[GRASS-user] working with vectors

[achim]

Hi,

its not as easy as it should be...

1) Having two sets of lines (one from a set of polygons)
2) Patch them
3) Clean them (break, ..)
4) Lines to boundaries
5) add centroids
...
6) ..do not get the areas that should be areas

Even if they are geometrically closed.
I think its a problem of topology, in particular line directions eg. ?

I spend a lot of time with these things (including connecting db's, ex-
and importing shapes trying alternatives). Wouldnt it be cheaper to try
with arc gis? ;-) There I do know to run zonal statistics and to connect
tables from vector data. Well.. I like grass, ...I spend a lot of
time... Maybe there should be some things easier, more intuitive and
better connected that one is led better thought all these all days
procedures.

Thanks a lot,
achim


Hamish schrieb:
> Achim:
>>> , measure length of lines even in an latlong projection...
> Moritz:
>> v.to.db, although I'm not 100% sure how that deals with
>> lat-long. Only comment in the man page:
>>
>> "When calculating perimeters in Latitude-Longitude
>> locations, the geodesic distance between the vertices is
>> used."
>>
>> I would suppose that this is the same for line length...
> 
> actually ??, although I do get correct looking results in meters
> from v.to.db option=length, it looks like v.to.db/lines.c needs
> to have in a few other places:
> 
> - len = Vect_line_length(Points);
> + if (G_projection() == PROJECTION_LL)
> +     len = Vect_line_geodesic_length(Points);
> + else
> +     len = Vect_line_length(Points);
> 
> ??? not sure when that code is used though --
> it also uses length() instead (from v.to.db/calc.c) which uses
> G_distance() (LatLon-aware), which is probably where the
> reasonable output comes from.
> 
> 
> perhaps we need to make a new V_distance(Points) lib fn that,
> like G_distance(), automatically switches to geodesic mode when
> the location is lat/lon?
> 
> test:
> 
> test_lines.vasc
> L 2 1
>  166 -45
>  167 -45
>  1 1
> L 2 1
>  166 -46
>  166 -47
>  1 2
> L 2 1
>  167 -44
>  168 -45
>  1 3
> 
> v.in.ascii in=test_lines.vasc out=test_lines format=standard -n
> v.db.addtable test_lines col='length_m double'
> v.to.db test_lines opt=length colum=length_m
> v.db.select test_lines
> 
> vertical line should be 1 deg lat dist (~ 1852m * 60)
> horiz line should be 1 deg lon dist (~ 1852 * 60 * 0.707 here)
> 
> 
> ?,
> Hamish
> 
> 
> 
>       
>