Arc Pro 2.7 Calculate geometry calculations are incorrect

[KellyPilarski]

I have been attempting to calculate Square yards in Arc Pro 2.7 and have run into a issue with the results. I am getting approximately half of the expected value.  A one square yard area is coming up as .518. I tested the calculations on acres and square feet and they are all exporting incorrect values as well. I attempted to create new feature classes in several different spatial references to rule that out.

*Update: The Issue was resolved by having the same locale map frame and data spatial reference

We switched the map frame and the data to State Plane Michigan south and that seems to have done the trick. Thank you @jcarlson , @JoshuaBixby , and @DanPatterson for your time and assistance.

[JoshuaBixby]

Can you elaborate more on how exactly you are going about your calculations, and how are you determining the outputs are incorrect?  For example, are you creating basic square polygons of 1 yard x 1 yard and looking at the area field ArcGIS calculates?

[KellyPilarski]

After we realized we were having a issue, we started testing on a 1 yard by 1 yard  square. We also  opened the featureclass in Arc Map 10.7, and  calculated the geometry in that program and the areas came out correct. 

We also attempted to calculate field with a python script (!shape.area@squareyards!)  and it came up with the same number. 

[jcarlson]

I'd reiterate @JoshuaBixby's ask for how the calculations are being done. There are ways of calculating area that rather than depend on the spatial reference of the layer, depend on that of the map view the layer is added to.

[jcarlson · Accepted solution]

For issues with area / length calculations, the first step should always be checking the layer's spatial reference. What projection is your layer in? Not all projections are appropriate for what you're doing.

Try calculating the same shapes at different latitudes, see if that changes the output.

[KellyPilarski]

The map and data are currently in in State Plane Michigan South International feet. We attempted creating new feature classes in Michigan GeoRef Meters, and the original default of the web Mercator after reading an article that indicated web Mercator was not appropriate for calculations.

I moved the 1 sq yard area to northern Canada and it did change my calculations.  

Would you be able to assist on what an appropriate spatial reference would be for calculating area? Also, why is it different in ArcMap? 

[DanPatterson · Accepted solution]

You can try a simple UTM or State Plane projection for your data.  It is widely used, and anything is better than Web Mercator.

You must use the Project tool to get a file in the correct projection and not rely on alternate methods.  My advice is to only perform any area and distance calculations with projected data in a projection suitable (and/or widely used) for your area.