Arcade area expressions form builder

[Anonymous User]

I want to configure a field in the form builder with arcade that calculates area using the expression area ($feature, 'hectares') or ($layer, 'hectares'). My layers are in British National Grid and the base map is the ESRI world imagery layer which is WGS84.

I know that my BNG layers will be re projected to WGS and I think %feature uses the base map co ordinate system/projection to calculate area.

I wondered if $layer would use the layer's defined co ordinate system/projection or the basemap co ordinate system/projection?

I also wondered if it would be possible to define the co ordinate system/projection for the area calculation in the expression?

Please and thank you.

[RobertBorchert]

I think that is beyond the capabilities of the Form Builder. 

It seems mostly restricted to Contingent Values

However, I do feel the ability to use calculations, like in Survey123, would be  tremendous step forward for forms.