Why are areas calculated in ArcGIS Pro different to the same calculation in ArcMap?

LindsayRaabe_FPCWA

I have 2 datasets that were put through a union and using the output I have then calculated the geometry to get a Hectare area figure. It's a broad layer covering a vast area of Western Australia. 

 

In ArcMap, we calculated the geometry using 3 different coordinate systems and got 3 different areas (as we expected we would). 

 

I have then repeated the process in ArcGIS Pro (same data sources and process) and got the below results - and not only do the figures differ from the ArcMap results, but they are the same from projection to projection (this is what originally got us looking closer as we expected different numbers). 

The Albers figure is closest to matching between the ArcMap and ArcGIS Pro figures which makes sense as it's a broader scale projection compared to Zones 50 & 51, but still differs. 

Can anyone explain either what we're doing wrong or why the results in ArcGIS Pro are seemingly consistent when they shouldn't be?

 

sjones_esriau

Hey Lindsay,

"Or will geodesic fix this too?" 

Yes, Kind of. 

 

In the example above I've calculated the area in both Planar and Geodesic.

When calculating the planar area of the data in Zone 51, the geometry is projected from the globe to a flat extended plane which is centered on the central meridian of zone 51, somewhere to the east. 

 

This is why the data gets distorted the further you are away from the central meridian of the zone and why calculated planar area for Zone 51 is larger than Zone 50 (roughly where the source data is).

For the geodesic calculations, the geometry is still being projected to the zones, but now the extended zone wraps around the globe rather than being a flat plane. Since the GDA zones are uniform, the area and length calculations of features will be the same for geodesic measurements between zones.

This doesn't negate the need for zones however as Pro map views also operate in planar space unless specific tools or operations provide the options to create or calculate data differently. (e.g. create geodesic in editing, geodesic viewsheds, calc geometry in geoprocessing etc.). 

The mind bending thing is, like projections, all these area calculations are mathematically correct. You just need to use the one that is fit for purpose and record it when reporting. For large area calculations within Australia you can also try the GDA 94 Geoscience Australia Lambert projection or Albers.

(also upgrade Pro, if you are not seeing the dropdown options)