Inconsistent Flow Direction Values After DEM Preprocessing – Seeking Guidance

Question

Hello ESRI Community, I’m currently working on DEM preprocessing for hydrological modeling and encountering an issue with inconsistent flow direction values after running the Flow Direction tool in ArcGIS. Despite filling sinks (using the Fill tool), some cells in my output raster (FlowDir_FIL3) show unexpected values like 241, which don’t align with standard D8 codes (1, 2, 4, 8, etc.). Here’s what I’ve done so far:

Steps Taken:

* Input Data: Used a DEM (Fill_tif1) in NAD 1983 Geographic (attached screenshot shows values).

* Preprocessing: Applied the Fill tool to remove sinks.

* Flow Direction: Ran the Flow Direction tool with default settings (D8 method).

Issue:

* The output flow direction raster contains cells with value 241, which is not a valid D8 code.

* Expected values should be powers of 2 (e.g., 1, 2, 4, 8...128) representing cardinal directions.Has anyone encountered similar non-standard flow direction values? Attached is a screenshot of my workflow and output values. Any insights or troubleshooting tips would be greatly appreciated!

Regards

Shishir Bhusal

Screenshot 2025-08-13 134433.png

AltinPojani · Answer

HI

How can topographic map with contours and hillshade etc...  from digital elevation model

TaniaLopezCantu · Answer

Hi Shishir,

Thank you for reaching out!

Flow Direction values out of the D8 directions, (1,2,4,8,16,32,64,128) mean that the cell has an undefined flow direction. This can happen, for example, when the elevation change is the same in multiple directions. Think of a sink cell, where the south (4) and south-east (2) direction have the same change in elevation value. At this cell, flow direction will be undefined, because it can go either south or south-east, and the value in the output Flow Direction raster will be equal to 6 (the sum of the two directions).

Typically, using Fill tool would help address this problem. Did you run Fill tool with default parameter values? or did you specify a value in the Z limit parameter?

Also, what is the size of your input raster? You may consider running Derive Continuous Flow tool (if your goal is to obtain flow direction raster) - this tool uses an improved algorithm to handle sinks and depressions. Compared to Flow Direction tool, it requires a machine with more RAM, but a regular machine with 16 or 32 GB RAM may be enough to process your data.

Please let me know how it goes!

Thank you,

Tania