                                                                                                                                                       1.5 day repeat viewing with pointable sensor                  launched and on other specialized sensors such as
                                                                                                                                                                                                                                                       Satellite operational since 1999.                             DMSP, ATRS, TRMM, MISR, and QuikScat can

                                         Africa: 253,000                                                  Africa: 31,000,000                                                                                                                                                                                         be found at
                                         Costa Rica: 420                                                  Costa Rica: 50,000
                                                                                                          (with 1 m pixels)

                                         Sanaa, Yemen                                                     Sanaa, Yemen
                                                                                                                                                                                           0.5                1.0                     5.0   10.0
                                                                        Image courtesy of Space Imaging                                    Image courtesy of Space Imaging

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