Raster terrain analysis

Aspect

Calculates the aspect of the Digital Terrain Model in input. The final aspect raster layer contains values from 0 to 360 that express the slope direction: starting from North (0°) and continuing clockwise.

../../../../_images/aspect.png

Aspect values

The following picture shows the aspect layer reclassified with a color ramp:

../../../../_images/aspect_2.png

Aspect layer reclassified

Parameters

Label Name Type Description
Elevation layer INPUT [raster] Digital Terrain Model raster layer
Z factor Z_FACTOR

[number]

Default: 1.0

Vertical exaggeration. This parameter is useful when the Z units differ from the X and Y units, for example feet and meters. You can use this parameter to adjust for this. The default is 1 (no exaggeration).
Aspect OUTPUT [raster]

Specify the output aspect raster layer. One of:

  • Save to a Temporary Layer (TEMPORARY_OUTPUT)
  • Save to File…

The file encoding can also be changed here.

Outputs

Label Name Type Description
Aspect OUTPUT [raster] The output aspect raster layer

Hillshade

Calculates the hillshade raster layer given a Digital Terrain Model in input.

The shading of the layer is calculated according to the sun position: you have the options to change both the horizontal angle (azimuth) and the vertical angle (sun elevation) of the sun.

../../../../_images/azimuth.png

Azimuth and vertical angle

The hillshade layer contains values from 0 (complete shadow) to 255 (complete sun). Hillshade is used usually to better understand the relief of the area.

../../../../_images/hillshade.png

Hillshade layer with azimuth 300 and vertical angle 45

Particularly interesting is to give the hillshade layer a transparency value and overlap it with the elevation raster:

../../../../_images/hillshade_2.png

Overlapping the hillshade with the elevation layer

Parameters

Label Name Type Description
Elevation layer INPUT [raster] Digital Terrain Model raster layer
Z factor Z_FACTOR

[number]

Default: 1.0

Vertical exaggeration. This parameter is useful when the Z units differ from the X and Y units, for example feet and meters. You can use this parameter to adjust for this. Increasing the value of this parameter will exaggerate the final result (making it look more “hilly”). The default is 1 (no exaggeration).
Azimuth (horizontal angle) AZIMUTH

[number]

Default: 300.0

Set the horizontal angle (in degrees) of the sun (clockwise direction). Range: 0 to 360. 0 is north.
Vertical angle V_ANGLE

[number]

Default: 40.0

Set the vertical angle (in degrees) of the sun, that is the height of the sun. Values can go from 0 (minimum elevation) to 90 (maximum elevation).
Hillshade OUTPUT [raster]

Specify the output hillshade raster layer. One of:

  • Save to a Temporary Layer (TEMPORARY_OUTPUT)
  • Save to File…

The file encoding can also be changed here.

Outputs

Label Name Type Description
Hillshade OUTPUT [raster] The output hillshade raster layer

Hypsometric curves

Calculates hypsometric curves for an input Digital Elevation Model. Curves are produced as csv file in an output folder specified by the user.

A hypsometric curve is a cumulative histogram of elevation values in a geographical area.

You can use hypsometric curves to detect differences in the landscape due to the geomorphology of the territory.

Parameters

Label Name Type Description
DEM to analyze INPUT_DEM [raster] Digital Terrain Model raster layer to use for calculating altitudes
Boundary layer BOUNDARY_LAYER [vector: polygon] Polygon vector layer with boundaries of areas used to calculate hypsometric curves
Step STEP

[number]

Default: 100.0

Vertical distance between curves
Use % of area instead of absolute value USE_PERCENTAGE

[boolean]

Default: False

Write area percentage to “Area” field of the CSV file instead of the absolute area
Hypsometric curves OUTPUT_DIRECTORY [folder]

Specify the output folder for the hypsometric curves. One of:

  • Save to a Temporary Layer (TEMPORARY_OUTPUT)
  • Save to File…

The file encoding can also be changed here.

Outputs

Label Name Type Description
Hypsometric curves OUTPUT_DIRECTORY [folder]

Directory containing the files with the hypsometric curves. For each feature from the input vector layer, a CSV file with area and altitude values will be created.

