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NAME
r.geomorphon - Calculates geomorphons (terrain forms) and associated geometry using
machine vision approach.
KEYWORDS
raster, geomorphons, terrain patterns, machine vision geomorphometry
SYNOPSIS
r.geomorphon
r.geomorphon --help
r.geomorphon [-me] elevation=name [forms=name] [ternary=name] [positive=name]
[negative=name] [intensity=name] [exposition=name] [range=name] [variance=name]
[elongation=name] [azimuth=name] [extend=name] [width=name] search=integer
skip=integer flat=float dist=float [comparison=string] [coordinates=east,north]
[profiledata=name] [profileformat=string] [--overwrite] [--help] [--verbose]
[--quiet] [--ui]
Flags:
-m
Use meters to define search units (default is cells)
-e
Use extended form correction
--overwrite
Allow output files to overwrite existing files
--help
Print usage summary
--verbose
Verbose module output
--quiet
Quiet module output
--ui
Force launching GUI dialog
Parameters:
elevation=name [required]
Name of input elevation raster map
forms=name
Most common geomorphic forms
ternary=name
Code of ternary patterns
positive=name
Code of binary positive patterns
negative=name
Code of binary negative patterns
intensity=name
Rasters containing mean relative elevation of the form
exposition=name
Rasters containing maximum difference between extend and central cell
range=name
Rasters containing difference between max and min elevation of the form extend
variance=name
Rasters containing variance of form boundary
elongation=name
Rasters containing local elongation
azimuth=name
Rasters containing local azimuth of the elongation
extend=name
Rasters containing local extend (area) of the form
width=name
Rasters containing local width of the form
search=integer [required]
Outer search radius
Default: 3
skip=integer [required]
Inner search radius
Default: 0
flat=float [required]
Flatness threshold (degrees)
Default: 1
dist=float [required]
Flatness distance, zero for none
Default: 0
comparison=string
Comparison mode for zenith/nadir line-of-sight search
Options: anglev1, anglev2, anglev2_distance
Default: anglev1
coordinates=east,north
Coordinates to profile
profiledata=name
Profile output file name ("-" for stdout)
profileformat=string
Profile output format
Options: json, yaml, xml
DESCRIPTION
What is geomorphon:
Geomorphon is a new concept of presentation and analysis of terrain forms. This concept
utilises 8-tuple pattern of the visibility neighbourhood and breaks well known limitation
of standard calculus approach where all terrain forms cannot be detected in a single
window size. The pattern arises from a comparison of a focus pixel with its eight
neighbors starting from the one located to the east and continuing counterclockwise
producing ternary operator. For example, a tuple {+,-,-,-,0,+,+,+} describes one possible
pattern of relative measures {higher, lower, lower, lower, equal, higher, higher, higher}
for pixels surrounding the focus pixel. It is important to stress that the visibility
neighbors are not necessarily an immediate neighbors of the focus pixel in the grid, but
the pixels determined from the line-of-sight principle along the eight principal
directions. This principle relates surface relief and horizontal distance by means of
so-called zenith and nadir angles along the eight principal compass directions. The
ternary operator converts the information contained in all the pairs of zenith and nadir
angles into the ternary pattern (8-tuple). The result depends on the values of two
parameters: search radius (L) and relief threshold (d). The search radius is the maximum
allowable distance for calculation of zenith and nadir angles. The relief threshold is a
minimum value of difference between LOSs angle (zenith and nadir) that is considered
significantly different from the horizon. Two lines-of-sight are necessary due to zenith
LOS only, does not detect positive forms correctly.
There are 3**8 = 6561 possible ternary patterns (8-tuples). However by eliminating all
patterns that are results of either rotation or reflection of other patterns wa set of 498
patterns remain referred as geomorphons. This is a comprehensive and exhaustive set of
idealized landforms that are independent of the size, relief, and orientation of the
actual landform.
Form recognition depends on two free parameters: Search radius and flatness threshold.
Using larger values of L and is tantamount to terrain classification from a higher and
wider perspective, whereas using smaller values of L and is tantamount to terrain
classification from a local point of view. A character of the map depends on the value of
L. Using small value of L results in the map that correctly identifies landforms if their
size is smaller than L; landforms having larger sizes are broken down into components.
Using larger values of L allows simultaneous identification of landforms on variety of
sizes in expense of recognition smaller, second-order forms. There are two additional
parameters: skip radius used to eliminate impact of small irregularities. On the contrary
flatness distance eliminates the impact of very high distance (in meters) of search radius
which may not detect elevation difference if this is at very far distance. Important
especially with low resolution DEMS.
OPTIONS
-m
All distance parameters (search, skip, flat distances) are supplied as meters instead
of cells (default). To avoid situation when supplied distances is smaller than one
cell program first check if supplied distance is longer than one cell in both NS and
WE directions. For LatLong projection only NS distance checked, because in latitude
angular unit comprise always bigger or equal distance than longitude one. If distance
is supplied in cells, For all projections is recalculated into meters according
formula: number_of_cells*resolution_along_NS_direction. It is important if geomorphons
are calculated for large areas in LatLong projection.
elevation
Digital elevation model. Data can be of any type and any projection. During
calculation DEM is stored as floating point raster.
search
Determines length on the geodesic distances in all eight directions where
line-of-sight is calculated. To speed up calculation is determines only these cells
which centers falls into the distance.
skip
Determines length on the geodesic distances at the beginning of calculation all eight
directions where line-of-sight is yet calculated. To speed up calculation this
distance is always recalculated into number of cell which are skipped at the beginning
of every line-of-sight and is equal in all direction. This parameter eliminates forms
of very small extend, smaller than skip parameter.
