I used this youtube video: https://www.youtube.com/watch?v=FSblORbDyhc&list=PLWJ2X2wvq78QRGzl8Un5ADF0_4m5t99iD&index=11 to learn how to run flow accumulation and was successful. I realize it calculates where the watershed will accumulate flow (streams/rivers etc.) I just don't understand what changing the symbology to classified (2 values) in order to show it represents. Once flow accumulation is calculated, what can it show you, and how is that done?

  • Free cost of flow accumulation in latitude and longitude measure of a one gis software for area calculate with Google Earth map accuracy
    – user64195
    Dec 20, 2015 at 10:07

2 Answers 2


By just changing the symbology you can inspect the results visually, nothing else. By doing that you simply set a flow accumulation threshold what you want to see. This of course depends on the case and there are no exact answers what is right amount of flow accumulation for channel initialization. This visualization trick is of course very useful to detected any errors that might be in the original data (DEM) or with the algorithm. Note that there are several different algorithms for calculating flow accumulation. SAGA GIS is an excellent (free) software for calculating these, instead of ArcGIS.

Check these questions out too for more background regarding flow accumulation algorithms:Q1, Q2


Personally I've never found looking at the flow accumulation raster directly to be all that interesting. The purpose of messing with the cartography is to simply show you where the greatest flow accumulation is (the below example classifies it into two groups, one of which is transparent, the other is blue) from this you check if the major parts of your rivers look correct (like the below example). If there's oblivious problems then you would reprocess the flow accumulation raster. Flow carto example

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