Can somebody explain to me what units of measurement do I get in the results of analysis, ground morphometry in SAGA GIS software or through a plugin in QGIS? for example:

  1. Terrain ruggedness index (TRI)? (m,%, feet, etc)
  2. Topographic Position Index (TPI)? ...
  3. TPI Based Landform Classification
  4. Terrain surface Classification (Iwahashi and Pike)?


I have read the works related to the algorithms shown, but how do I interpret the results in the legend?

Topographic Position Index (TPI)

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This is numeric units of Topographic Position Index (TPI)

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Without digging through the scientific papers here is what they mean.

  1. Terrain Ruggedness Index (TRI): No specific units. The closer you are to 0 the less rugged the terrain likely is. The bigger the number is, eg 105, then the terrain is likely to be more rugged.
  2. Topographic Position Index (TPI): Again no specific units. The lower the numbers are the lower areas in the landscape. The higher numbers are the higher areas in the landscape.
  3. See 4
  4. Terrain Surface Classification: Is a classified map whereby the inputs(eg Slope, Elevation, Convexity, Texture) are meshed together to build landforms. Each landform has its own special characteristics, eg a High Ridge would be: higher in elevation and convex(shaped like a convex mirror), whereas a Valley would be lower in elevation and concave(shaped like a bowl).

If you can put the results on a 3d model it will help you interpret the results and if you take the maps you generated and go and look at the landscape it will help you understand even more.

  • You helped me a lot to eliminate the doubts. You understand well in this scientific discipline. I wish there was a classification in the form of rankings. Figure 1 shows the landforms and in Figure 2 the numerical value but with the equal rank value. For example, the upper slope and midslope must have a range of classifications. Sorry about my English – nagib Apr 5 '18 at 17:02
  • The values in Fig 2 are dynamic due to scaling and grid resolution in the case of the TPI based landform classification. Every different digital elevation model run through this algorithm will not have the exact same numbers as in Fig 2. This Link may aid your understanding: jennessent.com/downloads/tpi-poster-tnc_18x22.pdf as well I encourage you to dig through the SAGA GIS forums and documentation if you want to understand it even more. – Dan B Apr 5 '18 at 17:16

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