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When running a point-in-polygon operation, is it valid to run that with polygon features in straight up WGS84 and a point in lat/lng, or should both be projected into a non degree based coordinate system?

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    WGS84 is fine for this operation... projections would be more useful for getting area/lengths, etc. – DPSSpatial Nov 20 '14 at 17:57
  • Are you using ArcGIS for Desktop to do this? – PolyGeo Nov 20 '14 at 20:11
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It is entirely valid, but it may not produce the results you expect in any Cartesian coordinate space. So it isn't a problem with degrees or WGS84, but rather the mapping from the ellipsoid to a plane.

If the polygon is very large, the mapping of the curved surface on the ellipsoid to the plane can produce some artifacts where a point looks like it would be just inside (or outside) the polygon, but isn't quite. That can also be an issue if some of the polygons cross the anti-meridian (the -180/180 part). If those might be an issue for your data, and you can handle the performance impact, consider something like a PostGIS geography approach.

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    Mainly depends on how the software connects two points in a geographic CRS. Are the points connected with a geodesic or normal section or a 'Cartesian' line. Ex. between two points on the same latitude line, the Cartesian line is the latitude line. The geodesic/normal section might pass above or below the latitude line. – mkennedy Nov 20 '14 at 21:40
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    It's not the size of the polygon so much as the size of the gaps between vertices in the polygon. If the intervertex distances are set to approximate a great-circle route, then even a Cartesian point-in-poly will be "accurate" in a GCS (within the definition of "accurate" established for the polygon). – Vince Nov 21 '14 at 2:24

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