Brendan Harmon

Geomorphometry in GRASS GIS

A tutorial on quantitative terrain analysis in GRASS GIS.

Contents


Geomorphometry

Geomorphometry is the quantitative analysis of topography. Geomorphometric analyses include slope, aspect, curvature, and landforms. In this tutorial we will using the modules r.param.scale and r.geomorphon to automatically classify the landforms on Governor’s Island.

This tutorial uses the Governor’s Island Dataset for GRASS GIS. Download, extract, and move this geospatial dataset for Governor’s Island in New York City to your grassdata directory.

Start GRASS GIS, set the GRASS GIS database directory to grassdata directory, select nyspf_governors_island as your location, and create a new mapset called geomorphometry. Zoom in on the landforms in the southwest of the island. Either set the computation from the display using the various zoom options dropdown or run g.region and set the boundaries for the region. Save the region. Then set a mask to the vector map shoreline with the module r.mask.

g.region n=189850 s=189100 e=978550 w=976850 save=landforms
r.mask vector=shoreline
Digital Elevation Model
Elevation

Topographic Parameters

The module r.param.scale calculates the morphometric parameters of topography using calculus. Terrain is parameterized based on the first and second derivatives of quadratic surfaces fit to a moving window using least squares. The scale of the morphometric parameterization depends on the size of the moving window. Parameters include slope, aspect, curvature, and morphometric features. Morphometric features, i.e. landforms, are defined by relationship of a cell to it neighbors in terms of convexity and concavity, the second derivatives of the topographic surface. r.param.scale can identify six generals landforms - peaks, ridges, passes, planes, channels, and pits.

Use the module r.param.scale to automatically classify landforms. Set the moving window size to an odd number.

r.param.scale input=elevation_2017 output=landforms size=33 method=feature --overwrite
Landforms
Landforms

Try running r.param.scale with different moving window sizes. Either change the name of the output map or set the --overwrite flag. Add a legend. Note how this module characterizes most of the landforms here as either ridges or channels.


Geomorphons

The module r.geomorphon automatically recognizes and classifies landforms using machine vision. Landforms are classified as either flats, peaks, ridges, shoulders, spurs, slopes, hollows, footslope, valleys, or pits based on their visibility from 8 cardinal and ordinal directions.

Landforms
Landforms

Geomorphons has been used for diverse application such as characterizing submarine dunes on the ocean floor and creating global landform maps.

Classify landforms using r.geomorphon. Experiment with the search, skip, and flat parameters. The search radius determines the scale of the landform features, the flatness threshold set the angle at which ground is considered flat, and the skip radius eliminates small landforms, reducing noise.

g.region region=landforms
r.geomorphon elevation=elevation_2017 forms=geomorphons search=36 skip=6 flat=12 --overwrite
Geomorphons
Geomorphons

Note how r.geomorphon has clearly identified the ridge lines and their peaks and has classified the pathways as either valleys or footslopes.


Landform Visualization

Compute shaded relief from the digital elevation model using r.relief. Set the units to US survey feet and optionally the vertical scale to 2 or higher. Then drape a map of landforms over the shaded relief using r.shade.

r.relief input=elevation_2017 output=relief_2017 zscale=2 units=survey
r.shade shade=relief_2017 color=geomorphons output=shaded_geomorphons brighten=45
Geomorphons with Shaded Relief
Geomorphons

Landform Extraction

Use map algebra to extract the ridges from either map of landforms. In the raster map calculator, use an if, then, else statement. If cells in the geomorphons raster equal 3, then write a value of 1 in the new raster, else write null values.

r.mapcalc expression="ridges = if(geomorphons==3,1,null())"

Set a color for the maps of ridges. Right click on ridges layer in the layer manager and choose set color interactively. Assign a category value with r.category. In the Define tab enter category value directly as 1|ridge

r.category map=ridges separator=pipe
Ridges
Ridges

To make a simpler, cleaner vector map of the ridges, first convert the raster map to a vector using the module r.to.vect. Then remove small areas from the vector map with the module v.clean. All areas smaller than the threshold parameter will be removed. Then simplify the boundaries of the areas using v.generalize with the reumann method. Smooth the boundaries of the areas using v.generalize with the snakes or hermite method. Remove the intermediate cleaned and simplified maps with g.remove.

r.to.vect -s input=ridges output=ridges type=area
v.clean input=ridges output=ridges_cleaned type=point,line,area tool=rmarea thres=2
v.generalize input=ridges_cleaned type=area output=ridges_generalized method=reumann threshold=2
v.generalize input=ridges_generalized type=area output=ridges method=snakes threshold=2 alpha=1 beta=1
g.remove -f type=vector name=ridges_cleaned,ridges_generalized
Vector Ridges
Ridges