Understanding ArcGIS Online published layers Shape Length and Shape Area fields

[Nick_Creedon]

In ArcGIS Online, I host a majority of our layers in our local state plane (published from ArcGIS Pro), their shape_length and shape_area units are in feet.

These shape lengths match up with measuring tools in ArcGIS Online, ArcGIS Pro, and out in the field.

I would like to publish some layers from ArcGIS Online (not ArcGIS Pro). When it comes to running statistics on polygon shape_area and line shape_length, they never make sense. I was under the impression that layers published in ArcGIS Online are in meters, but if you run the conversion, it doesn't add up.

Is there another conversion? 

Example of what I'm seeing (I don't have snapping on, but it's close enough):

The first picture shows my hosted layer published in our state plan with feet as units- this is correct.

The second picture is a layer published from ArcGIS Online in Web Mercator with units as meters- measurements are not the same.

[RyanUthoff]

The reason it appears to be off is because of the method AGOL is using to calculate the lengths.

When using the measure tool, Esri uses a geodesic calculation. https://doc.arcgis.com/en/arcgis-online/get-started/measure-mv.htm

This is actually something you can validate in ArcGIS Pro. Take the layer that you published in AGOL in Web Mercator and export it to a FGDB and bring it into ArcGIS Pro. Then, open the measurement tool and switch between Geodesic and Planar measurement. Geodesic should match the measurement tool in AGOL, while Planar should match the length in your attribute table.

[Nick_Creedon]

I have conducted a test in ArcGIS Pro.

Line Test Layer made in ArcGIS Online, Shape_Length is 119.72  meters

My other hosted layer is the exact same length but is 301.18 feet (True Measurement) 

Map, Projected in Web Mercator: 

Test Layer Projected in Webmercator

Planar Measurement: 119.72 meters (Matches Attribute Table, Not True Measurement)

Geodesic Measurement: 91.8 meters (True Measurement, No Match to Attribute)

My State plane line layer

Planar Measurement: 392.78 feet (No match to Attribute, Not True Measurement)

Geodesic Measurement: 301.18 feet (Matches Attribute Table and True Measurement)

Map, Projected in Local State Plane:

I get 301.18 Feet  and 91.8 Meters in both Planar and Geodesic, mainly because of the extent of my measurement. 

Layers Project on the Fly in ArcGIS Online and there are also transfomations in ArcGIS Pro.

This is the confusion I have had for a while with layers created in ArcGIS Online, the only answer is for us to create them locally in our cordinate system and publish them as such so when we create data, the shape length is true and the measuring tools work without having to bring stuff back into ArcGIS Pro.