3

I have a DEM in a Mosaic Dataset covering a large area (300 x 200 km). I want to create a classified slope raster showing slopes of say 0 - 10 degrees, 10 - 20 degrees, and then 20 - 90 degrees. Because the dataset is so large I want to avoid having to create a separate slope raster dataset, but instead use the Image Analysis so I can visually see where the slope classes are.

Classified raster

In the Image Analysis window I can create a Slope Function for my dem and display it using the Stretched color ramp in the Symbology tab of the Layer Properties. I want to use the Classified renderer, but Arc requires the data to have a histogram. When I say Yes, it chugs away computing the statistics and histogram. For really large datasets it can take a very long time. I'm wondering where the histogram and statistics are stored for a Image Analysis layer because it this Compute Histogram dialog keeps re-appearing.

Compute histogram

I thought there could be a way to create this with a Function chain. There are functions called: Classify, Colormap, Remap that sound promising, but I haven't gotten any of them to work

The Classify function wants a .ECD or .ACT file as input. How do I create a ECD or ACT file or is there a better way to accomplish this?

Function Chains:

Functions

  • Do you have spatial analyst? Or 3D analyst? Both have slope tools that work well. – jbalk Sep 15 '17 at 0:20
  • I do have both SA and 3D analyst. I should have mentioned that I want to be able to do this against a Mosaic Dataset – Mattropolis Sep 15 '17 at 0:34
  • 1
    If your rasters are all the same number of bands and same pixel depths, you should use 'mosaic to new raster'. I'm pretty sure you need a continuous raster to calc the slope. Also it would probably solve the histogram issue. You'll have issues doing this if your rasters have different numbers of bands or different pixel depths. – jbalk Sep 15 '17 at 4:39
  • Have you considered using ArcGis pro to generate the slope raster? – Hornbydd Sep 28 '17 at 19:31
1
+50

if I understand well, you want an "on the fly" slope calculation in order to avoid building a large dataset of the slope. This could be a good idea, but only if you don't need to work with a large scale factor (otherwise it is better to create your slope raster and build pyramids on top of it).

With your 3 classes, you should use Remap function (manually enter the min and max values of your groups, no need to compute statistics) then adjust the colour with a stretched renderer. (FYI: Classify assign class values based on training samples when you do not know the boundaries of your classes, and colourmap apply colours based on unique codes of a single band raster)

  • I do want an on the fly slope class calculation. I will try the Remap function when I get back to the computer. – Mattropolis Sep 30 '17 at 3:51
1

I have Canadian DEM data (whole country) that I created a mosaic dataset out of. Adding a slope function chain to it produces a good working slope raster (percent rise). I have tried adding a Remap function that includes 3 classes: 1) 0-30, 2) 30-50, and 3) >50. The goal is to only see the 2nd and 3rd classes, and colour them appropriately. I don't want a colour for the first class, so I checked NoData in the Remap function properties.

All this being said, the Remap function does not work as I thought, and it seems like a bad choice to classify pixel ranges. At a minimum, I added a min-max value of 0-30 with no output and NoData checked. This removes the 'good' slope so I can see the ground below it.

  • Great workaround and solution if you only need classes for visual presentation. Could you elaborate a bit what you thought was wrong with Remap function please? – Mat Jan 16 at 19:13
  • 1
    This was indeed for visuals only. Adding the slope and then remap function chain worked to an extent, but to see the two classes 30-50 and <50% slope, the elevation raster had to be in 32-bit float. I ended up using 30m SRTM for this, and then did the slope (no Remap function needed!). Published to a service and it works online now. – cptstubing Jan 18 at 0:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.