# How to know if flow accumulation map reflects true catchment areas?

I have calculated flow accumulation (img file) using this DEM (img file), which is 0.5 degree resolution, using arcmap.

I am wondering if the cell accumulation value can directly be translated into drainage areas in square kilometers or not, and if these values reflect true catchment areas?

For example, if the flow accumulation value is 140 and my resolution is 0.5 * 0.5 degree = 56km^2, the upstream area is 140*56 = 7840km^2.

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There seem to be two mistakes in the calculation "0.5 * 0.5 degree = 56 km^2". One is that the area of a square degree varies with latitude, which is not accounted for here. The other--which is a huge mistake--is that a cell of 1/2 degree will have dimensions (approximately) 111/2 km = 56 km in the north-south direction and (approximately) 111/2 * cos(latitude) in the east-west direction, for a total area of about 3080 * cos(latitude) km^2, which is orders of magnitude greater than 56 km^2! – whuber Nov 5 '12 at 15:49
A more fundamental mistake reflected here is to compute flow accumulation using such a poor projection (the Plate Carree or "geographic"). This biases the results (see Calculating Flow Direction and Delineating Basins from Projected vs. Unprojected DEM Data). Rather than translating the results into areas, it is far better to redo the flow accumulation using a better projection, which will provide flow accumulation areas directly and with less bias. – whuber Nov 5 '12 at 15:56
Thanks ,how did you know the projection is geographic?what is better projection? – Jonsson Nov 5 '12 at 16:30
The link in my second comment goes to a detailed, extensive thread in which all these things are discussed and your question about a better projection is thoroughly answered. – whuber Nov 5 '12 at 22:02
I'm sorry, I am not going to take a look at it. In my first comment I have already explained why you are experiencing these differences in areas. The links in my second comment fully answer your other questions. More generally, this is not a place to ask others to do your work for you (although many kind community members often go a long way in that direction). Questions here are expected to be self-contained insofar as they provide sufficient context and showing what preliminary efforts and research have been done towards resolving the problem. – whuber Nov 6 '12 at 17:21