Does burning streams introduce error

Question

I am trying to model a watershed at a finer resolution than my states NHD shows.  I understand the processes of Hydro but was wondering why it is acceptable to burn the streams into the DEM when doing so not only alters the stream section of the dem by 10M but also smooths the surrounding cells.  It seems that in doing so would introduce error from the start.

After modeling with and without burning the NHD into the DEM there are clearly differences in the resulting stream layers.  Observation points match closer to the burned results.  Is this just the trade off?  That the total error is lessened by introducing slight error at the start?

Thanks

LornaMurison · Answer

If you burn the streams only in selected areas like at road crossings (like you are suggesting) or in particularly flat areas, where filling sinks eliminates the stream itself (like I may have to do) don't you then introduce sinks into the stream itself? Can that then be corrected by using fill sinks?

Thanks,

MarkBoucher · Answer

The goal of the DEM reconditioning using Spatial Analyst or Arc Hydro tools is to get the flow direction right. When I work on the stream burning step, I try to minimize the width of the burn by making the stream buffer only 2 cells wide. You may be able to make it just one cell wide, but I vaguely recall that not working well so I've stayed with 2. I work with a 10x10 dem. The walls have to be at least 2 cells wide or you will get "leakage" via the diagonal.

I also minimize where I put the streams to burn. If the rawdem is good for the flow direction, I don't put a line in it. I only put in lines at bridges and culverts. If I have a stream layer, I will buffer the road layer and use that to clip the stream layer so I only have short segments at the stream crossings. Of course I have to review the crossings to make sure the stream polyline is "correct".

So, does burning streams introduce error. Yes. Does the rawdem have errors already? Yes. If you are diligent enough you can make the rawdem better for flow direction determination and minimize the introduced error.