Defining Coordinate Systems in ArcMap

By CAST

 

An ESRI shapefile has 3 absolutely necessary files - *.shp (shape), *.dbf (table) & *.shx (index) ... otherwise it won't be a "complete" shapefile.

In addition, a *.prj file is very handy - this is a projection file that tells ArcGIS what coordinate system the shapefile is in, so that all of your data lines up correctly.

Sometimes when you download your data from the internet it will already have one of these (if you're lucky!) ... but not always! (If you export your data out of GPS Pathfinder Office then you won't have a *.prj file either!). When you add a shapefile that doesn't have a *.prj file to ArcMap you will get an error saying:

This is easy to fix ... first of all you need to know what coordinate system your data is in -

If you downloaded the data from the Internet (Geostor, ESRI, CaSIL, Terraserver, etc):

  • Internet Downloading: look on the site where you downloaded your data from OR:
  • Internet Downloading: look in the "metadata" file- it will have come with your shapefile and is usually an *.html file.
  • GPS collected: when you export your data out of GPS Pathfinder Office click properties at the bottom right corner of the export window and look under the "Coordinate System" Tab.

Common coordinate systems include:

  • Latitude/Longitude - measured in degrees
  • UTM (Universal Transverse Mercator) - measured in meters
    • Arkansas = Zone 15
    • California = Zone 10 or 11
  • State Plane - usually measured in feet
  • Teale Albers Equal Area (for California) - one coordinate system for the whole state (rather than 2 zones for UTM)

You also need to be aware of the Datum (an estimation of the shape of the earth) ... common datums include:

  • WGS 1984 (World)
  • NAD 1983 (North American Datum calculated in 1983 - very close to WGS 1984)
  • NAD 1927 (North American Datum calculated in 1927 - not as accurate as NAD 1983!)

Once you know both your coordinate system and datum you can create the *.prj file:

  • In either ArcMap or ArcCatalog start ArcToolbox. Click the plus sign next to "Data management Tools" and again next to "Projections and Transformations". Double Click define projection.

  • In the Define Projection" window select your "Input Dataset or feature class" (the file that you need a *.prj file for).
  • Click the icon that looks like a hand holding a sheet of paper to select your coordinate system ...

  • In the spatial references window hit select to browse for a coordinate system. In the new window you'll have a choice of two folders - "Geographic Coordinates Systems" and "Projected Coordinate Systems" . If your coordinate system is Lat/Long choose "Geographic Coordinates Systems", otherwise select "Projected Coordinate Systems".
  • In the Geographic Coordinate System choose the folder that refers to your datum
    • World = WGS 1984
    • North America = NAD 1983
  • In the Projected Coordinate System choose the folder that describes the coordinate system
    • State Plane = State Plane - select the datum folder and then the .prj file
    • State System = Teale
    • UTM = UTM (Universal Transverse Mercator) - select the datum folder and then the .prj file (look for the zone number).
  • Click Add, and OK on the spatial reference window, and again on the define projection window.
  • The following windows will appear during the creation of the *.prj file and after the file has been succesfully created

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Provided by your Geospatial Support team from:
Center for Advanced Spatial Technologies (CAST)
University of Arkansas