Flow Direction or Aspect: Which one is more suited to predict surface flow path?

[AdityaSharma1]

I am new to GeoNet community and had tried to search for the answer to my question, but couldn't find one. So I am writing it down here to my GIS community members, asking for their help on it.

If I was to depict the surface flow in a relatively small area of about 4 sq. Km area in a hilly terrain, would a flow direction raster (Hydrology ToolBox) be a better option or the aspect raster from (Surface Toolbox)? Or is my question redundant because practically both give almost the exact same output, barring a few outlier pixels?

Also, does the choice between the two, depend on how big or small the area under consideration is?

Thanks a lot to the community members.

Aditya Sharma

Environmental Researcher

India

[DanPatterson_Retired]

Use flow direction

Flow Direction—Help | Documentation n 

there is way more to flow direction than aspect or D8 calculations

How Flow Direction works—Help | Documentation 

[AdityaSharma1]

Thanks Dan. Appreciate the help.

I was thinking if I did use Aspect for this particular exercise (about 4 sq Kms of Hilly Terrain), would I be off the mark, by a lot? Or would there be just some minor difference between the two rasters? 

The reason I ask this is because I have already wrapped up the work using Aspect. But then I thought of making sure if Aspect did give a correct representation of the surface flow?

Thanks

Addy

[DanPatterson_Retired]

It would only take a few seconds to compare the two, then subtract the two and compare the difference

[AdityaSharma1]

Hi Dan... you are right... it didn't take long for me to run it and compare the two.. I noticed that the Flow Direction (Hydrology Tbx) was little bit fuzzier at local level, than the Aspect raster..What I mean to say is, that suddenly a pixel or a couple of them would show up in the middle of an otherwise perfectly contiguous family of pixels in the Flow Direction raster..

But even then about 90-92%% directions matched absolutely.

Thanks for all the help.

Addy

[NawajishNoman]

Hi Aditya,

By definition the Aspect tool identifies the direction the downhill slope faces. Whereas the Flow Direction tool creates a raster of flow direction from each cell to its downslope neighbor, or neighbors, using D8, Multiple Flow Direction (MFD) or D-Infinity (DINF) methods. 

Even though they sounds very similar, they are different. The difference lies in how they are calculated too. 

Please review the following two documents on Aspect and Flow Direction.

https://pro.arcgis.com/en/pro-app/tool-reference/spatial-analyst/how-aspect-works.htm

https://pro.arcgis.com/en/pro-app/tool-reference/spatial-analyst/how-flow-direction-works.htm

Put it simply, Aspect uses the elevation of all 8 neighboring cells for computation. Where as Flow Direction (D8) checks the change in elevation in all 8 cell but the result indicates the direction of steepest descent. 

Depending on the application (type of analysis you are doing) Flow Direction or the Aspect will be more appropriate. 

If you are trying to depict the Surface Flow (the direction what will flow) over a landscape, small or large, I would go with Flow Direction because that is commonly used and well understood. However, if the purpose of application is something different, you have to make the decision based on the application, intended use and the audience.

Hope it helps!

Noman

Esri, Redlands, California    

[AdityaSharma1]

Hey Norman

Thanks for such a detailed reply. I did read the extensive literature on this subject matter and understood the implications of using one over the other.

Thanks a ton for the help.

Addy