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slope & aspect (Zevenbergen & Thorne)

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#1
Jim

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I am trying to compute slope and aspect on grids, using the method of Zevenbergen & Thorne (1987). If I?m not mistaken, this method was formerly available through ARC/Info GRID, but I haven't been able to find a way to implement it using ArcGIS 9.x. The ArcView extension ?DEMAT? employs this algorithm (and the one by Horn (1981)), but my grids exceed ArcView?s maximum size. The TAPES-G Terrain Analysis software (available as a download from the USC GIS Lab) operates as an extension in ArcGIS and utilizes the Z&T algorithm, but from my experimentation it seems that it only works on grids with ?whole number? cell sizes (e.g. 30 meters). I downloaded DEMs from the USGS Seamless site (geographic coordinates), and reprojected them to Plate Carree (so that x, y, and z are in meters). Specifying a 10-meter resolution during the reprojection caused another set of problems that I won?t go into, but the bottom line is that my grid cell size is 10.3 meters.

Does anyone know of a way to compute slope and aspect using the Zevenbergen & Thorne algorithm? Any help would be greatly appreciated!
Jim

#2
paul

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You need either Spatial Analyst or 3D Analyst for ArcGIS 9.x to create slope or aspect grids. You can download a free two-month trial of either of these extensions through the ArcGIS Desktop Manager to see if it will work.

#3
Lui

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I would suggest SAGA GIS. It has strange user interface and some very powerfull DEM tools, but it is also memory hungry. Did I mention that it is free?

Lui

#4
Jim

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Thanks for the suggestion. I successfully executed SAGA GIS on a small sample grid. I was hoping to be able to do the slope and aspect calculations in ArcGIS (using Spatial Analyst), since using SAGA involved importing and exporting grids, a conversion from radians to degrees, and a projection definition. But it's good to know I have the ability. I appreciate your help!

#5
Charlie Frye

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Jim,

Both the Slope and Aspect tools in ArcGIS reference:

Burrough, P. A. and McDonell, R.A., 1998. Principles of Geographical Information Systems (Oxford University Press, New York), p. 190.

So, I checked that, and it looks like we use Horn (1981). The text went on to say that the Zevenbergen & Thorne algorithm was shown by Skidmore (1989) to be superior, but Horn's, among others, produced results that were of "little difference". Another comparison (Hodgson, 1995) showed similar, but went on to say that Horn's algorithm was best for rough surfaces, while the Zevenbergen & Thorne algorithm (among others) was best for smooth surfaces. Just what constitutes smooth or rough was not discussed.

Last, I'm no mathematician, but the formulae given in the "about" help topics looks like it matches to the formulae in the Borrough and McDonell book for Horn (1981). The notation differs, but I think the math is the same.

I hope that helps.
Charlie Frye
Chief Cartographer
Software Products Department
ESRI, Redlands, California




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