[pct-l] computer/droid help

[lancem at wvi.com]

Scott,

>From my little bit of experience I can share this,

For a map image to be used by GPS software, calibration data must accompany
the image so the software knows the latitudes and longitudes represented on
the map.

Calibration data for raster images (jpg, bmp, png, tif, and others) assigns
specific latitudes and longitudes to a few spread out pixels in the image. (A
digital camera generates jpg images.)

The GPS software uses mathematical algorithms to interpolate the lat/lon of
all the other pixels.  This is how the software is able to place waypoints and
display moving maps.

Another issue is file size.  Often GPS software will chop images into smaller
pieces (tiling) for better memory management.  For instance, a small map of
1275x1650 pixels may be tiled into 300x400 pieces.  Halfmile’s maps are about
this size.  A 7-1/2minute USGS topo might be 3200x4050 pixels.

I assume all this applies to vector image files as well, but takes different
software.

The BackCountry Navigator home page article "Using Calibrated Digital Maps for
Android Navigation" describes a way to calibrate and use your own map images.
 It looks like you would have to convert Halfmile’s maps from pdf to another
format such as jpg or bmp then calibrate and tile each map using MAPC2MAPC.


I've converted, calibrated, and tiled some of Halfmile’s maps for a Blackberry
GPS application.  I used Omniformat to convert the pdf files to bmp files, Oziexplorer
to calibrate and TBcutter to tile the images.  It takes 10-15 minutes per map
after a bit of practice.  To do all of Halfmile's maps would take days.

An easier solution is to simply load Halfmile's gpx waypoint files to your droid
and carry paper copies of the map images.  If you need to know exactly where
you are or where to go, many GPS apps can use the gpx files to tell you how
far and in what direction it is to the nearest waypoint.  Then look at your
map for that waypoint and follow your compass.  

-Lance 





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