GRASS GIS Manual: v.transects.py

NAME

v.transects.py

DESCRIPTION

v.transects creates equally spaced geometries along input lines. The geometries can be lines or quadrilateral areas. Lines and areas are generated to be perpendicular to the input line.

NOTES

Input vector lines that are shorter than transect_spacing are ignored.

transect_spacing, dleft, and dright are interpreted to be in horizontal map units (e.g., degrees in the WGS84 projection).

v.transects may fail for a network of lines in Windows.

EXAMPLES

In these examples, the Nags Head (19MB) mapset is used to generate a shoreline and shore-perpendicular geometries. To use the mapset, unpack it into the nc_spm_08 (150MB) Location or the gisdemo (47MB) Location.

Example 1) - Generate line transects along shoreline

Generate 20 cross-shore transects along 2008 shoreline (1m contour)

g.region rast=NH_2008_1m
r.contour input=NH_2008_1m output=NH_2008_1m level=1 cut=100
v.report map=NH_2008_1m option=length
# cat|level|length
# 1|1|1037.86684790028
# 1038m / 20transects = 52m per transect (value for transect_spacing)
v.transects input=NH_2008_1m output=NH_2008_transects transect_spacing=52
v.info NH_2008_transects

Example 2) - Generate line transects specifying the left and right length

Generate longer, more parallel transects by specifying dleft and dright and smoothing the input line

g.region rast=NH_2008_1m
r.contour input=NH_2008_1m output=NH_2008_1m level=1 cut=100
v.generalize input=NH_2008_1m output=NH_2008_1m_smoothed \
  method=sliding_averaging look_ahead=201
v.transects input=NH_2008_1m_smoothed \
  output=NH_2008_transects_long_smoothed transect_spacing=52 \
  dleft=20 dright=300

Example 3) - Generate area transects along shoreline

Generate longer, more parallel transects by specifying dleft and dright and smoothing the input line

g.region rast=NH_2008_1m
r.contour input=NH_2008_1m output=NH_2008_1m level=1 cut=100
v.transects input=NH_2008_1m output=NH_2008_areas \
  transect_spacing=52 dleft=20 dright=300 type=area
v.db.addtable NH_2008_areas
v.db.addcolumn map=NH_2008_areas columns='demStats DOUBLE PRECISION'
v.rast.stats vector=NH_2008_areas raster=NH_2008_1m column_prefix=NH2008
v.db.select NH_2008_areas

AUTHOR

Eric Hardin, Helena Mitasova, Updates by John Lloyd

Last changed: $Date$

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