topic MeasureAction: geodesic area? in ArcGIS API for Silverlight Questions

MeasureAction: geodesic area?

MarkCederholm — Mon, 19 Jul 2010 18:43:47 GMT

I've noticed that in the online MeasureAction sample, the perimeter is correct but the area is off compared to a similar measurement in a projected coordinate system. Is a cartesian or geodesic calculation used for the area? I've adapted the geodesic area code from GRASS and the results compare favorably to the projected results.

Re: MeasureAction: geodesic area?

dotMorten_esri — Mon, 19 Jul 2010 20:12:43 GMT

Geodesic calculation is only used when using a map with geographic coordinates. For anything else, euclidean calculations are used, so "beware of projection distortion".

Re: MeasureAction: geodesic area?

MarkCederholm — Mon, 19 Jul 2010 20:16:18 GMT

By "online" sample I mean the one published by ESRI at the API for Silverlight resource center. It uses the Streetmap_2D service, which is in geographic coordinates. I was assuming knowledge of this in my original post. To verify the measurements, I brought the same service into ArcMap and projected it to UTM (WGS 84) for the area I was testing. In the sample, the MeasureAction's map units are set to DD, and the perimeter calculation is correct, so obviously the geodesic distance algorithm is used in that case. It is the area calculation that I am bringing into question.