topic Re: Calculate degrees for go/slow-go/no-go in ArcGIS Spatial Analyst Questions

Calculate degrees for go/slow-go/no-go

SimonSeamount — Thu, 24 May 2012 16:59:35 GMT

I want to calculate degrees for go/slow-go/no-go.

I ran the raster through SLOPE tool.

First I used option DEGREES, and resulting values were all somewhere between 89.5 and 89.9.

Second I used option PERCENT OF SLOPE, and resulting values were between 0 and 17,000,000 with media of about 38,600.

The help page on Calculating Slope seems to assume that I will get values like 58.
http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Calculating_slope

I cannot find anywhere an explanation for how to get from 38,000 to the degree it apparently represents.

Thanks for any guidance.

Re: Calculate degrees for go/slow-go/no-go

EricRice — Fri, 25 May 2012 14:06:29 GMT

Simon,

The values you are receiving indicate that you are most likely mixing units of x & y with z. I would guess your DEM is in a geographic coordinate system using units of decimal degrees, and your z (pixel) values are in meters/feet. To ensure accurate slope values you need to have x,y, and z all be in the same units. So, what you need to do is run Project Raster and specify a coordinate system that has the same linear unit as your z units in the DEM.

Regards,
Eric

Re: Calculate degrees for go/slow-go/no-go

SimonSeamount — Fri, 25 May 2012 14:32:32 GMT

That sounds reasonable. The z values are in meters. What projection converts decimal to meters?

Re: Calculate degrees for go/slow-go/no-go

DanPatterson_Retired — Fri, 25 May 2012 14:36:30 GMT

You can use a UTM projection, an Albers, Lambert conformal conic etc etc, there are thousands, so you will need to do some basic reading on projections and/or find out what is the most suitable projection for your area. Many governments use standardized projections which might give you a start if you study area is small