Calculate geometry problem

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07-02-2021 03:07 PM
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ErikFernandez
New Contributor II

I have a data frame in utm and a layer in WGS. When I open the attribute table and right click to calculate acres the calculation result is wrong if I use the default coord system of the data source. It's correct if I switch to the data frame. The results were off by almost half.   I just upgraded from 10.6 to 10.7 arcmap desktop and never had this issue before. Is there a way to default to using the data frame coord system?   Thank you in advance! 

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DanPatterson
MVP Esteemed Contributor

web mercator will produce erroneous results for anything requiring distance and area measurements.

it should only be used in AGOL for "looking" at stuff, and not be used for anything requiring analysis.

Always use a well established coordinate system that renders useful geometric measures... or leave everything in the geodesic/unprojected WGS-84 datum coordinate system and project as needed or use geodesic calculations.

 


... sort of retired...

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DanPatterson
MVP Esteemed Contributor

Use Add Geometry Attributes (Data Management)—ArcMap | Documentation (arcgis.com)

you have better control of inputs and outputs


... sort of retired...
ErikFernandez
New Contributor II

Thanks for the tip, appreciate the help.  Using "calculate geometry" in the attribute table is much easier (fewer clicks), so prefer that. It works if I select the coord system of the data frame (not the layer itself).  If I use default (the layer) it generates incorrect acreage.  I tried the add geometry attributes tool and interestingly that didn't produce accurate results regardless of coord system selected.

I'm wondering if the projection of the layer was causing the issue (it's not one I usually use): WGS_1984_Web_Mercator_Auxiliary_Sphere .  I reprojected into utm and everything is working well again. Hoping it's isolated to that one projection and not something new from upgrading to 10.7. 

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DanPatterson
MVP Esteemed Contributor

web mercator will produce erroneous results for anything requiring distance and area measurements.

it should only be used in AGOL for "looking" at stuff, and not be used for anything requiring analysis.

Always use a well established coordinate system that renders useful geometric measures... or leave everything in the geodesic/unprojected WGS-84 datum coordinate system and project as needed or use geodesic calculations.

 


... sort of retired...
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ErikFernandez
New Contributor II

Thank you for confirming that is the issue. Now I just need to remember to keep an eye out any time I download a new layer to ensure it's not in that projection. Learned something new. Thank you! 

ScottCorwin
New Contributor III

I'll also add to this, even though a solution has been posted.  I have an open ticket with a similiar issue, using a point dataset  that has a local coordinate system of NAD 1983 HARN StatePlane Washington North FIPS 4601 (US Feet), in ArcPro I get the same results using Add Geometry Attributes or Add XY Attributes but get different results using Calculate Geometry Attributes tool or right click Calculate Geometry option.  If I repeat this workflow in ArcMap, the results of all 3 operations are the same.  Using WGS_84 with no transformations.

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ScottCorwin
New Contributor III

I still don't think this applies fully as I'm using a projected system, unless by default the data frame uses a GCS in Pro, but they are aware of it.

https://support.esri.com/en/Technical-Article/000018807

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