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Generating DEM's

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#1
Tommo

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Hi Guys

I'm after a bit of advice regarding the generation of DEM's. The area involved has rolling topography (convex slopes for all the geomorphology geeks.) Within the study area are some old opencast mining pits that are one of the foci for the end product. What I'd like to generate is a DEM reflecting the smooth hills, but that still shows the sharp edges and benches of the pits.

I've tried reducing the cell sizes to keep the detail where it exists, but then the rest needs smoothing. Is it possible to nest one detailed DEM within another coarser one?

I'm using ArcView with 3D Analyst, and havn't cracked into the trial version of Spatial Analyst yet. I've also got access to Illustrator, but I'm not too crash hot at driving it.

I'd like to use the end result as a hillshaded base map onto which exploration data (drilling grids, drillhole traces, etc) can be overlaid.

Thanks guys.

Tommo

#2
Martin Gamache

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One option would be to generate a TIN first with breaklines where your sharp discontinuities are. You could then generate a raster surface from the TIN in the next step.

You can certainly smooth your DEM based on slope attributes, I would probably use something like photoshop with a slope mask to do this but this can certainly be done in the GIS environment as well using a median filter with a proper window size and again a slope mask. This will allow you to maintain details over your sharp slopes and reduce details over the smoother convex hill slopes. Make sure your slope selection is a fuzzy one so as not to introduce any artificial discontinuities with your smoothing filter.

mg

#3
Charlie Frye

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I gave a presentation on this at NACIS in Portland, ME. I created a DEM for 1775 Boston; as today's Boston is very different. I had three known points of elevation and hatchured maps to work from to determine hillshape. I used the Spatial Analyst.

I hand entered 3,300 elevation points (10-12 hours a little at a time over a month). I did that, rather than use the TopoGrid tool because I wanted to nail the complex coastline formed by the various docks around Boston and Charleston. To do so, I needed to add points to "pin" the surface excessively where I wanted a near vertical portion of the surface to result when I interpolated the points.

So, if you don't have any complex faces or shapes in your surface, and you have a few known elevations, just create some contour lines and use the TopoGrid tool. Though, I expect your open pit mines will cause you to want to create points.

There are three attachments, one of just the DEM, a composite landcover and some relief shading, and one with just the DEM and elevation points.

The points in the water, I got from a period map of soundings, so I had better data for the water. You can see I did some concentric rings of points on one of the hills--not necessary, in fact it creates artifacts. Bunker Hill and Breeds Hill I did with fewer points. I generated DEMs and Hillshades freqently to check my work though.

I used the Spatial Analyst's IDW tool to interpolate the DEM. I fussed around with the parameters until I got what I was after, terrain character-wise. Initially, I just used that small hill with the concentric rings of points to test and expanded from there.

I've also tried, with no success, to mix DEMs of varied resolutions--I figure some of its user error, though I think some of it is pure incompatibility of information at different resolutions. Filling small holes like those typically found in SRTMs is relatively easy, but doing large holes is what I needed. In theory I think it could be done, but a high degree of control is needed.

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Charlie Frye
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#4
MapMedia

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Have you experimented with masks in Spatial Analyst? You can mask the mine areas then smoothen the rest of the dem as desired. You can then mask the boundary btw mine and smoothed dem and do some light smoothing here. Then create hillshade & overlay.

Sounds like this is a GIS based project where you are now generating a basemap, and will add project data to produce a map series?

#5
Charlie Frye

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Have you experimented with masks in Spatial Analyst? You can mask the mine areas then smoothen the rest of the dem as desired. You can then mask the boundary btw mine and smoothed dem and do some light smoothing here. Then create hillshade & overlay.

Sounds like this is a GIS based project where you are now generating a basemap, and will add project data to produce a map series?


Masking can be pretty tricky with respect to getting conformance at the edges. The technique, involves, once you create figure and ground masks (so to speak) subtracting from or adding to one of them without disturbing the edge. The subtracting/adding involves creating a grid that contains the values to add (negative values end up getting subtracted). This is what I tried with little success, though I was trying to integrate two different data sources which turned out to be much harder.

What I was told by our Spatial Analyst experts was that when I do the add or subtract, I would need dummy zero values at the edges to "pin" things down. The more complex the edge, and more complex the terrain that intersects the edge, the harder that becomes. So if you can make your mask along a contour line, that would seem, to me, to be potentially easiest route to take.
Charlie Frye
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ESRI, Redlands, California

#6
merft

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I had a similar problem with a bunch of DEMs for a mine-mouth coal-fired power plant. I needed to merge a variety of 2-foot contours from the mine that were not shown on the USGS DEM. I actually contoured out the USGS DEM at 10 foot contours and smooth them. With the mine contours I tossed out all the 2 foot contours and retained the 10-foot contours. I did have spot elevations and created so others where necessary. Then edge-matched the USGS and mine contours manually and generated a new DEM. It actually worked well but was very labor intensive.

An alternative would be to generate the two DEMs separately, rolling hills and mines then overlay the mines on the rolling hills in Global Mapper and export the new DEM. There will probably be a seam where they overlay, but if all you are doing is creating a shaded relief generate your shaded relief and manually edit out the seams in Photoshop.

-Tom

#7
Tommo

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Thanks Guys.

A wealth of information fell out of that question. Seems everyone has a favourite method for this kind of problem.

I feel kind of guilty that after all those responses I just generated a TIN of the area using both high and low detail data sets, then converted to a DEM with a cell size small enough to support the high detail. Now I'm in the proccess of manually smoothing the hills using Photoshop.

Thanks for the suggestions team.

Tommo




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