Planform Curvature values much too high

[StanaforthHopkins]

I am having trouble producing a planform curvature output.  I'm unsure what units to use and the spatial analysis documentation seems sketchy.  The values should be >-0.1 (convex), -0.1 to -0.4 (planar), and >-o.4 (concave).  My values are much larger as shown in an attachment. 

The intent is to employ the technique used in "Modeling Steep Terrain Harvesting Risks Using GIS" also attached.

Any guesses?

Stan Hopkins

[WilliamHuber]

Yes, but not a billion!

I think you're on the low side, Stan: there are about 10^5 meters per degree and curvature is a squared unit, so failing to project your data will introduce an error of about (10^5)^2 = 10^10 = ten billion.  Multiply that by 100 and you're up to an even trillion ;-).  Anyway, whenever topographic calculations are off by this many orders of magnitude, it's invariably due to keeping raster data in decimal degrees instead of projecting them: that insight is what prompted Dan's initial response.

[Anonymous User]

Original User: stanhopkins

Sorry for the delay - family matters.

Thanks to Bill Huber and Dan Peterson - projection to UTM and a Z value of 0.01 gives reasonable figures.

[AndrewStauffer1]

I am still having some difficulty with this output.  I downloaded a 10m DEM from USGS's TNM and projected it to UTM zone 18N.  I opened a new ArcMap project and opened the DEM.  I ran the curvature tool and got the following outputs:

Curvature: +/- 1.#INF
Profile: 3.40282e+038 - 1.17549e-038
Plan: same as Profile Curvature

I'm not sure why this isn't working.  I had the same thing occur in ArcInfo 9.3 and ArcView 10.  Any insights?  I tried to adjust the z value that delivered the same results, and this way seems like more of a cosmetic fix anyway.

Also, in Arc 10, ArcMap crashes at 99%, but creates an unprojected curvature raster.  Just wanted to know if anyone was experiencing this or if my Arc10 needs reinstalled.

Thanks

[Anonymous User]

Original User: whuber

The largest value of 10^38 is close to the limits of single precision floats, Andrew.  This suggests that (a) NoData values were present in the original grid, (b) they were coded as values with extremely large sizes, (c) for whatever reason they were not recognized as NoData during the import procedure, and (d) these values were (therefore) treated as genuine numbers during the resampling process that necessarily accompanies reprojection.

If this diagnosis is correct, the fix is to return to the original grid, force all out-of-range data (say, any elevations less than -1000 m or more than 10,000 m) to be true NoData values (use SetNull), and then reproject and recompute the curvature.

[AndrewStauffer1]

Thanks Bill!

I have identified the problem thanks to you.  I downloaded my data from the USGS's The National Map and my study area was broken into 6 DEM pieces.  When I projected the 6 pieces, I now noticed that the projection had created a 1 cell wide gap in some places that persisted when the pieces were merged together.  This was throwing the curvature tool off.

Thanks for your input.

[Anonymous User]

Original User: stanhopkins

Not sure of some of the problems expressed by others since I started the thread, but I have perhaps found a pragmatic solution to my latest problem (recommended values not "too high" for planar, concave and convex curvatures any longer, but not agreeing with the topography that I can see in the map).

I ignored the values in the attached "Steep Inclines" article and jiggered the planar, concave and convex values until they agreed with the landscape.  Works like a charm.

Stan Hopkins

[MeganSchofield]

I was wondering if you could help me.

I have curvature values of 4.6967e+012 --> -4.88592e+012, and once exported give mean values of -53136000614 to 73871998976
, which are obviously too high.

I really don't know what I am doing wrong. I (think I) have attached a picture of it for you to visualise better. Thanks.