Measured area inconsistent with Area() and Shape__Area

[jwb]

I am using a hosted feature service, maintained by my county assessor, that uses spatial reference, 102100 (3857), the units of which are "esriMeters". It is hosted here if you want to see the metadata:

https://services5.arcgis.com/ROBnTHSNjoZ2Wm1P/ArcGIS/rest/services/Parcels/FeatureServer

In my map on ArcGIS Online, I added an expression Area($feature, 'square-feet'). For a feature that is 7287 square feet in fact, the result is 11247. This isn't consistent with foot/meters confusion because because the ratio is wrong. If I measure the shape with the area measurement tool, I get the correct figure in square feet. I gather that the measurement tool uses the basemap spatial reference. See screenshot below.

The question is: what's the problem here? Is the metadata of the hosted feature layer incorrect? Or do I need to do something in the map to correct it?

 

[DanPatterson]

FAQ: Why Does ArcGIS Online Use a Deprecated Spatial Reference (102100) for Hosted Service (esri.com...

in any event, you shouldn't be using web mercator for measuring area or length, it will give erroneous results since it is not a conformal, equal area or equidistance coordinate systen

[jwb]

I am not "using" webmercator in any particular sense, as I assumed that the Area() function will paper over this issue or refuse to operate on a CRS where area is nonsensical. For example if I load this feature layer into geopandas it would refuse to to give the area unless I project it onto CEA or something. 

But that wasn't really my question. My question is given that this layer is not produced or hosted by me, what can I do about it, so that Area() works sensibly on my map? In the past I have downloaded the entire feature layer and pre-processed it offline, but that's kind of the nuclear option.

[DavidPike · Accepted solution]

possibly https://developers.arcgis.com/arcade/function-reference/geometry_functions/#areageodetic

[jwb]

That appears to do the job!