Constraining area of hillshading from DEM/TIN

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06-17-2014 01:33 AM
HughRice
New Contributor
Hi

I have made hillshading from contour data of an island I am working on but I cannot ???remove??? the grey fill area with a Pixel Value of 180 that fills in the sea. I tried Layer Properties ??? Symbology ??? Display Background Value ???180??? as ???white???, which I have used in other places to remove backgrounds, but of course this made all areas with a value of ???180??? go white, destroying the hillshading, so it is obviously the wrong approach.

To make the hillshading, I first made a TIN of the contour data, then a DEM from the TIN. I removed the background value in the DEM by the method described above (set background to white) and then made hillshading from the DEM. However, I can see that the TIN fills in the sea area in bays with triangles (TINs) and I wonder if it is these that are causing my problem.

I suspect I need to either restrict the formation of the TIN or the formation of the hillshading to within my coastline. I tried various options but none that I tried worked.

Any suggestions?

If this is not the cause of the problem, any other suggestions would be welcome.

Thanks
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IanMurray
Frequent Contributor
There is actually a good tool for this.

Check out Delineate Tin Data Area.

Usually since the DEM is flat for bodies of water, as there are no elevations points there for interpolating, they end up with the largest edge lengths, so you can set a threshhold length for your tin edges.  If they are longer than the threshhold, they will not be include in your output TIN.

Let me know if this works for you
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HughRice
New Contributor
Hi

Thanks for the reply

Perhaps a stupid question - in the Delineate TIN Data Area, how do I estimate the Maximum Edge Length? What units are these? I looked for a 'Attributes Table' type of thing showing the data but could not find one.

I also tried putting lengths of 1, 100 and 10000 in, but this seemed to make no difference.

Thanks
Hugh
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IanMurray
Frequent Contributor
The units are usually whatever units your data frame is in, so if you are in inches, and we are talking a large waterbody, it might need to be a pretty large edge length, though you can change your data frame units.  You can do a Tin Edge conversion to convert your tin to lines and measure some of your lines to check about what length to use.  Also, if these are internal watebodies, lake, pond, etc, you will need to the ALL Method, so it check the length of all lines, not just ones on the perimeter which is more for coastlines, where only one side of the map really needs to be cut out.
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HughRice
New Contributor
Hi

This does not going to work - I tried it - the coast is complex (it is in Greece, but imagine Norwegian fjords) so some of the triangles outside the coastline are smaller than some of the triangle inside it. Whatever I do, I'm never going to get a perfect match.

I cannot believe there is not an option constraining it to the shape of the coast itself.....

In the end, I guess I'll have to go to quick and dirty and just stick a white layer above the coast in CorelDraw, when I do the final text etc.

Thanks for your patience and help, anyway.

If anybody from esri reads this.....make the option I suggested! Cannot be that difficult.

(And for interest, the data has to be dragged from Catalog, not the Table of Contents, for the process to work)

Hugh
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IanMurray
Frequent Contributor
One last gasp at this.

See if you can do a TIN Edge, to convert it to lines, then clip the lines by a the coastline boundaries(if you have them), then use create TIN using the clipped lines as your input feature class.  I've never tried this so no idea if it works.

Or perhaps do a clip, if you had the coastline boundaries as a shapefile or feature class to cut out all of that part of the raster?
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HughRice
New Contributor
Hi

Many thanks for the further suggestions - but my understanding of ArcGIS is too limited to implement them - I am a geologist looking to make a map as a one off (or rare) event, not a person who is employed as a cartographer. I am already struggling to have got as far as I have.

Hence I will do my very inelegant 'quick and dirty' solution in CorelDraw. It will take 10 seconds to do, rather than several hours trying to puzzle out the processes you suggested.

But many thanks, anyway.

Hugh
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