I've got a polygon featurelayer in ArcMap 10.

The coordinate system of the featurelayer's featureclass is web mercator.

The dataframe of the map uses an equal-area projected coordinate system.

I've got a field on the featureclass called Acreage.

I'd like to calculate Acreage based on the polygon's re-projected area (into that of the dataframe's projection). I don't want to create a new featureclass to do this.

Is there an easy way to do this in ArcMap 10?


3 Answers 3


If you are doing this within Arcmap and you have an open table, have you ruled out using the Calculate Geometry option after right-clicking on the field. If your dataframe is already set in the projection that you want you should be able to use its coordinate system without the prior projection issue.

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There is a way to simple calculate the geometry using Python.

import arcpy
arcpy.CalculateField_management('INPUT_SHAPEFILE.shp','ACRES_FIELD','!shape.area@acres!', 'PYTHON')

This will calculate based on the projection information of the dataset.


XTools Pro, an extension for ArcGIS has a function which will calculate geometry based on any projection you choose regardless of the projection of your data frame.

It's free to download, and there are some features which expire after the trial. But this feature (as well as many others) remain free after the trial period.

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