topic Re: Hydrology Tool in ArcGIS Spatial Analyst Questions

Hydrology Tool

xintan — Tue, 01 Apr 2014 06:57:34 GMT

I have 0.5m DEM generated from Lidar data and I try to do water detection on that data.
It seems the Flow direction and Flow accumulation function do not work so well on my data.
Some of the big rivers and lakes are not detected, instead small ditches and tracks are detected.
I tried to use 'Aggregate' to resample the data, the result is better at some area, but still not so satisfying.
Is there anyone know to to optimize the result in this case?

Re: Hydrology Tool

DuncanHornby — Tue, 01 Apr 2014 11:01:55 GMT

Have you run your DEM through the fill sink tool before you attempt to use the other tools?

Re: Hydrology Tool

xintan — Wed, 02 Apr 2014 10:49:31 GMT

Have you run your DEM through the fill sink tool before you attempt to use the other tools?

Yes, I did. I have 0.5m DTM. I have a few stones in the stream, which seems to stop the water flow in the analysis.
I think the real problem is the flow direction and accumulation is not so good at detecting flat water.
Because at small ditches, both sides of the border is flowing inward and the surrounding area is quiet flat which leads to more accumulation.
However, at lakes and streams, the water surface is quiet wide and flat which means at the borders only one side is flowing inward. And the border is really steep but narrow, which kind of explains why it doesn't get so much accumulation.

Re: Hydrology Tool

MarkBoucher — Wed, 02 Apr 2014 16:12:42 GMT

Try using raster math to subtract your original dem (rawdem) from your fill dem (fil): fil - rawdem. This will reveal where the fll has been placed. Symbolizing this with a blue color ramp and making the 0 values transparent is helpful in this analysis.

If your rawdem has bridges or culverts over the rivers, your rivers would be filled in behind them. If your raster is relatively flat, this filling in would make the rivers "disappear".

What you may need to do before filling is to "burn in" a stream through the bridges and culverts in the rawdem. Here is one response to another question that I think explains how to burn in streams. I use Arc Hydro, but it can be done "manually".
http://forums.arcgis.com/threads/103449-How-you-work-if-not-have-a-streams-layer?p=368696&viewfull=1#post368696

You can search the hydro forum or the Arc Hydro forum with the word "burn" and you will find several other posts on burning.