[ gpsdrive ] Map projection

[Ryan J. Earley]

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.

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

hope that helps,
-ryan