Fwd: Re: [GRASS-dev] VTK export for lat-long locations
"Sören Gebbert"
soerengebbert at gmx.de
Tue Mar 6 13:00:26 EST 2007
Hi Benjamin,
-------- Original-Nachricht --------
Datum: Tue, 06 Mar 2007 17:50:41 +0100
Von: Benjamin Ducke <benjamin.ducke at ufg.uni-kiel.de>
An: GRASS devel <grass-dev at grass.itc.it>
CC:
Betreff: Re: Fwd: Re: [GRASS-dev] VTK export for lat-long locations
> OK, so as Markus has pointed out, we have the necessary funcs in
> lib/vector/Vlib/line.c.
Well, it uses the functions from lib/gis/distance.c and so should i.
> And as you pointed out, the basic scaling information changes with every
> raster row.
> I assume that you export a GRASS raster map in some loop construct where
> the outer loop goes through the rows in the GRASS map and the inner loop
> goes through the columns?
Yes.
> Then the code could be something like this:
>
>
> intialize geodesic distance measures (?)
yes:
G_begin_distance_calculations();
>
> for all rows in current map do
>
> get width of one cell (region east resolution)
> use Vect_line_geodesic_length() to get metric length cell
length = G_geodesic_distance (x1, y1, x2, y2);
>
> for all columns in current row do
> scale z using current metric length
> output x,y,z to VTK file
I dont think this is a good idea. It is a nice and easy approach
indeed, but IMHO the length of the z height does not depend on
the length of a line on the surface.
So the z elevation is independend from the positon
on the surface (i know the earth is not a perfect sphere ... ;) ),
in this case the row number.
The approach which i have in mind is to compute metric coordinates for
each point (center of the cell) without scaling the z elevation.
LL X,Y
_______ ____
| | / \
| | --> / \
| | / \
|_______| /__________\
But the problem which i have is: VTK does not know the meaning of
a world projection or a location like grass.
So i do not know how to transfrom the position on the earth
geoid in LL in a 3d space with x,y,z starting from 0,0,0.
So which of the point is 0,0,0 if converted from lat/lon?
The equator, the upper left cell, the lower right? I dont know. :/
I hope that i'm wrong and that there is a quite easy approach.
eg: scaling the elevation with a fixed value?
And my english is to bad,
to explain my problem in a meaningful sense.
Best regards
Soeren
btw.:
The next problem which i have is the transformation of LL volume data
into the x,y,z VTK format. In this case the size of one cell depends
on the row and on the height.
> done
> done
>
>
> Did I overlook anything?
>
>
>
>
> Sören Gebbert wrote:
> > Hi Benjamin,
> > -------- Original-Nachricht --------
> > Datum: Tue, 06 Mar 2007 16:45:14 +0100
> > Von: Benjamin Ducke <benjamin.ducke at ufg.uni-kiel.de>
> > An:
> > CC: grass-dev at grass.itc.it
> > Betreff: Re: Fwd: Re: [GRASS-dev] VTK export for lat-long locations
> >
> >> Wow, that explains the result I got!
> >> So I guess you need to know exactly how many meters a lat/long cell
> >> dimension is in the current projection, so you can scale the Z axis
> >> accordingly?
> >
> > Yes, thats the point. And i need to know where to start.
> >
> >> Does this vary between cells? Does GRASS have a function to calculate
> >
> > It changes with every raster row.
> >
> >> metric lengths between two points in a lat-long system? If so, I guess
> >> you would need to take the the metric distance between corner points of
> >> a lat-long cell and use the value for Z axis scaling?
> >
> > AFAIK these functions are implemented in grass (lib/gis/area* and
> stuff).
> >
> > The point is, im not very familiar with this kind of coordinate
> transformation, and even if i know the distance between each point,
> > i dont know where to start with 0,0 in metric coordiantes. :/
> >
> > Maybe im a bit dumb?
> >
> > Best regards
> > Soeren
> >
> >> Sören Gebbert wrote:
> >>> -------- Original-Nachricht --------
> >>> Datum: Tue, 06 Mar 2007 16:06:18 +0100
> >>> Von: "Sören Gebbert" <soerengebbert at gmx.de>
> >>> An: Benjamin Ducke <benjamin.ducke at ufg.uni-kiel.de>
> >>> CC:
> >>> Betreff: Re: [GRASS-dev] VTK export for lat-long locations
> >>>
> >>> Hi Benjamin,
> >>> please dont use the vtk export modules with lat/lon projections.
