[GRASS-user] HEC-RAS

[Micha Silver]

Hamish wrote:

> Micha Silver wrote:
>
>   
>> I'm trying to get geared up to do flood plain modeling with HEC_RAS. I
>> have a vector layer of the drainage channel and its banks. I need to
>> make a set of cross section line segments, such that each segment is
>> perpendicular to the channel line, and long enough to cross both banks.
>> These cross sections should be spaced at regular intervals along the
>> channel line, say every 100 m. Any hints how to proceed?
>>     
>
> v.lrs.* in GRASS 6.1?
>
>
> Hamish
>   
Hello Hamish,
I've installed 6.1 and started trying to work with the v.lrs.* modules. 
I'm stuck on the order of points along the line:

First I constructed a set of points along the line with v.segment. 
Here's a clip of the input file I piped to v.segment:
GRASS 6.1.cvs (Israel):~/geodata > head -5 k_pts_L
P 4 1 7500 200
P 5 1 7400 200
P 6 1 7300 200
P 7 1 7200 200
P 8 1 7100 200

Note that I've added a 5th column for the "side-offset" to get the 
points 200 m. orthogonal to the line. This is not working for me. The 
points appear every 100m along the line and placed *on* the line. Can 
anyone point out what's wrong?

Next, I use the resulting point vector layer, together with the line 
vector of the river channel to run v.lrs.create, but here's the results:
GRASS 6.1.cvs (Israel):~/geodata > v.lrs.create in_lines=kinet_channel 
out_lines=kinet_lrs points=kinet_pts_l lidcol=cat pidcol=PID 
rstable=kinet_rstable --o
WARNING: start of 1. MP >= start of 2. MP for points' cats 4, 5
WARNING: start of 1. MP >= start of 2. MP for points' cats 5, 6
WARNING: start of 1. MP >= start of 2. MP for points' cats 6, 7
WARNING: start of 1. MP >= start of 2. MP for points' cats 7, 8
....
WARNING: start of 1. MP >= start of 2. MP for points' cats 77, 78
WARNING: start of 1. MP >= start of 2. MP for points' cats 78, 79
WARNING: Incorrect order of points along line cat 1
Building topology for output (out_lines) map ...
Building topology ...
0 primitives registered
0 areas built
0 isles built
Attaching islands:
Attaching centroids: Topology was built.
Number of nodes     :   0
Number of primitives:   0
Number of points    :   0
Number of lines     :   0
Number of boundaries:   0
Number of centroids :   0
Number of areas     :   0
Number of isles     :   0

So nothing is created in the LRS. However from v.db.select it looks to 
me that the start_mp are correct:
GRASS 6.1.cvs (Israel):~/geodata > v.db.select map=kinet_pts_l
cat|PID|start_mp|end_mp|start_off|end_off
4|1|100|200||
5|1|200|300||
6|1|300|400||
7|1|400|500||
8|1|500|600||
.....

I also tried remaking the point vector layer with distances 
100,200,300... along the line, instead of 7500,7400,7300... as above. 
Same result.

Can anyone help me to understand this Linear Reference System better?
Again, my goal is to create a set of line segments which are 
perpendicular to the river channel, and spaced every 100 m.  along the 
channel.

Many thanks,
Micha