Using Map Projections and Coordinate Systems by oVWwKJu

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```									Using Map Projections and
Coordinate Systems

Melita Kennedy
Vivian Mitchell
In memory of
Dr Werner Flacke,
ESRI-Germany

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Objectives

• Learn about the spatial reference
– Geographic versus projected coordinate systems
– Storage and processing parameters
– Setting appropriate values
• Maintaining consistency in the geodatabase
• Identifying an unknown coordinate system
• Picking a geographic/datum transformation

Note: Presentation will be available on the Proceedings CD

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Projection & datum overview

• Earth is three-dimensional
• Map (screen) is 2-D
• Geographic coordinate system (datum) locates
in 3-D
• Map Projection converts 3-D to 2-D
• 3-D to 2-D causes distortions

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Geographic coordinate system

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Geographic coordinate system

(gcs, geogcs)

• Name (European Datum 1950)
• Datum (European Datum 1950)
–Spheroid (International 1924)
• Prime Meridian (Greenwich)
• Angular unit of measure (Degrees)

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Projected or Cartesian coordinate system

Y

• Linear units                            Data
X-           X+
• Lengths, angles,            Y+           Y+
and areas                            usually here
X
are constant
X-           X+
• Shape, area, and distance   Y-           Y-
may be distorted

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Projected coordinate system

(pcs, projcs)

• Name (NAD 1983 UTM Zone 11N)
• GCS          (NAD 1983)
• Map projection (Transverse Mercator)
• Projection parameters (central meridian, latitude of
origin, scale factor, false easting)
• Linear unit of measure (Meters)

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Geographic versus projected Demo

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Key questions before starting a project

• Purpose or uses
• Area of interest
– For local datasets, use projected coordinate systems
– For global/small scale use geographic coordinate systems
• Required accuracy
• Characteristics and attributes
– Raster/vector
– Point/line/polygon
– Distances/areas/??

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How well do you know your data?

• GIS data is designed for a specific scale
• Decisions regarding storage, accuracy, etc. are
influenced by the intended scale
• Metadata is invaluable
• If 2 datasets don’t overlay, can you tell which one is
wrong?

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Understanding coordinate systems

• What coordinate systems are used by the data?
• What coordinate system will the end product use?
• Choice of coordinate system is important
– Units
– Datum
– Projection

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Choosing the right coordinate system

• What does your boss think?
• What are other government agencies/partners using?
• For what purposes are the data going to be used?
• Minimize projecting data on the fly
– Impacts performance

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Coordinate system gotchas

• Defining a coordsys updates the metadata ONLY
– Doesn’t affect the coordinate values
– Define data in its current coordinate system, then project
• Datum transformations are important!
– Omit or choose the wrong one—up to 200 m
– Multiple ones exist—up to you to decide which one is best
– See Knowledge Base article #21327
• Raster and CAD data may need georeferenced
5,5                                432619,
3877223

0,0                          432499,
3877103            NAD83 UTM 11N
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Unknown coordinate systems

• ALWAYS define the coordinate system
• Good professional practice—help your successor
• Units are unknown
• Map scale is incorrect
• Geodatabase tools can’t use default values

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What do I do if I don’t know my data’s coordinate
system?
• Check the data provider or source
• Check any existing metadata
• Similar data types
• What coordinate systems are used in the area?
– http://www.epsg.org
• Try using ArcMap to figure it out
– See Article ID #: 24893
HowTo: Identify an unknown coordinate system using ArcMap
– Live Training Seminar (free)
Working with Map Projections and Coordinate Systems in
ArcGIS

http://training.esri.com/acb2000/showdetl.cfm?did=6&Product_i
d=826&2                                                         16
Familiarize yourself with common coordinate
systems

• Know what coordsys are used in your area of
interest
• Learn what the layer extents should be

San Diego, California
NAD 1983             X / longitude   Y / latitude

Geographic                   -116.67 °         33 °

UTM zone 10N                 530,000 m      3,650,000 m

State Plane (CA zone 6)     1,960,000 m     593,000 m

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Spatial reference

• Coordinate system (projection)
• Tolerance
• Resolution
• Domain (extent)

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Maintaining coordinate consistency in ArcGIS

• Tolerance information stored as part of feature class
schema
– Key piece of information
– Cannot be changed once created
– Value used throughout system
•   Map’s spatial reference
•   Spatial selections/queries
•   Topology
•   Geoprocessing

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Tolerance

We have always used a tolerance (based on the
resolution)
to evaluate spatial relationships

• 9.1 — Fixed tolerance
– 2x resolution
• 9.2 and later — User-defined tolerance
– Must be ≥ 2x resolution

