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Is it the case that when I use Arc GIS to compute nearest distance from a point to a poly-line/polygon, I need to project them both in an equidistant projection? Note: I am using the near tool.

What happens, for example, if I do not project them at all and so the everything is just in the Geographic Coordinate System (WGS1984 in my case)?

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The documentation for Near Analysis indicates "NEAR_DISTANCE: The distance between the input and near feature. The value is in the linear unit of the input features coordinate system, or Meters when the Method parameter is set to GEODESIC and the input is in a geographic coordinate system."

So my understanding of that statement is:

A Geographic Coordinate System using the Planar would give you a distance value most likely in decimal degrees, while a Geographic Coordinate System using the Geodesic method would give you a distance in meters.

  • Does it matter then to project everything in an equidistant projection like equidistant conic? Or will distances be accurate without using any projected coordinate system? – user52932 Jul 26 '15 at 0:16
  • As a general rule that I use, if I want the results to be in meters/feet, then I use a projection that would give me that result. For me it reduces the what ifs. If you were starting out in GIS (I am not sure of your experience), then I would be saying to project the data, as I generally avoid taking measurements with data in Geographic Coordinate System – TsvGis Jul 26 '15 at 1:17
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    No projected CRS is great at not distorting distances. Even in equidistant conic, only the standard parallels and the longitude lines have correct distances. Using the geodesic method will give you true distances on the ellipsoid surface. – mkennedy Jul 27 '15 at 1:42
  • Hi user52932 and @TsvGis, what version of ArcGIS Desktop are you on (tag says 10.0)? Just wanted to mention in 10.2 and above, the Near tool will give you the option to calculate geodesic distances (i.e. great circles based on the spheriod) rather than planar (i.e. Cartesian straight-line) distances. My understanding is: using a non-equidistant projection with PLANAR is worst. Using an equidistant project with PLANAR is better. And using the GEODESIC option with a geographic coordinate system in 10.2+ (no projection necessary) is best since the measurmt. line curves with the earth model. – John Jul 27 '15 at 13:13
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    There's a good post on calculating distances and areas in PLANAR and GEODESIC measurements with respect to geographic and projected coordinate systems by @dmahr at this link. – John Jul 27 '15 at 16:41

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