default shape area different than calculated area

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03-20-2020 04:43 PM
DryCreekEng
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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DryCreekEng
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. 

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DryCreekEng
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.

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DryCreekEng
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. 

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