Environment: ArcGIS Pro 3.5
Coordinate System: GCS_China_2000 (EPSG:4490, Lat/Lon)
Data: A large polygon (~40 sq km) with long straight segments defined in GCS.
The Issue:
I have noticed a significant inconsistency when calculating "Geodesic Area" for the same feature using different methods provided by ArcGIS Pro. The term "Geodesic" seems to imply different underlying algorithms depending on where it is used.
Test Results:
I compared four methods to calculate the area for the exact same feature. The results split into two distinct values (Value A and Value B):
Group 1: Result = 4687.6969 ha (Value A)
1. GUI Tool "Calculate Geometry Attributes":
2. Python Attribute Access (Calculate Field):
- Expression: !Shape.geodesicArea! / 10000
Group 2: Result = 4687.7674 ha (Value B)
3. Python Method Call (Calculate Field):
- Expression: !Shape.getArea("GEODESIC", "HECTARES")
4. Arcade Function:
- Expression: AreaGeodetic($feature, 'hectares')
Observations:
There is a difference of approximately 0.07 hectares between Group 1 and Group 2.
Group 2 (Python Method/Arcade) seems to strictly enforce Great Circle segments between vertices (bulging outward), resulting in a larger area.
Group 1 (GUI/Property) seems to respect the shape as defined in the GCS (likely treating segments as straight lines in the Lat/Lon grid), resulting in a smaller area.
Crucially: If I run the Densify tool (Geodesic, 10 meters) on the polygon first, ALL four methods return Value A (4687.69 ha).
Questions:
Why does the property !Shape.geodesicArea! return a different value than the method !Shape.getArea("GEODESIC", ...)!? This is highly confusing as they share the same name "geodesic".
Does the GUI tool "Calculate Geometry Attributes" implicitly assume a planar representation (or Rhumb lines) for GCS data, even when "Geodesic" is selected?
Which method is considered the "standard" implementation of geodesic area in ArcGIS Pro when dealing with sparse GCS geometries?
Any clarification on the underlying definitions would be helpful.
