## NAME

v.segment

## DESCRIPTION

v.segment generates segments or points from input lines and from positions read from a text file or '`stdin`'.

The format is:

    P <point id>   <line cat> <offset> [<side offset>]
    L <segment id> <line cat> <start offset> <end offset> [<side offset>]

### EXAMPLE

The user could send to `stdin` something like:

    P 1 356 24.56
    P 2 495 12.31
    ...

(pipe or redirect from file into the command).

## NOTES

A segment is only created for the first line found of the specified category.

Points are generated along the lines at the given distance(s) from the beginning 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").

To place a point in the middle of a line, 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.

## EXAMPLES

Spearfish location:

    # extract lines from railroad map:
    v.extract railroads out=myrr list=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 n=4928200 s=4921100 w=605600 e=613200

    # show line, category, direction (to find the beginning)
    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

    # get points from a text file
    cat mypoints.txt | v.segment myrailroads out=myrailroads_mypoints

    # 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

    # 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.vect myrailroads_pt2km icon=basic/circle color=blue fcolor=blue size=5

    # 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.vect myrailroads_pt2kmO500m icon=basic/circle color=aqua fcolor=aqua size=5

## BUGS

There is a problem with side-offset parallel line generation for inside corners.

## SEE ALSO

LRS tutorial (Linear Referencing System),
d.vect, v.build.polylines, v.lrs.segment, v.parallel, v.split, v.to.db, v.to.points

## AUTHOR

Radim Blazek, ITC-Irst, Trento, Italy

Last changed: $Date: 2011-11-08 12:29:50 +0100 (Tue, 08 Nov 2011) $

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