![]() As mentioned before, these are designed to minimize distortion for just the regions or zones that they cover. If your application covers a small area, such as a city, county, or perhaps even a state or province, you can use a local coordinate system such as State Plane or UTM. You should make your decision based on the general area covered by your application. This begs the question, which coordinate system should you use for the measurement? There is no answer that fits every application. Once the measurement is complete, you return the correct result to the user. ![]() The re-projected geometry is used internally for measurement only you don’t display it on the map. When a user submits a geometry to your application for measurement, you can re-project that geometry into a more appropriate coordinate system for measurement. So, how should you measure?įortunately, you don’t have to perform your measurements in the same coordinate system used by the map. The farther away from the Equator you get, the more distorted your measurements will be. In cylindrical projections like this, errors are minimized along the standard parallel(s), and in the case of the Mercator projection, this standard parallel is at the Equator. If you perform measurements in the Mercator projection or the Google/Bing/ArcGIS Online variation thereof, your results will likely be much larger than you intended. This is evident from the enormous dimensions of Greenland and Antarctica relative to land masses closer to the Equator: The projection sacrifices some accuracy because it is based on a perfect sphere (the earth is better approximated by a spheroid), but the biggest problem is the heavy vertical and horizontal stretching at extreme latitudes. This projection fit the entire globe (well, most of the latitudes anyway) into a square area that could be covered by 256 x 256 pixel tiles. Instead, it was engineered for convenience in working with cached map tiles. The modified Mercator projection used by Google, Bing, and ArcGIS Online is not designed to minimize distortion at all. Other projections (such as UTM and State Plane) are designed for focused areas of the globe in order to keep the distortion minimal. Some projections such as Robinson or Winkel Tripel attempt to minimize distortion across the world through some compromise of all those factors. ![]() Why is the Google/Bing/ArcGIS Online projection not acceptable for measurements?Įvery map projection causes distortion of shapes, areas, directions, and/or distances. You should instead re-project user-submitted geometries into a more appropriate coordinate system before you perform a measurement. If you use the new ArcGIS Online services, avoid the temptation to perform measurements of polylines and polygons in Web Mercator. Recently, ArcGIS Online services became available in the same “Web Mercator” projection used by Google Maps and Bing Maps.
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