SRTM DEM slope calculations

Question

Hi,
I am trying to calculate the slope of SRTM data. When I tried to calculate the slope in Decimal degree using z factor 1, it gives me 
strange values though in the range of 0 to 89.998680 which is normal, the distribution is quite strange as most of the area appears as steep slope except for extremely flat areas (like lakes) i.e, only two classes. On contrary when I use the percentage, it works fine. I 
tried several times but no luck. I even tried to use the UTM projections for my area but it did not work either. Can anybody help me 
to fix the problem as my rest of the analysis depends upon the slope radians.
Thanks
Rakhshan

RakhshanRoohi · Answer

The z units are most likely meters and your UTM should also be meters, unless they have been modified. Have you tried ground truthing against known elevations like benchmarks?  As long as the units are the same you should be fine and a z factor will not be necessary. If you have feet elevations then the issue will still occur.

Hi Jeff,

Thanks for the suggestion. I used the projected coordinate system (UTM Zone 54) along with the units in meters but it was somehow not working. Later I set the Z coordinate system (Vertical Coordinate Systems) as Australian Height Datum and it did work.

Now I am having another problem to get the radiance values of Y-grid of the image in ArcGIS 10. I tried to convert the Landsat mage into polygon with cell size of 30x30 with label using FishNet. Later I added the XY coordinates to point layer and then joined the table of polygon to point layer. Later I converted the polygon layer back to raster layer. In this way I did get the Y grid of raster layer but the problems are 1) it is quite long way to achieve Y grid and 2) it does not work for large raster datasets.

Can you please suggest any easy and straight forward way to get the Y-Grid of raster dataset so that I could calculate the 
radiance.

Cheers

JeffreySwain · Answer

The z units are most likely meters and your UTM should also be meters, unless they have been modified. Have you tried ground truthing against known elevations like benchmarks?  As long as the units are the same you should be fine and a z factor will not be necessary. If you have feet elevations then the issue will still occur.

RakhshanRoohi · Answer

You need to project your data (eg.  UTM) and ensure that your X, Y and Z values are all in the same units (eg. meters)

Hi Dan,
Thanks for suggestion. As I mentioned that I tried UTM projections but it did not work. I am not sure about how to check the units for Z values. can you please comment on this.

Rakhshan