z-factor in slope tool

DanielBigelow1

Hello,

I am trying to run the slope tool for an elevation raster that covers Sonoma and Napa counties in CA. I'd like to figure out the appropriate value to use for the z-factor. My elevation is measured in meters, and the coordinate system for the DEM is in degrees. The latitude in this area ranges from 38 to 39 degrees. Using the midpoint, 38.5, combined with the information on this Arc help page: http://resources.arcgis.com/en/help/main/10.1/index.html#//009z000000vm000000, I have determined that the appropriate z-factor is .0000115075. I did this using a simple linear interpolation for the 30- and 40-degree values referenced in the table contained in the link.

Does this seem like a sound way to determine the appropriate z-factor? If not, could anyone suggest an alternative?

Thanks,

Dan

Luke_Pinner

I did this using a simple linear interpolation for the 30- and 40-degree values referenced in the table contained in the link.

Does this seem like a sound way to determine the appropriate z-factor? If not, could anyone suggest an alternative?

No, it's not a linear relationship, but it's not that much of an issue as long as the z factor is roughly correct.

To calculate it properly:

z=1/(111320*COS(latitude*pi/180))
z=0.00001148 where latitude = 38.5

In Excel you can use the following formulas:

=1/(111320*COS(38.5*PI()/180))

or

=1/(111320*COS(RADIANS(38.5)))

See this page for further info.