NAME

       d.out.gpsdrive  - Export display monitor to a GpsDrive compatible backdrop image

KEYWORDS

       display, export, GPS

SYNOPSIS

       d.out.gpsdrive
       d.out.gpsdrive help
       d.out.gpsdrive [-j] output=string  [--verbose]  [--quiet]

   Flags:
       -j
           Make JPEG instead of PNG image

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       output=string
           name for new map image (lives in ~/.gpsdrive/maps/)

DESCRIPTION

       d.out.gpsdrive exports the current GRASS display monitor to a GpsDrive compatible backdrop
       image  and  updates  the  GpsDrive  image  list  with  the   appropriate   geo-referencing
       information.

       Use  at  your  own risk. Do not use as a primary means of navigation.  This software comes
       with absolutely no warranty.

NOTES

       Maps are saved in the user's ~/.gpsdrive/maps/ directory.  Geo-referencing information  is
       added to the user's ~/.gpsdrive/maps/map_koord.txt file.

       JPEG output requires the pngtopnm and pnmtojpeg programs from the NetPBM tools.

       GpsDrive  assumes northings are not rotated compared to true-geographic north.  If you are
       using a projection with significant curvature away from the central  meridian,  or  a  map
       datum  with  a  significant  rotational  component,  then  you  will  likely end up with a
       distorted or inexact background map!  Keeping the area small will lessen  the  error,  but
       not  eliminate it, if necessary you could reproject the map into a custom projection (such
       as tmerc) centered directly on the center of your map. You can check the local convergence
       angle (difference between grid north and true north) with "g.region -n".

       To avoid distortion, anything more global than 1:150k to 1:500k should come from a lat/lon
       location. Anything more local than that will look better coming from  a  projected  system
       such as UTM.

       The    extent    of    a   map   given   a   target   scale   can   be   calculated   with
       x_ext=scale*(1280/2817.95). e.g. 1:50,000 translates to approx a  22.7km  x  18.2km  image
       window in the GIS.

       For your convenience (calculations are rough, but nominal):
       1:50,000 uses a region 22.7km x 18.2km.
       1:75,000 uses a region 34.1km x 27.3km.
       1:100,000 uses a region  45.4km x 36.3km.
       1:175,000 uses a region  79.5km x 63.6km.

       Maps  exported  from  lat-lon locations will be given a "top_" prefix.  Maps exported from
       locations of other projections will be given a "map_" prefix. This  is  done  so  GpsDrive
       knows how to scale the image correctly.

       GpsDrive  requires backdrop images to be 1280x1024 pixels in size. While this script takes
       care of that automatically, to avoid annoying bands on the sides of  your  image  you  may
       want  to  set  up your display monitor at half-scale (same aspect ratio) and use d.zoom to
       select the full frame. For example:

       export GRASS_WIDTH=640
       export GRASS_HEIGHT=512
       d.mon x0

       Map scaling is set from the region  settings  so  should  work  correctly  even  when  the
       display's aspect ratio does not match that of the output image.

   Batch export
       It  may be desirable to create a series of image tiles covering a large area.  An easy way
       to do this is to run d.out.gpsdrive in a shell loop.   Here  is  an  example  Bash  script
       contributed by Manuel Morales:

       #!/bin/bash
       # map scale is determined by "panels" by dividing the N-S
       # region extent into that number of maps. Note that the
       # generated maps overlap by 1/2 along the N-S axis and by
       # approximately 1/2 along the E-W axis.
       panels=3
       iter=$((panels*2-1))
       eval `g.region -eg`
       eval `g.region -g`
       north=$n
       south=$s
       west=$w
       east=$e
       unit_ns=$(echo "scale=8; $ns_extent / $panels" | bc)
       unit_ew=$(echo "scale=8; $unit_ns * 4/3" | bc)
       panels_ew=$(echo "((2*$ew_extent / $unit_ew +.5 ) -1) / 1" | bc)
       for i in `seq 1 $panels_ew`;
       do
         east=$(echo "scale=8; $west+$unit_ew" | bc)
         for j in `seq 1 $iter`;
           do
           g.region n=$(echo "scale=8; $north-($j-1) * $unit_ns/2" | bc)      \
               s=$(echo "scale=8; $north-($j-1) * $unit_ns/2-$unit_ns" | bc ) \
               w=$west e=$east ewres=$ewres nsres=$nsres
               d.redraw
               d.out.gpsdrive -j tile_${i}_${j}
           done
         shift_west=$(echo "scale=8; ($ew_extent-$unit_ew)/($panels_ew-1)" | bc)
         west=$(echo "scale=8; $west+$shift_west" | bc)
       done

       Note  that  to  get  a  smoother  transition  between  backdrop  maps  this script creates
       overlapping tiles. For best results at least 1/3rd overlap should be maintained.

SEE ALSO

       d.info, d.grid, d.out.file, d.out.png, d.save, g.region, v.in.garmin
       The GRASS PNG driver
       The GpsDrive project
       The gpsd personal GPS server project

AUTHOR

       Hamish Bowman

       Department of Marine Science
       University of Otago
       New Zealand

       Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $

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