Area Calculations - Different totals when comparing Original Layer and Sum of Clipped Layers

[StevenSGIS]

Hi,

I was tasked with something that I thought would be simple, but is giving me a hard time.

When comparing the acreage totals between the original layer and the sum of clipped layers from the original layer, it comes out with a huge discrepancy.

1036972.541

1231231.148

The sums were calculated outside in excel.

I've double checked other threads and looked at potential solutions, such as making sure the layers are projected under the same coordinate system - but after having done that, the results have not changed.

Other Details:

Working in NAD 83 CA Teale Albers.

The statewide acreage totals to LESS than the sum of when I clip the acreages into (7) different regions in California.

[StevenSGIS · Accepted solution]

After banging my head against the keyboard for the past 6 hours.

I've figured out the issue.

To explain:

I've been calculating the geometry BEFORE clipping.

After clipping, I noticed that the geometry was NOT being updated - which resulted in polygons that were shared among the different regions having the same acreage.

The solution - after clipping, recalculate the geometry.

[BarryNorthey]

Your problem might be a Data Type issue. Pro creates and manages the Shape_Length and Shape_Area fields in fGDB polygon feature classes so they cannot be directly edited in the attribute table by a user. If for example, you Split an fGDB polygon into two parts (an edit), Pro would automatically recalculate the Shape_Length and Shape_Area information for both new polygons and being an edit, can be saved or discarded.

A shapefile requires a minimum of 3 fields: FID, Shape and 1 other. A polygon shapefile feature class does not require fields relating to area or length. Pro does not automatically create or manage these. A user can manually create these as number fields (Type: Double) or they might appear in a shapefile exported from another format that had these fields but they are now treated as any number field by Pro. If for example, you Split a shapefile polygon that had a field called Area, the two new polygons would have the same value in the Area field as the original polygon as Pro does not tie the values to geometry directly and treats them as any number field. You can then Calculate Geometry (Area) on that field and Pro will update the Areas.

From my viewpoint it's one of the many reasons to work in fGDB vs shapefile. You can always export to shapefile if needed for data exchange.