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FAQ: Why are some polygon areas different in ArcGIS 10 than in previous versions of ArcGIS?

Question

Why are some polygon areas different in ArcGIS 10 than in previous versions of ArcGIS?

Answer

Area calculations made by the ArcSDE sg library - the geometry library that performs such tasks as SE_shape calculations and geometry processing in ArcSDE and some ArcGIS clients - use internal integer coordinates. These integers can be large for shapes that contain coordinates near the upper range allowed by a high-precision coordinate reference. In ArcGIS 10, the sg library has been altered so that it offsets the integer coordinates by the first coordinate in the polygon, thereby reducing the scale of the integers, reducing the amount of rounding error, and improving the accuracy of area measurements on these high-precision shapes.

Therefore, when the area for an extremely high-precision polygon is calculated using the ArcSDE 10 sg library (which is present in ArcSDE and ArcGIS 10 clients), the resultant value may be slightly different than the area returned for the same shape in a previous version of the software.

Note:
Because the sg library is also used for some calculations in ArcGIS client applications, connecting from an ArcGIS 10 client to an ArcSDE 9.3.1, 9.3, or 9.2 geodatabase, may also return this difference in area calculation.

The shapes themselves have not been altered, and only the area calculation is more accurate than it was previously.