Object thickness is evaluated by creating transects along the median lines of the raster objects, clipping them with object themselves and evaluating their lengths.
Optionally, r.object.thickness can save a CSV file containing the complete list of the lenghts of the parts of all created transects inside the objects. It is possible to save a maps containing the median lines of the objects, in both raster and vector format, a vector map containing the transects and a vector map containing the clipped transects.
The v.transects addon must be installed to run this module.
The expected maximum lentgth of the transects is used to create the transects before clipping them with the raster objects. It must be chosen large enough to contain the longer cross secton of the biggest object. The module issues a warning if the maximum evaluated thickness is less than or equal to the expected maximum: in this case at least one transect has not been clipped because it does not intersect the object boundary. Therefore the expected maximum size parameter must be raised.
Transects spacing controls the distance between transects along the median line. It must be chosen so that at least one transect is created on each median line. Smaller values can provide slightly more accurate results but require more processing time. As a rule of thumb, a good starting point is setting transects spacing abount 1/50 of the expected maximum size, but the minimum value can change. If the the transects spacing value is too low no transect is created and no thickness can be evaluated: in this case the module issues an error and stops.
It is possibile to choose the direction (N-S or E-W) of the region resolution used to convert the estimated lenghts in pixels. The choiche is irrelevant for regions with square cells.
Optional maps containing the median lines of the objects, in both raster and vector format, the vector map containing the transects and the vector map containing the clipped transects are created only if a name is provided for them. In the same way, a CSV file containing the complete list of the lenghts of the parts of all created transects inside the objects is also created only if a file name is given.
# set the region on the landuse map g.region rast=landuse@PERMANENT # evaluate the thickness of water bodies (categoy 6) in the landuse map # create a vector map containing the median lines called median # create a vector map containing the transects inside the water bodies called transects_in r.object.thickness input=landuse@PERMANENT category=6 tsize=4000 tspace=100 vmedian=median itransects=transects_in
Thickness in map units: min 1.525433 max 2962.446155 mean 301.059197 Thickness in pixels: min 0.053524 max 103.945479 mean 10.563481
© 2003-2021 GRASS Development Team, GRASS GIS 7.8.7dev Reference Manual