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DEM resolution - hillshading

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#1
gregory

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Hi

I wonder if 20 meters resolution DEM can be used for hillshading in map with 1:50 000 scale? I know that from 30 meters DEM i can get countours with 20 meters interval, what meet countour intervals using at 1:50 000, but 30 meters seems to be not enough to do a shading for such scale, what about 20 meters?

thanks in advance for any comments
Gregory

#2
Hans van der Maarel

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I wonder if 20 meters resolution DEM can be used for hillshading in map with 1:50 000 scale? I know that from 30 meters DEM i can get countours with 20 meters interval, what meet countour intervals using at 1:50 000, but 30 meters seems to be not enough to do a shading for such scale, what about 20 meters?


It seems a bit coarse, although I have to admit my method for determining that is kinda hypotethical...

Back in college, we were taught that the smallest size object the human eye could still easily distinguish once printed on paper is 0.2 mm. DEM cells of 20 meters would be shown as 0.4 mm if printed at 1:50k scale, so again hypotethically speaking, you could be seeing some artifacting.

Whether or not this would be an issue is largely dependent on the map itself and how you display the hillshading. I would think that if you blur the hillshading slightly and add your other map information, it wouldn't really show. Also, if time and budget permits, you could touch it up manually.

[wonders whether there's anything in Imhof's book about this]
Hans van der Maarel - Cartotalk Editor
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#3
frax

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It might also depend on the topography of the area you try to map, and the quality/accuracy of the DEM.

Gregory - you are also confusing me a bit - you talk about contours and hillshading, but it is just a shaded backdrop you want? (no contours?)
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#4
Charles Syrett

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My approach to this kind of situation is first to prepare the basic relief image from the DEM, then edit in Photoshop, because it's almost inevitable that you'll see artifacts and/or pixelation. Make a duplicate of the image on another layer, and then start playing with filters. Filter>Noise>Median almost always gives good results; at the right setting, it seems magically to filter out unwanted details and leave larger forms intact. It's always good to edit to drainage lines as well, since it almost never fits very well. That means rasterizing the drainage linework onto another layer (which you will later turn off or discard), and use the airbrush tool to touch up. Excellent resources:
http://www.reliefshading.com/ -- this one very nicely shows the limitations of unedited analytical relief.
http://www.shadedrelief.com/ -- encyclopedic in scope!

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#5
gregory

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Charles;
Thanks for the the tips.

Frax;
Information about countours intervals was just an general indication for me what DEM should i consider to use, however I will need them both.

regards
Gregory




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