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

[DanPatterson_Retired]

Are the coordinates and the elevations all in the same units? ie meters or feet
Or are the coordinates in decimal degrees?  I would suspect that you must be working with projected data and x,y and z must all be the same units

[Anonymous User]

Original User: whuber

Curvature values are artificially multiplied by 100, Stan: http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=How%20Curvature%20works .

[DanPatterson_Retired]

Grief...now why would they do that, without giving the option to the user to omit the scaling

[Anonymous User]

Original User: whuber

Grief...now why would they do that, without giving the option to the user to omit the scaling

I have always supposed this scaling was selected because for "typical" earth elevation datasets the sizes of the resulting values are typically not huge (e.g., most of them 10^2 or larger) or tiny (e.g., most of them 10^-2 or smaller).

Watch out for the help.  The pages describing curvature exhibit both the best and worst of ESRI documentation: they are excellent in clearly displaying the formulas used (which cuts through a lot of potential confusion) and horrible because many of the English statements made about the formulas are incorrect!

[WilliamHuber]

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.

These thresholds are unusual.  Curvatures always, in my experience, change sign to differentiate between some form of local convexity and local concavity, with zero always corresponding to flat.

[Anonymous User]

Original User: stanhopkins

Dan_Patterson;35779 wrote:
Are the coordinates and the elevations all in the same units? ie meters or feet

Or are the coordinates in decimal degrees?  I would suspect that you must be working with projected data and x,y and z must all be the same units[/QUOTE

Both the map and the elevation file are in decimal degrees and both are unprojected (GCS_North_American_1983)

[StanaforthHopkins]

Curvature values are artificially multiplied by 100, Stan: http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=How%20Curvature%20works .

Yes, but not a billion!

[DanPatterson_Retired]

Stan
If the coordinates are in decimal degrees, the elevation will not be (doesn't make sense, elevation is either in meters or feet).  This is what I was questioning.  since you indicate that the coordinates are in decimal degrees, the file(s) should be projected so that the coordinates ( X and Y) are in the same units as elevation ( Z ) whether it be meters or feet.  Bill indicated that the software (arbitrarily??) multiplies the resultant by 100, essentially expressing values as a percentage.  Try projecting the data to a suitable projection, redo the analysis and see if the resultant makes more sense (accounting for the scaling)

[Anonymous User]

Original User: stanhopkins

Stan
If the coordinates are in decimal degrees, the elevation will not be (doesn't make sense, elevation is either in meters or feet).  This is what I was questioning.  since you indicate that the coordinates are in decimal degrees, the file(s) should be projected so that the coordinates ( X and Y) are in the same units as elevation ( Z ) whether it be meters or feet.  Bill indicated that the software (arbitrarily??) multiplies the resultant by 100, essentially expressing values as a percentage.  Try projecting the data to a suitable projection, redo the analysis and see if the resultant makes more sense (accounting for the scaling)

I'm sorry, I was careless in my reply.  Both are in meters.