Merging DEMs with selected smooth/blended/feathered borders?

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07-23-2011 07:25 AM
AnthonyWestphal
New Contributor
Hi!

I'm trying to find a specific function in ArcMap 9.3 that is available in Global Mapper for example.
I'm loading two DEM files that have the same extends but different resolutions. The higher resolution DEM is on top.
Because the high resolution DEM is full of artifacts I want to select the mountainous areas only by covering those with polygons for example. The next step would be to make the areas outside the polygons invisible in the highres DEM so that the lower resolution DEM becomes visible outside the mountainous areas.
The highres DEM needs to merge seamlessly with the lowres DEM with smoothed/blended/feathered borders (whatever you like to call it).
GlobalMapper can do this by default and easily but takes ages to calculate the result. Therefore I'm trying to find a way to do the same in ArcMap, which is usually much faster in processing.

Thanks!
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RobertBerger
New Contributor III
Greetings,

I think what you're looking for is called "mosaicking". There are a few tools you can try to use. Play with the mosaic operator to see if you can get what you want.
There is some help documentation online.
Hope this helps.

Robert
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AnthonyWestphal
New Contributor
Sorry that I have to dig this up after such a long time. I finally had access to ArcGis again and had another look at the Mosaic tools without the expected result.

Here is to explain what you can see on attached screenshots:

#1
TMP01.tif: This is supposed to be merged on top of TMP02.tif

#2
TMP02.tif

#3
This is what the result should look like - with a smooth blend between both of course - which you can't see here because I didn't archive this yet.

#4
While above screenshots show what the TIFFs look like in Global Mapper this is what TMF01.tif looks like in ArcMap. ArcMap completely ignores the "cut out" areas which have no data and are defined as -32767 meters in Global Mapper.

#5
This is what the result looks like in Global Mapper after I mosaiced both files with the MocaicToNewRasterTool.
ArcMap obviously can't see the no data areas and therefore blends both rasters along TMP01.tif's actual file coverage area but not the data coverage area.

Is there anything I'm missing here? How can I define no data areas in ArcMap to let the Mocaic tool blend both DEM files along the data corners and instead the file coverage area?
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RobertBerger
New Contributor III
Greetings,

A couple things that will help you:
- To make the data display not as a black box you need to http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//0017000000m3000000.htm.
- To define nodata you need to use the copy raster tool and set the nodata value as part of the copy.
- Then you want to play with the mosaic operators to get the desired output. You have a couple tools available to you:
Mosaic
Mosaic to new
Raster Catalog to dataset
Workspace to dataset

In essence they do the same thing, the difference being how you load the data initially and if you just attach the pixels to an existing dataset or create a new output dataset. I think, if you only have two datasets, then the mosaic or mosaic to new tool should be plenty.

Good luck.

Robert
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AnthonyWestphal
New Contributor
- To define nodata you need to use the copy raster tool and set the nodata value as part of the copy.


Fantastic! That's what I was missing, thank you!
I just had to type in -32767 as no data value and could mosaic both faster than I thought.

Unfortunately the resulting DEM doesn't look better than from Global Mapper (it's stepped along the merging line) but at least it's much, much faster. The same task that sometimes takes days (!) to process for Global Mapper is done by ArcMap in 5 minutes!
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