DEM Reconditioning problem.

Question

Hi all,
  I am using Aster DEM having 30m resolution n i wana use DEM reconditioning (AGREE) option. but i am little confuse with the following inputs;
   Stream buffer -------
   Smooth drop/raise --------
   Sharp drop/raise --------
  Kindly guide me that what should be the above inputs and which would be suitable for this DEM.

LaylaBz · Answer

My SA license is not available for me to look at the commands in detail.

My watersheds are generally not as flat as Florida. In the flat valley areas I do have, I find the need to "build" lots of walls to make the flows go in the right directions.

I think there is an option in the burn streams tool that tells Arc Hydro to bring the negative dem values up to zero. If you do have zero values and can't find that option, but still want to see if the negative numbers make a difference, you could use the CON() function to make anything negative 0.0 or slightly above. Then you could use that raster for the rest of the process. I also don't recall negative numbers being a problem. I'm in the SF Bay area and my elevations aren't all that high. I use the default burn values which I think the deepest is 1000 ft. and I know many of my projects have gone to the bay and I've never had a problem.

Since I have steeper terrain, my tendency is to minimize the width of the smooth and sharp rise values. The purpose of the smooth drop is to eliminate parallel stream right next to your main stream.

It might be a trial and error situation for you to figure out what values to use. When I do trial and error, I do a trial noting the parameters I used, get out of Arc Map, delete the Arc Hydro file geodatabase and raster folder, start up Arc Map again and reprocess using different smooth/sharp drop values. Seems to be a few extra steps, but makes things run with fewer hiccups due to overwrite conflicts etc. This is where model builder comes in handy.

thank you, I am very new to ArcHydro and I am still learning. I would have to read through everything you said and research more on it to be sure I understand what you said. I will get back to you. Thanks a lot.

MarkBoucher · Answer

My SA license is not available for me to look at the commands in detail.

My watersheds are generally not as flat as Florida. In the flat valley areas I do have, I find the need to "build" lots of walls to make the flows go in the right directions.

I think there is an option in the burn streams tool that tells Arc Hydro to bring the negative dem values up to zero. If you do have zero values and can't find that option, but still want to see if the negative numbers make a difference, you could use the CON() function to make anything negative 0.0 or slightly above. Then you could use that raster for the rest of the process. I also don't recall negative numbers being a problem. I'm in the SF Bay area and my elevations aren't all that high. I use the default burn values which I think the deepest is 1000 ft. and I know many of my projects have gone to the bay and I've never had a problem.

Since I have steeper terrain, my tendency is to minimize the width of the smooth and sharp rise values. The purpose of the smooth drop is to eliminate parallel stream right next to your main stream.

It might be a trial and error situation for you to figure out what values to use. When I do trial and error, I do a trial noting the parameters I used, get out of Arc Map, delete the Arc Hydro file geodatabase and raster folder, start up Arc Map again and reprocess using different smooth/sharp drop values. Seems to be a few extra steps, but makes things run with fewer hiccups due to overwrite conflicts etc. This is where model builder comes in handy.

LaylaBz · Answer

Yes, thanks I have seen that thread and checked out the links (only the second one opens for me though)
I think I understand what smooth/sharp drop/raise are. But I am not sure how to figure out what number to use for each. 
I am working on Brevard County in Florida, which has a minimum of -30 ft and max of 150 ft in elevation. (I work on 5 separate sets of RAW DEM data that each have different min and max, but I provided the min and max for the whole county)

My first question, is that since Florida is very flat (compared to a mountainous terrain) , do I need to take that into account when using default values for smooth (10 ft) and sharp(100ft) drop/raise factors?  
(For example, one of my sets of data has a min elevation of -4 ft and max of 55 ft )

Second, Since I Have negative values for my minimum elevations in a few of my RAW DEM layers, do I need to take that into account when figuring out what values to use for smooth and sharp drop/raise?

Thank you very much .