# Comparing watersheds generated with different DEMs

Hello, I’m working on a project comparing watersheds created with two different DEMs, where the difficulty lies in figuring out how to make their parameters equivalent, since the DEMs have different resolutions. I was wondering if this method makes sense:

I have two DEMS: A has a resolution of 1-arc second, and B has a resolution of 500m. A quick search showed that one arc-second = about 1860m of latitude. I know that the longitude of a square geographical unit differs depending on where on the globe one is, but for the purposes of this study, I’m just assuming that latitude and longitude are equivalent. So one cell of DEM A = 1860 squared = 3459600 sq m, and one cell of DEM B = 500 squared = 250000 sq m.

If I want to generate a watershed from DEM B with a stream cell count of 1000 for a stream threshold, that would equal 1000*250000 sq m of area. Since 1860 squared = 3459600 sq m, an equivalent flow accumulation for DEM A should be (1000*250000)/3459600, which equals 72.

Below are the results, with DEM B (500m) in the dashed red line and DEM A (1 arc second) in solid blue. Their similarity seems to confirm my calculations, but I was wondering if anyone had any comments.