The lower right corner of the flow accumulation example in ArcGIS Desktop's documentation has a flow accumulation value of 2.

However, I only see one cell flowing into the lower right corner (the cell immediately above):

enter image description here

Is there some nuance to the algorithm that I'm missing that would result in the flow accumulation being equal to 2 at this cell?

I don't care about the software being used - just the example/algorithm.

  • @PolyGeo, I'm not sure the argis-desktop tag is appropriate. I don't care about the software. Just teh example/algorithm.
    – Paul H
    Apr 26, 2015 at 21:28
  • I've edited accordingly but I think having the arcgis-desktop tag would have been more likely to bring your question to the attention of whoever authored and/or is responsible for that page of documentation.
    – PolyGeo
    Apr 26, 2015 at 21:35
  • @PolyGeo That's an excellent. I trust your judgement on that.
    – Paul H
    Apr 26, 2015 at 21:41
  • ESRI changed it's name/branding to Esri several years - but questions here are almost always about GIS software (or algorithms) rather than GIS companies so I would have left that out too.
    – PolyGeo
    Apr 26, 2015 at 21:46
  • 1
    In an answer at gis.stackexchange.com/a/63338/664 I worked through this example in detail, pointed out the error, and derived the correct answer. There is more than one error, BTW.
    – whuber
    Apr 27, 2015 at 21:25

1 Answer 1


It is an error. Your link and presumably image source is the 10.0 help. The error is still present in the 10.1 help. However in the 10.2 help, it has been corrected to show only a 1.

enter image description here

It's also worth noting that if you work through the math matrix as whuber has done at Arcmap 10 restrict Flow Accumulation, that error is apparent as well as the adjacent 35 value being incorrect, though that has not been corrected in the latest version of the graphic (it should be 27).

  • Thanks. I'm not an ESRI products user, and all of my googling took me to 10.0 and 10.1 pages.
    – Paul H
    Apr 26, 2015 at 23:46
  • @PaulH I've also noticed that Google results tend to favor 10.0 and to a slightly lesser degree 10.1. I imagine because of indexing and how long they've been around. But you do still run across 9.x and 10.2 results as well. Easiest thing to do is also specify a version as a search term - a specific one if you're looking for it or the 'latest' for 'current' help (though I notice 10.3 isn't showing up yet, possibly because of Esri page/site restructuring, time it's been available, or search engine mechanics).
    – Chris W
    Apr 27, 2015 at 19:18

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