Area surface not matching

Question

I am using ArcGIS Desktop 9.3 and WGS1984.

I have three municipalities with a total surface of 178 sq km. I want to analyze land cover of those municipalities using both LANDSAT images with a pixel size of 30 x 30 meters and SPOT with a pixel size 10 x 10 meters.

I have a raster of a whole county (> 1000 sq km) with land cover info derived both from LANDSAT and SPOT. When I clip the whole county raster with polygons of three municipalities, in case of clipping SPOT-based raster I get the correct surface of 178 sq km. That is the clipped raster area, the same as the surface of the three polygons/municipalities I am cutting with

If I clip the LANDSAT raster, the surface is 25% smaller, according to the attribute table! I know there should be differences because the pixel size is not the same and on the edges of the polygon I am cutting with some small discrepancies will appear.

But what happened with the 25% of the surface?

I really have no clue what to do, I have done such things before, but never had such different results.

Could it be something wrong with the data in the attribute table?

Answer 1

Two suggestions.

(1) I hate to be so obvious but arcgis does not auto-expand field in the attribute table. Try verifying manually by dragging the field to make it larger and see if more numbers appear.

(2) Find the exact x,y cell size (I suspect from the values your actually in UTM / WGS 1984) in raster properties and just multiply the numbers together and then multiply this number by the count in the field calculator to recalculate the area.