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I have a dataframe in arcMap 10.0.4 that is of Africa Albers Equal Area Conic PCS. I bring in a census shapefile of WGS 1984 CS and it is projected on the fly. I do 2 things:

  1. I use the calculate geometry option to overwrite the "Shape_Area" field in the census shapefile so that I may obtain the planar measurements of the shape area (i.e. square Km as opposed to Decimal Degrees).

  2. I 'union' an existing polygon grid projected as Africa Albers Equal Area Conic with the WGS'84 Census shapefile. The result of this union is a shapefile with the Africa Albers Equal Area Conic PCS.

I ask both these questions because they seem to me to stem from the same issue: error propagation. Am I creating measurement and geometric errors because I should be instead projecting the census shapefile to Africa Albers Equal Area Conic PCS? What is the capabilites of ArcMap to handle this issue?

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up vote 1 down vote accepted

As long as you use the "use the data frame's coordinate system" option in Calculate Geometry, the same algorithm will be used to calculate the polygon areas as if you permanently projected the data to the Africa Albers equal area conic.

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Thank you, now what about the union process? Will the algorithm also work as if the shapefile was permanently projected into Africa Albers Equal Area Conic PCS? –  XNSTT Apr 11 '13 at 7:40
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