Raster terrain analysis
=======================
.. only:: html
.. contents::
:local:
:depth: 1
.. _qgisaspect:
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.
.. figure:: img/aspect.png
:align: center
:scale: 50%
Aspect values
The following picture shows the aspect layer reclassified with a color
ramp:
.. figure:: img/aspect_2.png
:align: center
Aspect layer reclassified
Parameters
..........
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
* - 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]
Default: ``[Save to temporary file]``
- Specify the output aspect raster layer. One of:
.. include:: ../algs_include.rst
:start-after: **file_output_types**
:end-before: **end_file_output_types**
Outputs
.......
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
* - Label
- Name
- Type
- Description
* - **Aspect**
- ``OUTPUT``
- [raster]
- The output aspect raster layer
Python code
...........
**Algorithm ID**: ``qgis:aspect``
.. include:: ../algs_include.rst
:start-after: **algorithm_code_section**
:end-before: **end_algorithm_code_section**
.. _qgisdtmslopebasedfilter:
DTM filter (slope-based)
------------------------
|334|
Can be used to filter a digital elevation model in order to classify its cells into ground and object (non-ground) cells.
The tool uses concepts as described by Vosselman (2000)
and is based on the assumption that a large height difference between two nearby cells is unlikely to be caused by a steep slope in the terrain.
The probability that the higher cell might be non-ground increases when the distance between the two cells decreases.
Therefore the filter defines a maximum height difference (``dz_max``) between two cells as a function of the distance (``d``) between the cells (``dz_max( d ) = d``).
A cell is classified as terrain if there is no cell within the kernel radius
to which the height difference is larger than the allowed maximum height difference at the distance between these two cells.
The approximate terrain slope (``s``) parameter is used to modify the filter function
to match the overall slope in the study area (``dz_max( d ) = d * s``).
A 5 % confidence interval (``ci = 1.65 * sqrt( 2 * stddev )``) may be used to modify the filter function even further
by either relaxing (``dz_max( d ) = d * s + ci``) or amplifying (``dz_max( d ) = d * s - ci``) the filter criterium.
*References: Vosselman, G. (2000): Slope based filtering of laser altimetry data. IAPRS, Vol. XXXIII, Part B3, Amsterdam, The Netherlands, 935-942*
.. seealso:: This tool is a port of the SAGA `DTM Filter (slope-based) <https://saga-gis.sourceforge.io/saga_tool_doc/9.1.1/grid_filter_7.html>`_
Parameters
..........
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
:class: longtable
* - Label
- Name
- Type
- Description
* - **Input layer**
- ``INPUT``
- [raster]
- Digital Terrain Model raster layer
* - **Band number**
- ``BAND``
- [number] [list]
- The band of the DEM to consider
* - **Kernel radius (pixels)**
- ``RADIUS``
- [number]
Default: 5
- The radius of the filter kernel (in pixels).
Must be large enough to reach ground cells next to non-ground objects.
* - **Terrain slope (%, pixel size/vertical units)**
- ``TERRAIN_SLOPE``
- [number]
Default: 30
- The approximate terrain slope in ``%``.
The terrain slope must be adjusted to account for the ratio of height units vs raster pixel dimensions.
Used to relax the filter criterium in steeper terrain.
* - **Filter modification**
- ``FILTER_MODIFICATION``
- [list]
Default: 0
- Choose whether to apply the filter kernel without modification
or to use a confidence interval to relax or amplify the height criterium.
* 0 - None
* 1 - Relax filter
* 2 - Amplify
* - **Standard deviation**
- ``STANDARD_DEVIATION``
- [number]
Default: 0.1
- The standard deviation used to calculate a 5% confidence interval applied to the height threshold.
* - **Output layer (ground)**
Optional
- ``OUTPUT_GROUND``
- [raster]
Default: ``[Save to temporary file]``
- Specify the filtered DEM containing only cells classified as ground.
One of:
.. include:: ../algs_include.rst
:start-after: **file_output_types_skip**
:end-before: **end_file_output_types_skip**
* - **Output layer (non-ground objects)**
Optional
- ``OUTPUT_NONGROUND``
- [raster]
Default: ``[Skip output]``
- Specify the non-ground objects removed by the filter.
One of:
.. include:: ../algs_include.rst
:start-after: **file_output_types_skip**
:end-before: **end_file_output_types_skip**
Outputs
.......
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
* - Label
- Name
- Type
- Description
* - **Output layer (ground)**
- ``OUTPUT_GROUND``
- [raster]
- The filtered DEM containing only cells classified as ground.