flat
The difference (in degrees) between zenith and nadir line-of-sight which indicate flat
direction. If higher threshold produce more flat maps. If resolution of the map is low
(more than 1 km per cell) threshold should be very small (much smaller than 1 degree)
because on such distance 1 degree of difference means several meters of high
difference.
dist
>Flat distance. This is additional parameter defining the distance above which the
threshold starts to decrease to avoid problems with pseudo-flat line-of-sights if real
elevation difference appears on the distance where its value is higher (TO BE
CORRECTED).
comparison
Comparison mode for zenith/nadir line-of-sight search. "anglev1" is the original
r.geomorphon comparison mode. "anglev2" is an improved mode, which better handles
angle thresholds and zenith/nadir angles that are exactly equal. "anglev2_distance" in
addition to that takes the zenith/nadir distances into account when the angles are
exactly equal.
forms
Returns geomorphic map with 10 most popular terrestrial forms. Legend for forms, its
definition by the number of + and - and its idealized visualisation are presented at
the image.
Forms represented by geomorphons:
ternary
returns code of one of 498 unique ternary patterns for every cell. The code is a
decimal representation of 8-tuple minimalised patterns written in ternary system. Full
list of patterns is available in source code directory as patterns.txt. This map can
be used to create alternative form classification using supervised approach.
positive and negative
returns codes binary patterns for zenith (positive) and nadir (negative) line of
sights. The code is a decimal representation of 8-tuple minimalised patterns written
in binary system. Full list of patterns is available in source code directory as
patterns.txt.
coordinates
The central point of a single geomorphon to profile. The central point must be within
the computational region, which should be large enough to accommodate the search
radius. Setting the region larger than that will not produce more accurate data, but
in the current implementation will slow the computation down. For the best results
remember to align the region to the raster cells. Profiling is mutually exclusive with
any raster outputs, but other parameters and flags (such as elevation, search,
comparison, -m and -e) work as usual.
profiledata
The output file name for the complete profile data, "-" means to write to the standard
output. The data is in a machine-readable format and it includes assorted values
describing the computation context and parameters, as well as its intermediate and
final results.
profileformat
Format of the profile data: "json", "yaml" or "xml".
NOTE: parameters below are experimental. The usefulness of these parameters are currently
under investigation.
intensity
returns avarage difference between central cell of geomorphon and eight cells in
visibility neighbourhood. This parameter shows local (as is visible)
exposition/abasement of the form in the terrain.
range
returns difference between minimum and maximum values of visibility neighbourhood.
variance
returns variance (difference between particular values and mean value) of visibility
neighbourhood.
extend
returns area of the polygon created by the 8 points where line-of-sight cuts the
terrain (see image in description section).
azimuth
returns orientation of the polygon constituting geomorphon. This orientation is
currently calculated as a orientation of least square fit line to the eight verticles
of this polygon.
elongation
returns proportion between sides of the bounding box rectangle calculated for
geomorphon rotated to fit least square line.
width
returns length of the shorter side of the bounding box rectangle calculated for
geomorphon rotated to fit least square line.
NOTES
From computational point of view there are no limitations of input DEM and free parameters
used in calculation. However, in practice there are some issues on DEM resolution and
search radius. Low resolution DEM especially above 1 km per cell requires smaller than
default flatness threshold. On the other hand, only forms with high local elevation
difference will be detected correctly. It results from fact that on very high distance (of
order of kilometers or higher) even relatively high elevation difference will be
recognized as flat. For example at the distance of 8 km (8 cells with 1 km resolution DEM)
an relative elevation difference of at least 136 m is required to be noticed as non-flat.
Flatness distance threshold may be helpful to avoid this problem.
EXAMPLES
Geomorphon calculation: extraction of terrestrial landforms
Geomorphon calculation example using the EU DEM 25m:
g.region raster=eu_dem_25m -p
r.geomorphon elevation=eu_dem_25m forms=eu_dem_25m_geomorph
# verify terrestrial landforms found in DEM
r.category eu_dem_25m_geomorph
1 flat
2 peak
3 ridge
4 shoulder
5 spur
6 slope
7 hollow
8 footslope
9 valley
10 pit
Extraction of peaks
Using the resulting terrestrial landforms map, single landforms can be extracted, e.g. the
peaks, and converted into a vector point map:
r.mapcalc expression="eu_dem_25m_peaks = if(eu_dem_25m_geomorph == 2, 1, null())"
r.thin input=eu_dem_25m_peaks output=eu_dem_25m_peaks_thinned
r.to.vect input=eu_dem_25m_peaks_thinned output=eu_dem_25m_peaks type=point
v.info input=eu_dem_25m_peaks
REFERENCES
• Stepinski, T., Jasiewicz, J., 2011, Geomorphons - a new approach to classification
of landform, in : Eds: Hengl, T., Evans, I.S., Wilson, J.P., and Gould, M.,
Proceedings of Geomorphometry 2011, Redlands, 109-112 (PDF)
• Jasiewicz, J., Stepinski, T., 2013, Geomorphons - a pattern recognition approach
to classification and mapping of landforms, Geomorphology, vol. 182, 147-156 (DOI:
10.1016/j.geomorph.2012.11.005)
SEE ALSO
r.param.scale
AUTHORS
Jarek Jasiewicz, Tomek Stepinski (merit contribution)
SOURCE CODE
Available at: r.geomorphon source code (history)
Accessed: Mon Jun 13 15:09:20 2022
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