# default shape area different than calculated area

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03-20-2020 04:43 PM
Occasional Contributor

If I draw a polygon in an ArcGIS Online web map or in ArcPro using an AGOL layer, the default "shape__area" field is off by about a factor of 5.  I need to calculate the area (in acres) of field boundaries using the default "shape__area", but dividing by 43560 (sq. ft./ac) does not work. Please see the image below where you can see the default shape__area, manually entered Acres, and measured Area in ArcPro.  For some reason, the conversion from shape__area to (manually entered) acres is consistently around 8500.

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Occasional Contributor

For anyone else that finds themselves with this problem, the answer did lie in the projection. If I create a polygon feature layer in AGOL, it will have the WGS 84 coordinate system and give wonky area calculations.  To solve this, I opened the feature layer in ArcPro, assigned a State Plane coordinate system, and calculated the area in ArcPro in the layer's attribute table.  To check my work, I opened the feature layer in AGOL and used the "draw area" tool in a web map to verify the measured area matches the calculated area, and it did. I also created another data column and calculated the areas in AGOL, again to verify the areas are the same.

2 Replies
Occasional Contributor

Also - Im assuming this has something to do with coordinate systems since AGOL layers are in WGS 84 and my projection is Wa State Plane South.

Occasional Contributor

For anyone else that finds themselves with this problem, the answer did lie in the projection. If I create a polygon feature layer in AGOL, it will have the WGS 84 coordinate system and give wonky area calculations.  To solve this, I opened the feature layer in ArcPro, assigned a State Plane coordinate system, and calculated the area in ArcPro in the layer's attribute table.  To check my work, I opened the feature layer in AGOL and used the "draw area" tool in a web map to verify the measured area matches the calculated area, and it did. I also created another data column and calculated the areas in AGOL, again to verify the areas are the same.