Why is Spatial Analyst - Hydrology - FILL Performing this way?

JustinSmith

Hello,

I have been attempting to perform a Watershed analysis and can't quite get past the Fill tool.  I am working with a clipped DEM of 20cm resolution, see attached image with random selected points of elevation.  There are two rather large ponds on the site and so I attempted to use a Z limit of 1079 as the ponds are all under 1079m while everything else is above. (not 100% sure if I am using this option correctly).

However as can be seen no the AfterFill image almost the entire site obtains values of 1083.0378.  

Why is this occurring this way?  

Thanks,

Justin

DEM.png

AfterFill.png

キャプfsdfチャ.PNG

Esri help 2.jpg

ChrisDonohue__GISP

Actually, the Z-limit option performs a different function that what you were trying to use it for.  What it does is it constrains what gets "filled". 

In terms of setting the pond elevations, one possibility would be to employ Raster Calculator to recalculate the lower elevations.

Raster Calculator—Help | ArcGIS for Desktop 

Conditional Evaluation (CON)

Con (in_conditional_raster, in_true_raster_or_constant, {in_false_raster_or_constant}, {where_clause})

Con—Help | ArcGIS for Desktop 

So in Raster Calculator, it would be something like this:

CON (DEM filename, 1079, elevation field, elevation field <1079)

Caveat - the tricky part will be if there are other parts of the DEM that are below an elevation of 1079 meters, as this universal CON statement will modify those too.  If that is the case, you will need to select just the cells that need to be modified (the pond cells) and then apply the CON statement to only them.

Chris Donohue, GISP