The format is:
P <point id> <line cat> <offset> [<side offset>] L <segment id> <line cat> <start offset> <end offset> [<side offset>]
P 1 356 24.56 P 2 495 12.31 P 3 500 -12.31 P 4 510 -20% ...
Points are generated along the lines at the given distance(s) or percent(s) of the line length from the beginning or end, if offsets are negative, of the vector line.
The side offset is the orthogonal distance from the line. Positive side offsets are to the right side of the line going forward, negative offsets are to the left (d.vect with display=shape,dir shows the direction of vector lines). As the segment distance is measured along the original line, side-offset lines will be longer than the start-end segment distance for outside corners of curving lines, and shorter for inside corners.
All offsets are measured in map units (see "g.proj -p") or percents of the line length, if followed by a % character.
To place a point in the middle of a line, 50% offset can be used or the v.to.db module may be used to find the line's length. Then half of that distance can be used as the along-line offset.
Example: Extract line segment from 400m to 5000m from beginning of line 1:
# extract lines from railroad map: v.extract railroads out=myrr cats=1 # join segments into polyline and reassign category numbers v.build.polylines myrr out=myrr_pol v.category myrr_pol out=myrailroads option=add # zoom to an area of interest g.region vector=myrailroads -p # show line, category, direction (to find the beginning) d.mon wx0 d.vect myrailroads disp=shape,cat,dir lsize=12 # extract line segment from 400m to 5000m from beginning of line 1 echo "L 1 1 400 5000" | v.segment myrailroads out=myrailroads_segl d.erase d.vect myrailroads d.vect myrailroads_segl col=green width=2 # set node at 5000m from beginning of line 1 echo "P 1 1 5000" | v.segment myrailroads out=myrailroads_segp d.vect myrailroads_segp icon=basic/circle color=red fcolor=red size=5
Example: Create parallel 1km long line segments along first 8km of track, offset 500m to the left of the tracks.
v.segment myrailroads out=myrailroads_segl_side << EOF L 1 1 1000 2000 -500 L 2 1 3000 4000 -500 L 3 1 5000 6000 -500 L 4 1 7000 8000 -500 EOF d.erase d.vect myrailroads disp=shape,dir d.vect -c myrailroads_segl_side width=2
Example: A series of points, spaced every 2km along the tracks
v.segment myrailroads out=myrailroads_pt2km << EOF P 1 1 1000 P 2 1 3000 P 3 1 5000 P 4 1 7000 EOF d.erase d.vect myrailroads disp=shape,dir d.vect myrailroads_pt2km icon=basic/circle color=blue fcolor=blue size=5
Example: A series of points, spaced every 2km along the tracks, offset 500m to the right
v.segment myrailroads out=myrailroads_pt2kmO500m << EOF P 1 1 1000 500 P 2 1 3000 500 P 3 1 5000 500 P 4 1 7000 500 EOF d.erase d.vect myrailroads disp=shape,dir d.vect myrailroads_pt2kmO500m icon=basic/circle color=aqua fcolor=aqua size=5
Example: A series of points, spaced every 10% of the line's length along the tracks from the end of the line up to the middle point, offset 500m to the right
v.segment myrailroads out=myrailroads_pt10pctO500m << EOF P 1 1 -0% 500 P 2 1 -10% 500 P 3 1 -20% 500 P 4 1 -30% 500 P 5 1 -40% 500 P 6 1 -50% 500 EOF d.erase d.vect myrailroads disp=shape,dir d.vect myrailroads_pt10pctO500m icon=basic/circle color=red fcolor=black size=5
Last changed: $Date: 2015-05-10 17:11:34 -0700 (Sun, 10 May 2015) $
© 2003-2018 GRASS Development Team, GRASS GIS 7.2.3svn Reference Manual