Use at your own risk. Do not use for navigation. This software comes with absolutely no warranty.
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.
Best output is created from a Lat/Lon location or by keeping the output area small (1:50,000). This translates to approx a 22.7km x 18.2km image window in the GIS. [x_ext=scale*(1280/2817.95)]
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.
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
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 are some ideas to get you started: (contribution of full code welcome!)
#get region width and height (in map units)
eval `g.region -ge`
# shift the region by that amount
g.region e=e+$ew_extent w=w+$ew_extent
d.out.gpsdrive tile_${i}_${j}
i=`expr $i + 1`
etc.
ew_extent2=`echo $ew_extent | awk '{printf("%f", $1 / 2. )}'`
g.region e=e+$ew_extent2 w=w+$ew_extent2
Last changed: $Date$
