r.geomorphon(1grass) GRASS GIS User's Manual r.geomorphon(1grass)
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] [exposi-
tion=name] [range=name] [variance=name] [elongation=name] [az-
imuth=name] [extend=name] [width=name] search=integer skip=integer
flat=float dist=float [prefix=string] [step=float] [start=float]
[--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
prefix=string
Prefix for maps resulting from multiresolution approach
step=float
Distance step for every iteration (zero to omit)
Default: 0
start=float
Distance where search will start in multiple mode (zero to omit)
Default: 0
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 neigh-
bourhood and breaks well known limitation of standard calculus ap-
proach 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 continu-
ing 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 visi-
bility neighbors are not necessarily an immediate neighbors of the fo-
cus 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 calcula-
tion of zenith and nadir angles. The relief threshold is a minimum
value of difference between LOSs angle (zenith and nadir) that is con-
sidered 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 reflec-
tion of other patterns wa set of 498 patterns remain referred as geo-
morphons. 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 ter-
rain 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 irregulari-
ties. On the contrary flatness distance eliminates the impact of very
high distance (in meters) of search radius which may not detect eleva-
tion 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 sup-
plied distances is smaller than one cell program first check if
supplied distance is longer than one cell in both NS and WE direc-
tions. 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 projec-
tions is recalculated into meters according formula: num-
ber_of_cells*resolution_along_NS_direction. It is important if geo-
morphons are calculated for large areas in LatLong projection.
elevation
Digital elevation model. Data can be of any type and any projec-
tion. 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 de-
termines 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 calcu-
lated. 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 elimi-
nates 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 me-
ters 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).
forms
Returns geomorphic map with 10 most popular terrestrial forms. Leg-
end for forms, its definition by the number of + and - and its ide-
alized 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 pat-
terns 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 (neg-
ative) 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.
NOTE: parameters below are very 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 cal-
culated 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 es-
pecially above 1 km per cell requires smaller than default flatness
threshold. On the other hand, only forms with high local elevation dif-
ference 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 eleva-
tion 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 summit
3 ridge
4 shoulder
5 spur
6 slope
7 hollow
8 footslope
9 valley
10 depression
Extraction of summits
Using the resulting terrestrial landforms map, single landforms can be
extracted, e.g. the summits, and converted into a vector point map:
r.mapcalc expression="eu_dem_25m_summits = if(eu_dem_25m_geomorph == 2, 1, null())"
r.thin input=eu_dem_25m_summits output=eu_dem_25m_summits_thinned
r.to.vect input=eu_dem_25m_summits_thinned output=eu_dem_25m_summits type=point
v.info input=eu_dem_25m_summits
SEE ALSO
r.param.scale
REFERENCES
• Stepinski, T., Jasiewicz, J., 2011, Geomorphons - a new ap-
proach to classification of landform, in : Eds: Hengl, T.,
Evans, I.S., Wilson, J.P., and Gould, M., Proceedings of Geo-
morphometry 2011, Redlands, 109-112 (PDF)
• Jasiewicz, J., Stepinski, T., 2013, Geomorphons - a pattern
recognition approach to classification and mapping of land-
forms, Geomorphology, vol. 182, 147-156 (DOI: 10.1016/j.geo-
morph.2012.11.005)
AUTHORS
Jarek Jasiewicz, Tomek Stepinski (merit contribution)
SOURCE CODE
Available at: r.geomorphon source code (history)
Accessed: unknown
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