How do I get for each v.to.points point an ID in the database (DB).

So far I splited the line (each 10m) but it returns only one DB entry for each original line.

v.db.select line
1|1|Line 1
2|2|Line 2

# split the lines every 10m
v.to.points in=line out=line_points dmax=10

# check the result
v.db.select line_points
1|1|Line 1
2|2|Line 2

Each point should have its own DB entry. The result should be:

v.db.select line_points
1|1|Line 1
2|1|Line 1
3|2|Line 2
4|2|Line 2

Workaround: So far my workaround is, to export (v.out.ogr) the points as shape and import (v.in.ogr) it again. Then each point has his own ID. But there must be a smarter way to do that!

  • Not much of a GRASS user but what about v.category.add which is described as: "...attaches, deletes or reports vector categories to map geometry. These categories (IDs) are used to link geometry object(s) to attribute records (from attribute table linked to vector map)."
    – Joseph
    Jan 22 '16 at 15:18
  • I tried that before in GRASS 7 the function calls v.category. What I tried is the following command v.category in=line_points option=add out=line_points2. I posted the question 'cuz I have it already for quite a while and work since ever with my workaround, but I felt alway its not the best method to do it.
    – wittich
    Jan 23 '16 at 21:42
  • Have you looked at grass.osgeo.org/grass70/manuals/v.segment.html? It looks like it performs a similar function and might work better for creating separate segments rather than vertices.
    – TDavis
    Jan 27 '16 at 18:37

v.to.points creates a vector with 2 layers. With the second layer you should have unique ID.

You should thus simply specify the second layer with v.db.select or write into a new feature with v.category.

# return unique IDs
v.db.select line_points layer=2

# change layer 2 to 1
v.category input=line_points output=line_points2 option=chlayer layer=2,1

For more details see v.to.points description:

v.to.points creates points along input lines. The output is a vector with 2 layers. Layer 1 holds the category and attributes of the input lines; all points created along the same line have the same category, equal to the category of that line. In layer 2 each point has it's unique category; other attributes stored in layer 2 are lcat - the category of the input line and along - the distance from line's start.

  • One more time I should have read the manual more carefully. Thx @radouxju that's where I was looking for.
    – wittich
    Jan 30 '16 at 23:55

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