[ gpsdrive ] Map size limitation and map scale calculation

[Joerg Ostertag (gpsdrive Munich/Germany)]

On Mittwoch 24 Oktober 2007, Paweł Bednarski wrote:
> Let me say first that I'm totally green in terms of geodesy and
> cartography but I would like to use gpsdrive with self-made (in terms
> of gluing and calculating parameters) maps. So some questions arise.
>
> 1) Why there is a hardcoded 1280x1024 limitation for map size?
> Or there isn't? Could I for example put map in 2000x2000 size? (my
> tiles are 200x200 in size so easiest way to create map from them
> is glueing them together and there comes gpsdrive limitation I've
> read in man pages about)

The 1280x1024 size limit is historically. The problem is; it's hardcoded 
scattered all over the gpsdrive Code. Maybe we will solve this sometime in 
the future. But until the you'll have to create 1280x1024 images

> 2) What is definition of map scale in gpsdrive? How to calculate one?
> Digital raster maps have scale in for example meters/inches/foots per
> pixel. How is this scale interconnected with scale written in
> map_koord.txt? I obtained (for my personal use) raster map of Poland
> in scales 2, 4, 8, 16, 32 meters per pixel. Map is georefereced in
> '1992' coordinate system (EPSG2180 number for reference). I can
> recalculate it into WGS84 so there is no problem if transformation is
> linear... But even that I'm able to calculate lat/lon parameters of
> the center of the map I've created I can't calculate scale...

I can't exactly remember the formulas. But there was a post some time ago 
about how to convert the coordinates and zoom levels. 
Alternatively you might want to either:
	have a look in the source code of gpsfetchmap.pl or map-projection.c
	or try to rectify and get the scale of your maps by using the 
		Menue: Option-Maps-Import

I hope these hints helped at least a little bit.

-- 

Jörg (Germany, Munich)

http://www.ostertag.name/
Tel.: +49 89 420950304
Skype: JoergOstertag