> >>> The vtk export modules in grass are not
> >>> usable with lat/lon projections right now.
> >>> I think i should put this info to the html docs.
> >>>
> >>> The lat/lon coordiantes are used to define the coordinates for each
> >> point.
> >>> VTK interpret these coordinates as planimetric and not as lat/lon.
> >>> This is not that bad for x,y coordinates, but for the height (z
> >> coordinate).
> >>> Because if x and y lat/lon coordiantes are of type [54.001/34.002] and
> >> the
> >>> height varies from 0m - 500m you will get a senseless result.
> >>>
> >>> I still dont know exactly how to implement a meaningful
> >>> lat/lon ->planimetric coordinate transformation to get rid
> >>> fo this behaviour.
> >>>
> >>> Any suggestions are welcome.
> >>>
> >>> Best regards
> >>> Soeren
> >>>
> >>>
> >>> -------- Original-Nachricht --------
> >>> Datum: Tue, 06 Mar 2007 15:57:17 +0100
> >>> Von: Benjamin Ducke <benjamin.ducke at ufg.uni-kiel.de>
> >>> An: GRASS devel <grass-dev at grass.itc.it>
> >>> CC:
> >>> Betreff: [GRASS-dev] VTK export for lat-long locations
> >>>
> >>>> Hi everyone,
> >>>>
> >>>> I just tested r.out.vtk in a lat-long location and all I got when I
> >>>> loaded the VTK file in Paraview (2.6.0), was a single line of data
> >>>> projecting through the origin of the coordinate system.
> >>>>
> >>>> Does anyone have any experiences with VTK/Paraview and lat-long data?
> >>>>
> >>>> Thanks,
> >>>>
> >>>> Benjamin
> >>>>
> >>>>
> >>>> --
> >>>> Benjamin Ducke, M.A.
> >>>> Archäoinformatik
> >>>> (Archaeoinformation Science)
> >>>> Institut für Ur- und Frühgeschichte
> >>>> (Inst. of Prehistoric and Historic Archaeology)
> >>>> Christian-Albrechts-Universität zu Kiel
> >>>> Johanna-Mestorf-Straße 2-6
> >>>> D 24098 Kiel
> >>>> Germany
> >>>>
> >>>> Tel.: ++49 (0)431 880-3378 / -3379
> >>>> Fax : ++49 (0)431 880-7300
> >>>> www.uni-kiel.de/ufg
> >>>>
> >>>> _______________________________________________
> >>>> grass-dev mailing list
> >>>> grass-dev at grass.itc.it
> >>>> http://grass.itc.it/mailman/listinfo/grass-dev
> >> --
> >> Benjamin Ducke, M.A.
> >> Archäoinformatik
> >> (Archaeoinformation Science)
> >> Institut für Ur- und Frühgeschichte
> >> (Inst. of Prehistoric and Historic Archaeology)
> >> Christian-Albrechts-Universität zu Kiel
> >> Johanna-Mestorf-Straße 2-6
> >> D 24098 Kiel
> >> Germany
> >>
> >> Tel.: ++49 (0)431 880-3378 / -3379
> >> Fax : ++49 (0)431 880-7300
> >> www.uni-kiel.de/ufg
> >>
> >> _______________________________________________
> >> grass-dev mailing list
> >> grass-dev at grass.itc.it
> >> http://grass.itc.it/mailman/listinfo/grass-dev
> >
>
> --
> Benjamin Ducke, M.A.
> Archäoinformatik
> (Archaeoinformation Science)
> Institut für Ur- und Frühgeschichte
> (Inst. of Prehistoric and Historic Archaeology)
> Christian-Albrechts-Universität zu Kiel
> Johanna-Mestorf-Straße 2-6
> D 24098 Kiel
> Germany
>
> Tel.: ++49 (0)431 880-3378 / -3379
> Fax : ++49 (0)431 880-7300
> www.uni-kiel.de/ufg
>
> _______________________________________________
> grass-dev mailing list
> grass-dev at grass.itc.it
> http://grass.itc.it/mailman/listinfo/grass-dev
--
"Feel free" - 10 GB Mailbox, 100 FreeSMS/Monat ...
Jetzt GMX TopMail testen: www.gmx.net/de/go/mailfooter/topmail-out
More information about the grass-dev
mailing list