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I would like to calculate catchment area. I used D8 algorithm and running in both arcgis 10.2 and SAGA GIS 2.1.2. However, the results from two software are different. The lowest value of catchment area in arcgis = 0; high value = 109325, while SAGA GIS lowest value= 625, highest value=47820. I think the result should be similar and maybe I do something wrong. Could you please explain what I did wrong? These are what i am doing,

the input: DEM25 In arcgis, to calculate catchment area: Dem25, then run tool flow direction, then run tool flow accumulation, then 25 (cell size) * 25 * flow accumulation

In SAGA GIS, terrain analysis -- hydrology --catchment area (parallel), elevation = dem25, sink routes=no set, weight = not set, catchment area = create, Step =1, linear flow = no tick, linear flow threshold =500. convergence = 1.100000001

Thanks so much for your help.

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ArcMap uses the D8 routing algorithm for determining flow directions between cells, with no convergence exponent since it is a single direction algorithm, while the default in SAGA GIS (I believe - at least it is in mine) is MFD with the convergence you specify.

These methods differ and therefore will generate different catchment area results.

You could run SAGA GIS with the D8 algorithm (and a convergence = 1), and this should make the results match more closely, but I have the feeling that even then they will not be identical. Different implementations of the same algorithm (differences in the way they are coded up) should still lead to differences in the results.

Hope this helps.

Edit: See the comment from @WhiteBoxDev below as well.

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    Also, it is possible that they are measuring catchment area in different units, e.g. number of upslope grid cells, versus upslope area, versus specific contributing area (area per unit contour length). All of these are common units for measuring flow accumulation.
    – WhiteboxDev
    Sep 2, 2014 at 20:40

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