Tabulate intersection: strange error in area calcs

[NathanKarres]

Hi all,

I am trying to understand an issue with a relatively simple overlay analysis. I have two polygon feature classes: 1 for my zones (with 1 zone field), and 1 for my classes (no class fields). My global data are all WGS_1984, so I am projecting on-the-fly to get area calculations (Mollweide) in km2.

When I run the "Tabulate Intersection" tool, I get what appear to be accurate percentage values, but very much inaccurate area values. As an easy illustration of the issue, see the example values below for a zone with 100% intersection:

Tabulate Intersection PERCENT: 100%

Tabulate Intersection AREA: 116.2149223 km2

Zone polygon area (from calculate geometry): 96.86654592 km2

Intersect tool area (from calculate geometry): 96.866546 km2

Any clue what is going on here? What am I not understanding about the Tabulate Intersection tool that leads to this discrepancy?

[NathanKarres]

Some follow up to my own question — looks like Tabulate Intersection does not respect the selected on-the-fly coordinate system. I recall reading somewhere that the defacto ArcMap display projection for unprojected (geographic) data is roughly equivalent to Plate Carree. Sure enough, with Plate Carree selected as my projection, calc geometry for the polygon gives an area value of 116.605551 km2 — that's pretty close to the original Tabulate Intersection value.

And if I transform the projection of my zonal and class feature classes to Mollweide prior to running Tabulate Intersection, I end up with the expected area value: 96.866546 km2.

Looks like you can't take shortcuts with Tabulate Intersection, needing to transform data to the appropriate projection before running the tool. Good to know where the apparent error was originating, but is this the intended behavior for this tool?

[curtvprice · Accepted solution]

How were you projecting on the fly? It should work if you set the environment setting Output Coordinate System.

If you just set it in in the ArcMap data frame, that will not affect the coordinate system used in geoprocessing.

There is no "default" projection in ArcGIS. Some tools will calculate areas using geodesic area.

[NathanKarres]

Thanks Curtis, I don't often deal with calculating areas through geoprecessing tools and didn't realize the data frame setting is inconsequential.

I'd still be curious to know how ArcMap is calculating area values for GCS (unprojected) data with the Tabulate Intersection tool. Here's a comparison for one of my polygons: 

AreaKM2_Gdsic	AreaKM2_MollW	AreaKM2_PlateC	AreaKM2_TabInt
907.4346	912.9755	935.2109	932.078

My comment about a "default" projection was simply in reference to whatever projection Esri uses to visualize a geographic coordinate system on my 2D computer screen. See definition 2 here.