[ gpsdrive ] Map projection

[Petra Galuscakova]

On 8/4/07, Ryan J. Earley <ryan.j.earley at gmail.com> wrote:
> On 8/2/07, Petra Galuscakova <galuscakova at gmail.com> wrote:
> > Hello,
> > I would like to ask you, in which projection  the maps that are
> > displayed in Gpsdrive are. I`ve found, that it`s Miller projection.
> > Could you confirm that?
> >
> > I would like to use gdalwarp software to translate my map into the
> > projection which Gpsdrive maps uses, but I`m not sure how to do it. My
> > map is in S-JTSK projection (ESRI 102067).
> >  I have tried to translate it into Miller projection (EPSG 53002).
> > To do that I have used next:  gdalwarp -s_srs "+init=esri:102067"
> > -t_srs "+init=epsg:53002" map_file0025g.gif map_file0025t.gif
> > I`ve compared this map with expedia map from the same area and the
> > same scale (downloaded by Gpsdrive) and I found some differences.
> > Am I doing something wrong or are there possible some differences
> > between maps?  Am I using appropriate epsg number?
>
> As long as gdal understands your projection, you can use gdalwarp to
> go from your projection to WGS84/LAT-LON (EPSG:4326).  Then you should
> be able to treat the resulting maps as "top_" files in gpsdrive.
>
> Example: i recently had a dataset in NJ State Plane i needed in
> gpsdrive.  I used the following with  success:
> gdalwarp -s_srs "+init=nad83:2900 +datum=NAD83 +units=us-ft \
> +proj=tmerc +lon_0=-74d30 +lat_0=38d50 +k=.9999 +x_0=150000 +y_0=0 \
> +no_defs +ellps=GRS80 +towgs84=0,0,0" -t_srs "+init=epsg:4326 \
> +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs +towgs84=0,0,0" \
> INFILE.geotif     OUTFILE.latlon.gtiff
>
> if you replace the -s_srs parameters (inside the quotes) with your
> map's parameters, this conversion should "just work" and will give you
> a gpsdrive "top" file.

Thank you very much,
it`s exactly what was I thinking about.
I will try it tommorow morning.

>
> gpsdrive has 2 (so far?) projections -- "map" and "top".  here is one
> explanation:
>
> <Cut-and-paste from gdal_slice.sh
> http://hamish.bowman.googlepages.com/gdal_slice.sh >
> # Important! The maps must be named "map_*" for UTM-like projections
> # (lat:lon = 1:cos(lat)) and "top_*" for lat/lon Plate carree projection
> # (lat:lon = 1:1). The prefix is given so that gpsdrive knows how to
> # scale the maps correctly. Alternatively the maps can be stored without
> # prefix in subdirectories of $HOME/.gpsdrive/ which end in "_map" or
> # "_top".
> </Cut-and-paste>
>
> If you original map is not 1280x1024px, you can use Hamish's
> gdal_slice.sh, a geotiff-->gpsdrive tiler, to generate the gpsdrive
> tiles and map_koord entries quickly.
>
> it is available here:
> http://hamish.bowman.googlepages.com/gpsdrivefiles

I`ve got original map 1280x1024px, but I didn`t know about this utility.
It looks useful.

>
> hope that helps,
> -ryan

Yes, thank you,
Petra

> _______________________________________________
> GPSdrive mailing list
> GPSdrive at lists.gpsdrivers.org
> http://lists.gpsdrivers.org/mailman/listinfo/gpsdrive
>