[mkgmap-dev] Re: [PATCH] Creating contour lines from DEM data

Sat Jul 4 12:26:22 BST 2009

Am 04.07.2009 um 10:04 schrieb Toby Speight:

> P.S. it would be interesting to see contour regions (as opposed to  
> lines)
> suitable for elevation shading layers; another idea is to compute  
> slope
> aspect for a hill shading layer - maybe I'll look into that when I get
> time...

Actually I've done that already using octave (matrix calculation  
application, among other uses). I can share the scripts if you like.  
The problem there is not to generate the data, but to display it in a  
usable way on the GPS unit. The DEM maps with elevation and hill  
shading look "Wow" before you try to convert them into an .IMG file.  
Then they look like crap, because you have nowhere near enough  
different layers available.

There is an article about how to create a bigger number of shadings by  
dithering: http://www.cgpsmapper.com/download/Creating%20custom%20types%20to%20represent%20elevation%20data.pdf

But even with this method I think the resulting maps look crap,  
especially if you look at the maps that the GPS units shows if a DEM  
model for the map is available.

So, anybody interested in figuring out the Garmin DEM file format? I  
would assume that it might not be too complicated if the DEM data is  
stored in a sampled format like the SRMT data (hopefully not  
compressed). This could have the added benefit that you might also be  
able to view the elevation data and use the 3D display mode on the GPS  
unit.

Regards
Thilo