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I am trying to build a flow direction raster for an area from a 1m DEM created from LiDAR. I need this in order to build a flow accumulation raster, so I can use that to build a stream network. I have followed ESRI's guide (found here http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?id=3914&pid=3910&topicname=Creating_a_depressionless_DEM) on how to create a depressionless DEM so that flow direction is accurate. This works very well except in some areas that are quite flat. In these areas the Fill tool used in the depressionless DEM workflow fills in the streams on the DEM. This results in an inaccurate flow direction because the streams are filled to the same elevation of the surrounding area. As a result, flow accumulation is inaccurate in these areas and as a result of that the stream network built from it is as also inaccurate in these areas.

Inaccurate Flow Direction made from depressionless DEM with inaccurate streamlines (black lines) created from resulting inaccurate flow accumulation (shown to display inaccuracy of flow direction):

enter image description here

1m DEM with clearly visible streams. The blacklines are the stream lines created from the resulting inaccurate flow accumulation. Notice how they are very inaccurate compared to where the stream actually is on the DEM. This is due to the Fill tool filling in the stream, causing the flow direction to not flow into the right area.:

enter image description here

Does anyone know of a way to create a depressionless DEM without inadvertently filling in streams?

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    Pre-processing the input DEM to create outflow channels that prevent the depressions from being filled is the only way to deal with this scenario if you want to generate stream center lines starting with just a DEM. Once the depression has been filled, there is no information about flow direction and the software simply assumes a default direction for that region. – Apollo2020 Dec 6 '17 at 20:44
  • Thanks for your answer. What do you mean by pre-processing the DEM to create outflow channels? Could you be more specific about how this might be done? – Rdubya54 Dec 6 '17 at 20:49
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    The stream burning method mentioned by @Hornbydd is what I was referencing. Burning in refers to the use of linear vector features to create channels in an existing DEM. In a typical case you would have a complete stream network to "burn in". In your case, I would try to manually digitize the stream segments through the depression areas, burn them in, then process the new DEM. You may get some aberrations right at the beginning and end of the burned in sections depending on how closely your digitized lines actually match the DEM stream channel. – Apollo2020 Dec 6 '17 at 21:11
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These are common problems, typically resolved by stream burning, but that relies on you having a vector network to enforce drainage. I recommend you check out the free GIS software Whitebox GAT it has several alternative methods to fill and compute flow accumulation.

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