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ArcMap Desktop Introduction: Projection Quick Reference Guide

Notes

  • This quick cheat sheet may help a user properly overlay layers on a map in ArcGIS.  NOTE: You still may need to further define or re-project layers, and certainly those with Geographic Coordinates, in order to perform certain sorts of analysis (e.g., proximity analysis, distance measurement).
  • In the last two columns when it says "Then, set data view's projection," once the layers are properly defined the data view's projection can be set to anything (doesn't have to match).

Sample data

Sample shapefile data to play with:

  • Geographic Defined (tl_2009_37063_tract00):  Durham, NC, Tracts (polygons) from 2009 Census TIGER Shapefiles
  • Geographic Undefined: Durham, NC Roads (tgr37063lkA) and Railroads (tgr37063lkB) (lines) from ESRI Census 2000 files
  • Projected Defined (dotprimrds_def): NCDOT primary roads (lines)
  • Projected Undefined (dotprimrds_ukn): Same as above, with ".prj" file named differently to hide projection information

Projection Quick Reference Guide

Layer opened first
 		 Geographic
Defined		 Projected
Defined		 Geographic
Undefined		 Projected
Undefined
  Sets Data View Sets Data View Discerns Long/Lat Coordinates,
but Data View still undefined		 "Unknown Units"
Subsequent
layer		  
Geographic
Defined		 No overlay problems No overlay problems No overlay problems 1. Define Projection tool on undefined layer
2. Then, set data view's projection
Projected
Defined		 No overlay problems No overlay problems 1. Define Projection tool on undefined layer
2. Then, set data view's projection		 1. Define Projection tool on undefined layer
2. Then, set data view's projection
Geographic
Undefined

No overlay problems
Discerns Long/Lat Coordinates

No overlay problems
Discerns Long/Lat Coordinates

 No overlay problems
Discerns Long/Lat Coordinates		 1. Define Projection tool on one layer
2. Define Projection tool on other layer
3. Then, set data view's projection
Projected
Undefined		 Use Define Projection tool on undefined layers Use Define Projection tool on undefined layers 1. Define Projection tool on one layer
2. Define Projection tool on other layer
3. Then, set data view's projection

Overlays only if layers are really both the same units (just undefined); otherwise, must define each layer, then set data view