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					 Spatial Data

 Spatial data comes in many
 forms. So How does a GIS
work with the data so that it
can put the data in the right
      place on a map?

                            1
                       Outline
• Kinds of spatial data
  – Vector data
     • Points, lines, and polygons
     • Highway maps
  – Raster data and image data
     •   Raster representation of data
     •   Satellite data (Landsat)
     •   Aerial photographs
     •   Digital elevation data
• Real world problems associated converting
  data from earth’s spherical coordinates to
  coordinates on a flat surface
                                               2
       Compare Raster & Vector
  • Vector GIS                        • Raster GIS
     – Objects                               – AREA represented
       represented by:                         by:
         •   points                            • Grid cells
         •   lines                             • one value per cell
         •   polygons                          • Large number
         •   large database each                 thematic layers
             object




Looks like a
   map                                                       Looks like
                                                               what?
                           Forest     road

                           Cropland   stream
                                                                      3
             324
  “Kinds” of GI data
Point




                           2’ resolution
                           rectified aerial
                           photograph

            Aerial Photo

              Landsat 7 image         4
The Digital Elevation Raster




                               5
                Uses
• Vector data is most common because
  you can tie huge databases to
  features
• BUT
• Raster data is very good for
  continuous surfaces like
  – Elevation
• Images (dumb picture) data
                                       6
                 OK…

• Data on the globe is not very useful
  because you can’t put the globe in
  your report
• So you need a 2D map
• Thus you have to convert 3D data to
  2D
• Lets start with the spherical earth …

                                          7
Where are we?   8
   Earth’s Coordinate System
              X is Longitude and is measured
              E and W from Greenwich,
              England. West is negative, East
              is positive
               Y is latitude and is measured N
               and S from the equator. North
               is positive and S is negative.

These are called Geographic Coordinates
                                             9
  X, Y = Longitude, Latitude
                                            90
                 Stretch the top
                                            60
                                            30
Equator                                      0
                                             -30
                                             -60
                                             -90
      -180     Stretch the bottom
             -90        0       +90       +180
               Lines of constant Longitude
               Lines of constant Latitude

                                                   10
  X, Y = Longitude, Latitude
                   90E, 30N         +90, +30
                                           90
                                            60
                                            30
Equator                                      0
                                             -30
                                             -60
                     -90 -30
90W, 30S                                     -90
      -180   -90        0       +90       +180
               Lines of constant Longitude
               Lines of constant Latitude

                                                   11
    The world in Geographic
         Coordinates



    Is
Antarctica
Really that
  big?



                              12
             3D to 2D

• Geographic coordinates introduce too
 much distortion to be useful
• So we need to convert 3D coordinates
  into 2D coordinates
• But, there is a problem…


                                     13
The Problem
              14
15
16
The Mercator Projection
                  Making a
                  Projection




                               17
Some Projections




                   18
             Some Projections




It is pretty obvious that if
you have data in different
projections they are NOT
going to “line up” with each
other                           19
  Why different Projections?
• Spherical coordinates to flat surface
  produces distortions in:
  –   Shape
  –   Area
  –   Distance
  –   Direction
• So different projections for
  different purposes (Mercator for
  transportation (rumb line stright)
                                       20
            That means…
• Data in different projections will not
  line up or be congruent!
• This is something you have to be
  aware of …
• HOWEVER                             Data
                                      about
• ArcGIS will project on the fly so the
                                      Data
  problem is not great
• But only IF there is a metadata file
  for the data.                          21
Lets make life a bit more difficult

• In addition to the many projections
  that 2D data can be in…
• There are two Coordinate Systems
  that are in common use …
• For smaller areas (like ½ a state)
• Much of the data you will find useful
  will be in one of these systems

                                          24
       These systems are…
• The UTM coordinate system or
  Universal Transverse Mercator
  coordinate system
• And
• The State Plane coordinate system
  – Unique to each state



                                      25
        UTM Coordinate Systems
• The UTM Coordinate system is –
  – based on the Mercator projection
  – A world wide system                Central Meridian
• Except that the                      Errors are Zero!
  cylinder is now
  horizontal and so
  is tangent to the
  earth along a
  meridian
  which passes
  through the Poles

                                                 26
       UTM coordinate system
• Is a projected coordinate system that divides
  the world into 60 north and south zones, each
  six degrees wide.
• Why bother?
• Increase Accuracy and decrease distortion
• Because all the data for a zone is within 3
  degrees of the Central meridian it is pretty
  accurate!
• Can’t map within multiple zones
• New York is usually mapped in one zone         27
            Most of NY is in
             UTM Zone 18
UTM Zones




                         28
          UTM Coordinates
Northing(Y)

     •The units in UTM are usually Meters
     •The coordinates are Eastings & Northings
     •The zone has to be specified

                         Easting(X)
  • Example: Location of CCC is:
   373,800 Meters E & 4,756,000 Meters N
   in Zone 18, N
                                          29
  The State Plane Coordinate System
• A projected coordinate system used
  in the United States
• Divides each state into one or more
  zones
• Also known as SPCS and SPC.




                                        30
              State Plane
• Horizontal zones (Tenn) are in Lambert
  Conformal projections
• Vertical zones are in Transverse Mercator
  projections
• Each state has its own origins for its own
  system
• States may have multiple zones in
  different projections
• UNITS are usually feet BUT NOT
  ALWAYS
                                           31
                    NY East
                   Zone 4801

State Plane Zones
              NY Central
              Zone 4826
   NY West
  Zone 4851




              NY Long Island
                Zone 4876



                               32
                                    NY East
                                   Zone 4801

              State Plane Zones
                              NY Central
                              Zone 4826
                   NY West
                  Zone 4851


Transverse
 Mercator

                              NY Long Island
              Lambert           Zone 4876
             Conformal

                                               33
     Another niggling Problem

• The earth is only approximately spherical
• We can mathematically convert features
  on the 3D earth to a 2D map easily if the
  surface is spherical and smooth
• Oops - earth is pear shaped and rough
• So we have to introduce the idea of a
  datum

                                        34
       Spheroids & Datums
• A spheroid can be moved mathematically
  to fit different parts of the earth…




         Earth




                            Now we have 2
                 Spheroid   different datums
                                           35
                So what?
• The spatial properties of a GIS data
  layer specify both the projection or
  Coordinate system and the Datum
• Different datums will cause shifts in
  location of the order of 100 meters
• Not big but troublesome
• In ArcGIS on-the-fly projection takes
  care of both projection and datum
                                          36
              Some Datums
     NAD 27      NAD 83     WGS 84

• These are the common datums
• For Coordinate Systems the spatial
  properties are given in statements like…
  NAD_27_UTM _ZONE_18N

               NAD_83_SPC _ZONE_4826
                                         37
                Summary
• There are a variety of spatial data types
• Spherical Geographic Coordinate Systems
  are based on Spheroids
• Spherical data is projected onto 2D maps
• There are many Projections
• More commonly, you will run into the class
  of Projections called Coordinate Systems
  (UTM, SP)
• Projected data is based on a datum and
  data in different datums will not (usually)
  line up!
                                                38
             Summary
• The subject of projections and
  datums is the most confusing and
  complex area of using GIS.
• Take good notes and do your best to
  understand it.
• At GIS conferences sessions on this
  topic are always very crowded! That
  tells you something!

                                        39

				
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posted:12/4/2011
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