Aspect-slope map

By Aileen Buckley, Mapping Center Lead

Aspect-Slope - Thumbnail

An aspect-slope map simultaneously shows the aspect (direction) and degree (steepness) of slope for a terrain (or other continuous surface). Aspect categories are symbolized using hues (e.g., red, orange, yellow, etc.) and degree of slope classes are mapped with saturation (or brilliance of color) so that the steeper slopes are brighter. This will result in a map that has the colors shown to the right.

NOTE — This blog entry has been updated for ArcGIS 10.

The coloring is based on Moellering and Kimerling’s MKS-ASPECT scheme (GIS world 1991). Below is the aspect-slope map for the area around Crater Lake, Oregon.

Aspect-Slope - Thumbnail

Cindy Brewer and Ken Marlow describe the process for making this kind of map using ArcInfo GRID in their online article, Color Representation of Aspect and Slope Simultaneously.  Translating the instructions to ArcGIS 3D Analyst or Spatial Analyst is straightforward. To try this yourself, download this dataset. Included with the data is an .mxd file. Click here to download the style used to make the map above. These are the same colors Cindy and Ken originally used.

Here is how to achieve this effect yourself:

  1. First download the data and open the associated .mxd.
  2. Using Style Manager, reference the aspect-slope style. To make things easier, you can also “un-reference” the Esri style. If you have a lot of content in your personal style, you will have to wade through this when you want to use the aspect-slope style as there is no way to “un-reference” your personal style.
  3. Create a slope map from the Crater Lake DEM using the Slope tool (this is in ArcToolbox under Spatial Analyst Tools -> Surface).  Use the PERCENTRISE option for the Output measurement.
  4. Create an aspect map from the Crater Lake DEM using the Aspect tool (also in ArcToolbox under Spatial Analyst Tools -> Surface).
  5. Reclass the slope map using the Reclassify tool (also in ArcToolbox under Spatial Analyst Tools but this time, not surprisingly, in the Reclass toolset). Use these categories:
    0-5 : 10
    5-20 : 20
    20-40 : 30
    40-highest value : 40
  6. Reclass the aspect map using the Reclassify tool. Use these categories:
    0-22.5 : 1
    22.5-67.5 : 2
    67.5-112.5 : 3
    112.5-157.5 : 4
    157.5-202.5 : 5
    202.5-247.5 : 6
    247.5-292.5 : 7
    292.5-337.5 : 8
    337.5-359.5 : 1
  7. Combine the two reclassed grids into an aspect-slope grid using the Plus tool (this is in ArcToolbox under Spatial Analyst Tools in the Math toolset). The input rasters are the slope-reclass grid and the aspect_reclass grid. Call it “aspect-slope”. This will create a new output grid with values in the 20s for low slope areas, 30′s for medium slope areas and 40s for high slope areas.  The numbers will range from 21-28, 31-38 and 41-48 where the last digit relates to the aspect orientation (with 1 being north, 2 northeast, 3 east, and so on).  Numbers less than 20 are essentially flat areas and will be assigned a gray color.
  8. Now classify the results so that you can assign the appropriate colors:
    • Right-click the aspect-slope grid in the Table of Contents.
    • Click the Symbology tab.
    • Under Show on the left side of the dialog box, click Classified.
    • Under Classification, change the number of classes to 26 and click Classify. Use these numbers for the break values (the number on the left is the break value and the number on the right is the class number):
      19 : 1
      21 : 2
      22 : 3
      23 : 4
      24 : 5
      25 : 6
      26 : 7
      27 : 8
      28 : 9
      31 : 10
      32 : 11
      33 : 12
      34 : 13
      35 : 14
      36 : 15
      37 : 16
      38 : 17
      41 : 18
      42 : 19
      43 : 20
      44 : 21
      45 : 22
      46 : 23
      47 : 24
      48 : 25
      highest value : 26
  9. You are now ready to assign colors.  These are the RGB colors in the aspect-slope style and their associated class in the aspect-slope grid:
    19 : 161 161 161
    21 : 152 181 129
    22 : 114 168 144
    23 : 124 142 173
    24 : 140 117 160
    25 : 180 123 161
    26 : 203 139 143
    27 : 197 165 138
    28 : 189 191 137
    31 : 141 196 88
    32 : 61 171 113
    33 : 80 120 182
    34 : 119 71 157
    35 : 192 77 156
    36 : 231 111 122
    37 : 226 166 108
    38 : 214 219 94
    41 : 132 214 0
    42 : 0 171 68
    43 : 0 104 192
    44 : 108 0 163
    45 : 202 0 156
    46 : 255 85 104
    47 : 255 171 71
    48 : 244 250 0
  10. If you want to use the colors in the style that you downloaded, in the Table of Contents, click the color chip for each class and then hover over the color chips in the palette until you find the corresponding RGB values (a label will appear for each chip if you hover over it long enough).
  11. You can save yourself the time and effort of applying the colors one-by-one if you simply reference the colors in the aspect-slope layer that we put in the zip file for you. To use this:
    • In the Table of Contents, right click the aspect-slope grid you created.
    • Click the Symbology tab.
    • At the top right, click Import.
    • Select the aspect-slope layer.

