[GRASS-user] v.lrs.xxx

[ivan marchesini]

Dear users...
does someone has tried to work with v.lrs.xxx modules!!
I think they are really interesting but, after I have read this notes
http://www.j-geoinfo.net/Content/fulmar05/IJG_095-100.pdf
http://gisws.media.osaka-cu.ac.jp/grass04/viewpaper.php?id=50
http://grass.ibiblio.org/grass63/manuals/html63_user/lrs.html

I have probably (but I'm not sure) understood the theory but I have some
problem with practice...

the first step is to create a LRS... (v.lrs.create)
if I have well understood I need:
- a vector map with a line composed by some vector lines, each one
having a cat. this line represents our road...
- a vector map containing some points equispaced along this line (I can
create them with v.segment o v.to.points) these points need to be at the
junction between the lines described before. This points are milepost
along the road.
- both line and points need to have a column that contain a label
defining the road.. we can call this column LID (Line ID) in the lines
table and PID (Point ID) in the point table.

in my example I have this situation:

table of points
cat	PID
1	1
2	1
3	1
4	1
5	1
6	1

table of lines
CAT	LID
1	1
2	1
3	1
4	1
5	1


i.e. I have 5 lines (the segments that compose my road) and 6 nodes
(points or milepost).. one of this points is at the start of the road,
and another is at the end...

now the problems (I think..)

although v.lrs.create needs (from the manual) only these parameters:

in_lines=name out_lines=name points=name lidcol=string pidcol=string
rstable=string

in my case
in_lines=road out_lines=lrs_vect points=point lidcol=LID pidcol=PID
rstable=lrs_table

when I run v.lrs.create I obtain:

DBMI-DBF driver error:
Column 'start_mp' not found
Error in db_open_select_cursor()
GRASS_INFO_ERROR(18585,1): Cannot select point attributes from pnt_2.

this means that probably it is necessary to use the "start_mp",
"end_mp", "start_off", "end_off" parameters, that the manual so explain:

start_mp=string 
Column containing milepost position for the beginning of next segment. 
Default: start_mp

end_mp=string 
Column containing milepost position for the end of previous segment. 
Default: end_mp

start_off=string 
Column containing offset from milepost for the beginning of next
segment. 
Default: start_off

end_off=string 
Column containing offset from milepost for the endof previous segment. 
Default: end_off

this is the big problem.. the meaning of these parameters...
I have understood, but maybe I'm wrong, that:

start_mp is the number of the milepost "viewed" from the segment that
start from it

end_mp is the number of the milepost "viewed" from the segment that
finish to it

is this right???
all the problems start here...

I don't know which values I have to give to the mailepost attributes
start_mp e end_mp.

can someone give me some help???
may be Radim???  :-)

thank you very much!!!



  





-- 
Ivan Marchesini
Department of Civil and Environmental Engineering
University of Perugia
Via G. Duranti 93/a 
06125
Perugia (Italy)
e-mail: marchesini at unipg.it
        ivan.marchesini at gmail.com
tel: +39(0)755853760
fax: +39(0)755853756
jabber: geoivan73 at jabber.org