The file names start with histogram_, followed by layer name and feature ID.

Relief

Creates a shaded relief layer from digital elevation data. You can specify the relief color manually, or you can let the algorithm choose automatically all the relief classes.

../../../../_images/relief.png

Relief layer

Parameters

Label Name Type Description
Elevation layer INPUT [raster] Digital Terrain Model raster layer
Z factor Z_FACTOR

[number]

Default: 1.0

Vertical exaggeration. This parameter is useful when the Z units differ from the X and Y units, for example feet and meters. You can use this parameter to adjust for this. Increasing the value of this parameter will exaggerate the final result (making it look more “hilly”). The default is 1 (no exaggeration).
Generate relief classes automatically AUTO_COLORS

[boolean]

Default: False

If you check this option the algorithm will create all the relief color classes automatically

Relief colors

Optional

COLORS [table widget]

Use the table widget if you want to choose the relief colors manually. You can add as many color classes as you want: for each class you can choose the lower and upper bound and finally by clicking on the color row you can choose the color thanks to the color widget.

../../../../_images/relief_table.png

Manually setting of relief color classes

The buttons in the right side panel give you the chance to: add or remove color classes, change the order of the color classes already defined, open an existing file with color classes and save the current classes as file.

Relief OUTPUT

[raster]

Default: [Save to temporary file]

Specify the output relief raster layer. One of:

  • Save to a Temporary Layer (TEMPORARY_OUTPUT)
  • Save to File…

The file encoding can also be changed here.

Frequency distribution FREQUENCY_DISTRIBUTION

[table]

Default: [Skip output]

Specify the CSV table for the output frequency distribution. One of:

  • Skip Output
  • Save to a Temporary Layer (TEMPORARY_OUTPUT)
  • Save to File…

The file encoding can also be changed here.

Outputs

Label Name Type Description
Relief OUTPUT [raster] The output relief raster layer
Frequency distribution OUTPUT [table] The output frequency distribution

Ruggedness index

Calculates the quantitative measurement of terrain heterogeneity described by Riley et al. (1999). It is calculated for every location, by summarizing the change in elevation within the 3x3 pixel grid.

Each pixel contains the difference in elevation from a center cell and the 8 cells surrounding it.

../../../../_images/ruggedness.png

Ruggedness layer from low (red) to high values (green)

Parameters

Label Name Type Description
Elevation layer INPUT [raster] Digital Terrain Model raster layer
Z factor Z_FACTOR

[number]

Default: 1.0

Vertical exaggeration. This parameter is useful when the Z units differ from the X and Y units, for example feet and meters. You can use this parameter to adjust for this. Increasing the value of this parameter will exaggerate the final result (making it look more rugged). The default is 1 (no exaggeration).
Ruggedness OUTPUT

[raster]

Default: [Save to temporary file]

Specify the output ruggedness raster layer. One of:

  • Save to a Temporary Layer (TEMPORARY_OUTPUT)
  • Save to File…

The file encoding can also be changed here.

Outputs

Label Name Type Description
Ruggedness OUTPUT [raster] The output ruggedness raster layer

Slope

Calculates the slope from an input raster layer. The slope is the angle of inclination of the terrain and is expressed in degrees.

In the following picture you can see to the left the DTM layer with the elevation of the terrain while to the right the calculated slope:

../../../../_images/slope2.png

Flat areas in red, steep areas in blue

Parameters

Label Name Type Description
Elevation layer INPUT [raster] Digital Terrain Model raster layer
Z factor Z_FACTOR

[number]

Default: 1.0

Vertical exaggeration. This parameter is useful when the Z units differ from the X and Y units, for example feet and meters. You can use this parameter to adjust for this. Increasing the value of this parameter will exaggerate the final result (making it steeper). The default is 1 (no exaggeration).
Slope OUTPUT

[raster]

Default: [Save to temporary file]

Specify the output slope raster layer. One of:

  • Save to a Temporary Layer (TEMPORARY_OUTPUT)
  • Save to File…

The file encoding can also be changed here.

Outputs

Label Name Type Description
Slope OUTPUT [raster] The output slope raster layer