The colors should now be appropriately assigned on your map.

To learn more about this mapping technique and this unique coloring scheme, check out the following references:

  • Color Representation of Aspect and Slope Simultaneously, by Cynthia A. Brewer and Ken A. Marlow, available online at: http://www.personal.psu.edu/cab38/Terrain/AutoCarto.html.
  • GIS World, 1991, MKS-ASPECT Enhances Color Surface Renderings, GIS World, 4(October): 30-32.
  • Moellering, H., and A.J. Kimerling, 1990, A New Digital Slope-Aspect Display Process, Cartography and Geographic Information Systems, 17(2): 151-159.
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Leave a Reply

7 Comments

  1. jreades says:

    It’s a very nice map and a really useful explanation of what the user should doing to create it, but from a programming standpoint the raster is reclassification approach ‘backwards’: you’ve set it up so that the slope steepness is the ‘tens’ column and the direction is the ‘unit’ column in the resulting raster.

    In other words, a value of 19 means that a cell has steepness 1 and aspect 9, while a value of 48 means steepness 40 and aspect 8. The problem with this approach is that it’s much more likely that you’ll reclassify the slope than the aspect, and that every time you do so your entire scheme has to be re-entered from scratch because the overall range will change (e.g. from 19-48 to 15 to 59).

    If you switch this around, then the aspect of the slope can be 10-19 (north), 20-29 (northeast), …, 80-89 (northwest). This is much less likely to change since we’d not be able to use the colour wheel approach for more than 8 directions anyway. After that, you just need to decide how many classes you want to represent the amount of slope in that direction.

    In this approach you can work directly from the colour wheel that you show by creating 4 classes for each direction (the fourth being the light grey that represents ‘flat’) and working from the outer colour to the inner one. So it’s more logical and easier to make sense of where you are when setting up the classification. But if you end up wanting to use five classes instead of four then it’s pretty easy to change this by inserting the new classes and moving the boundaries of the existing ones.

    HTH.

  2. sara.lovtang@oregon.gov says:

    “download the a dataset from the ArcGIS Resources page of Mapping Center– click on the Data tab and download the Crater Lake Aspect-Slope Map data”

    I can’t even get out of the starting gate. When I click on the link, I do not see ArcGIS Resources page or Mapping Center or a Data tab or anything to do with Crater Lake Aspect Slope Map data…

  3. gregpatterson1961 says:

    Fantastic documentation and product. Have used this for teaching at the Canberra Institute of Technology here in Australia for some years.
    One of my students documented the full process using ESRI ArcGIS and another students documented the process using Safe Software Feature Manipulation Engine (FME). The process takes 12.6 seconds in FME.
    Greg Patterson

  4. abuckley says:

    Note that the colors in Step 9 were not correct. This is the correct list of colors (updated 1-25-17):

    19 : 161 161 161
    21 : 152 181 129
    22 : 114 168 144
    23 : 124 142 173
    24 : 140 117 160
    25 : 180 123 161
    26 : 203 139 143
    27 : 197 165 138
    28 : 189 191 137
    31 : 141 196 88
    32 : 61 171 113
    33 : 80 120 182
    34 : 119 71 157
    35 : 192 77 156
    36 : 231 111 122
    37 : 226 166 108
    38 : 214 219 94
    41 : 132 214 0
    42 : 0 171 68
    43 : 0 104 192
    44 : 108 0 163
    45 : 202 0 156
    46 : 255 85 104
    47 : 255 171 71
    48 : 244 250 0

    The colors in Step 9 were “preliminary colors in use before the map was exported to the Macintosh environment for color-scheme work in HVC”, as noted in the Brewer article.

    I have updated these in the blog.

  5. eleldrid says:

    Hi! Looking for some help with this. When I reclassify my aspect map as per above, I get 9 classes with the following labels (not the values I put it on the reclassify tool):

    -1
    1
    2
    3
    4
    5
    6
    7
    8
    359

    I think it is having troubles because the negative values are not included in the classification. Anyone able to tell me what I’m doing wrong? Thanks in advance!!

    • abuckley says:

      This worked for me — in the Reclassify tool, I see these classes that need to be changed:
      -1 – 0.000001 : 1
      -0.000001 – 22.5 : 2
      22.5 – 67.5 : 3
      etc.
      Select the first line and click Delete Entries to get rid of that class. fir the new first class, change the lowest value from -0.00001 to 0 and leave the rest. Change its New Value to 1. then simply change all the New Values for the other classes as shown in the blog post above (what used to be New Value 3 becomes 3, what used to be New Value 4 becomes 3, and so on). Be cause toe click the down arrow to the right of the New Value column to set the New Value for the last class to 1. Then run the tool. It should work for you.