* - **Output layer (non-ground objects)**
- ``OUTPUT_NONGROUND``
- [raster]
- The non-ground objects removed by the filter.
Python code
...........
**Algorithm ID**: ``native:dtmslopebasedfilter``
.. include:: ../algs_include.rst
:start-after: **algorithm_code_section**
:end-before: **end_algorithm_code_section**
.. _qgishillshade:
Hillshade
---------
Calculates the hillshade raster layer given an input Digital Terrain
Model.
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.
.. figure:: img/azimuth.png
:align: center
:scale: 50%
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.
.. figure:: img/hillshade.png
:align: center
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:
.. figure:: img/hillshade_2.png
:align: center
Overlapping the hillshade with the elevation layer
Parameters
..........
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
:class: longtable
* - 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]
Default: ``Save to temporary file``
- Specify the output hillshade raster layer. One of:
.. include:: ../algs_include.rst
:start-after: **file_output_types**
:end-before: **end_file_output_types**
Outputs
.......
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
* - Label
- Name
- Type
- Description
* - **Hillshade**
- ``OUTPUT``
- [raster]
- The output hillshade raster layer
Python code
...........
**Algorithm ID**: ``qgis:hillshade``
.. include:: ../algs_include.rst
:start-after: **algorithm_code_section**
:end-before: **end_algorithm_code_section**
.. _qgishypsometriccurves:
Hypsometric curves
------------------
Calculates hypsometric curves for an input Digital Elevation Model.
Curves are produced as CSV files 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
..........
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
:class: longtable
* - 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:
.. include:: ../algs_include.rst
:start-after: **directory_output_types**
:end-before: **end_directory_output_types**
Outputs
.......
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
* - 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.
.. figure:: img/hypsometric.png
:align: center
:scale: 50%
Python code
...........
**Algorithm ID**: ``qgis:hypsometriccurves``
.. include:: ../algs_include.rst
:start-after: **algorithm_code_section**
:end-before: **end_algorithm_code_section**
.. _qgisrelief:
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.
.. figure:: img/relief.png
:align: center
Relief layer
Parameters
..........
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
:class: longtable
* - 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.
.. figure:: img/relief_table.png
:align: center
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:
.. include:: ../algs_include.rst
:start-after: **file_output_types**
:end-before: **end_file_output_types**
* - **Frequency distribution**
Optional
- ``FREQUENCY_DISTRIBUTION``
- [table]
Default: ``[Skip output]``
- Specify the CSV table for the output frequency distribution.
One of:
.. include:: ../algs_include.rst
:start-after: **file_output_types_skip**
:end-before: **end_file_output_types_skip**
Outputs
.......
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
* - Label
- Name
- Type
- Description
* - **Relief**
- ``OUTPUT``
- [raster]
- The output relief raster layer
* - **Frequency distribution**
- ``OUTPUT``
- [table]
- The output frequency distribution
Python code
...........
**Algorithm ID**: ``qgis:relief``
.. include:: ../algs_include.rst
:start-after: **algorithm_code_section**
:end-before: **end_algorithm_code_section**
.. _qgisruggednessindex:
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.
.. figure:: img/ruggedness.png
:align: center
Ruggedness layer from low (red) to high values (green)
Parameters
..........
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
* - 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:
.. include:: ../algs_include.rst
:start-after: **file_output_types**
:end-before: **end_file_output_types**
Outputs
.......
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
* - Label
- Name
- Type
- Description
* - **Ruggedness**
- ``OUTPUT``
- [raster]
- The output ruggedness raster layer
Python code
...........
**Algorithm ID**: ``qgis:ruggednessindex``
.. include:: ../algs_include.rst
:start-after: **algorithm_code_section**
:end-before: **end_algorithm_code_section**
.. _qgisslope:
Slope
-----
Calculates the slope from an input raster layer. The slope is the
angle of inclination of the terrain and is expressed in **degrees**.
.. figure:: img/slope.png
:align: center
Flat areas in red, steep areas in blue
Parameters
..........
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
* - 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:
.. include:: ../algs_include.rst
:start-after: **file_output_types**
:end-before: **end_file_output_types**
Outputs
.......
.. list-table::
:header-rows: 1
:widths: 20 20 20 40
* - Label
- Name
- Type
- Description
* - **Slope**
- ``OUTPUT``
- [raster]
- The output slope raster layer
Python code
...........
**Algorithm ID**: ``qgis:slope``
.. include:: ../algs_include.rst
:start-after: **algorithm_code_section**
:end-before: **end_algorithm_code_section**
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