Define the tolerance based on data accuracy,
not storage accuracy

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Maintaining coordinate consistency in ArcGIS,
part 2

• Resolution information stored as part of feature class
schema
– Key piece of information
– Cannot be changed once created

• NOTE: Resolution = 1/precision

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Resolution and spatial domain

• Domain and resolution are complementary properties
– i.e., increase in resolution results in decrease in domain extent

Resolution
increases

• Domain defined using the valid extent of the horizontal
coordinate system

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Resolution

• ArcGIS supports resolutions < 1 micrometer
– Varies slightly by coordinate system

• Why don’t we just use the minimum resolution?
– Performance Impact
• Storage cost
• Processing performance

• Our default (1/10 mm) balances storage precision and
performance
– Defaults should be used in most cases

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Choosing an appropriate tolerance and resolution

• Question: How is my data going to be used?
– Only as accurate as source data (best case)
– Do I expect to improve my accuracy over time?

• If values are too small,
– Data won’t line up
– Adversely affects performance

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How accurate is my source data?

• My coordinates are stored in a coverage or shapefile as
double-precision numbers
– Does not mean your data is accurate to those values
– For coverages, fuzzy tolerance is a rough measure of the data’s
accuracy
• Accuracy does not equal storage resolution
– ArcGIS can support storage resolution of ~10 nanometers,
but data is never this accurate

– Coordinate accuracy and storage accuracy are now separate

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Map Scales and Resolution

1 inch =      1 cm =
1:500       41.67 ft      5m
1:1,000      81.33 ft     10 m
1:5,000     416.67 ft     50 m
1:10,000      833.33 ft     100 m
1:12,000       1000 ft      120 m
1:24,000       2000 ft      240 m
1:50,000    0.78914 miles   500 m
1:63,360       1 mile       634 m
1:100,000    1.5783 miles    1 km
1:250,000    3.9547 miles    2.5 km
1:500,000     7.891 miles    5 km
1:1,000,000   15.783 miles    10 km

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Putting it all together

• With the answers to the key questions, you can define
your GIS database schema

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Putting it all together - Example

• Source data – shapefile
• Coordinate system – NAD83 State Plane California VI
(6)
• Desired study area – San Diego County
• Desired accuracy – 1 meter
• Intended use – Identify possible routes to connect
existing bike trails

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Create Feature Dataset and Feature Class Demo

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Spatial References in ArcMap

• The map has a spatial reference
• Properties of this map determine processing
parameters
• Can be set explicitly
• Important if you have data with various accuracy
standards

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Editing in projected space

• Edit using a local coordinate system
– Data stored in decimal degrees
• Considerations when creating or editing features
• Snapping
– Use vertex or endpoint snapping in the editor
– Do not use edge snapping
• Managing distances
– Determined by the map’s spatial reference

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Understanding coordinates in ArcGIS

• Unexpected behavior:
– Example: ―I just typed in 50.01, but when I went back to check
it, the coordinate was 50.0099999!‖

• All coordinates in ArcGIS geodatabases are stored as
integers, but processed as double precision values

• Significance is based on your tolerance—if your data
isn’t this accurate then they are coincident

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Geographic/datum transformations

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Warning: different geographic coordinate
system…

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Why transformations matter

• Offsets can be significant

San Diego, California
Geographic           Longitude         Latitude

NAD 1927               -116.6691455°      32.9999533°

NAD 1983               -116.6700000°      33.0000000°

NAD 1983 HARN          -116.6700004°      33.0000000°

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Choosing an appropriate transformation demo

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Wrap-up

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Where to find help
–Melita’s schedule
• Geodatabase Management Island
– Wednesday 12:30 pm – 4:00 pm
(2-4pm: all members of projections team)
• Technical Support
–Wednesday 9:00 am – 10:00 am
–Thursday 12:00 pm – 1:30 pm

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More information

• Don't forget the Knowledge Base!
– http://support.esri.com
• 23025, 29129, 24893, 29035, 17420
• ESRI forums for user-to-user help
– http://forums.esri.com
• Virtual Campus
– http://campus.esri.com
– Live Training Seminar and Course

• http://www.epsg.org
– Database of coordinate systems & datums
– Guidance Note 7

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Books, etc.

• Snyder. Map Projections: A Working Manual
– http://pubs.er.usgs.gov/djvu/PP/PP_1395.pdf
• Flacke & Kraus. Coordinate systems in ArcGIS
• Elithorp & Findorff. Geodesy for Geomatics and GIS
Professionals
• Snyder & Voxland. An Album of Map Projections.
USGS PP 1453
– http://infotrek.er.usgs.gov/pubs
• Iliffe. Datums and Map Projections

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