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




                                                            7
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
                                                                   14
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




                                   33
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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