LiDAR DEM is too detailed, catchments are too small

DarleneWilcox

I have a 1m DEM derived from LiDAR.  I go through the usual process -- Fill, FlowDirection, Watershed.  The result is that sometimes I'm getting only part of the catchment (see the attached screen shots.  The blue outline is from a survey of the catchment area -- which may not be perfect, but it will be close). 

It appears that there may be too much detail in the DEM?  Small rises and ridges that are stopping the flow when in reality it is not a barrier.  Is there a threshold setting I'm missing?  Or repeat the Fill (will this result in good data being smoothed out as well as the bad?) 

I'm sure there is more artistry to be applied -- I just don't know what it is!  Any help would be appreciated.

only_part_of_catchment1.jpg

only_part_of_catchment2.jpg

RandyMcGregor_BMcD_2-1767208618670.png

SimonSchütte_ct_0-1622462387468.png

Anonymous User

Original User: MBoucher21

It appears that there may be too much detail in the DEM?  Small rises and ridges that are stopping the flow when in reality it is not a barrier.  Is there a threshold setting I'm missing?  Or repeat the Fill (will this result in good data being smoothed out as well as the bad?)

The fill function should only need to be run once unless you put a limit on the amount that should be filled. My guess is that the DEM is not too detailed unless you way made it more detailed (precise) than the LiDAR data is accurate. (I have no experience in converting LiDAR to DEMs, so I'm no help there and could be way off on this last sentence.)

If you follow the flow direction downstream from the point a few cells, does the generated watershed boundary better follow your surveyed watershed boundary?

One way to check the the DEM is to examine the flow direction grid and see if it makes sense. I use flow path trace tool in Arc Hydro. This allows me to click on a cell and it will draw graphical lines that follow the flow direction. It is a great tool for understanding the DEM accuracy and especially if one is modifying the DEM using other tools (Arc Hydro DEM Reconditioning tools are what I use). I don't directly use the SA hydrology tools much.