[GRASS-dev] Re: z-exag in wxnviz

Hamish hamish_b at yahoo.com
Sat Nov 26 01:14:16 EST 2011

Michael wrote:
> What needs to be done IMHO is to figure out what a 1:1
> horizontal/vertical ratio should be for any map and set that
> to 1 in the z-exag slider. BUT, we need to decide what a 1:1
> ratio really is. This is easy for a projected DEM. Using
> grass libraries, it should be pretty easy for a latlon DEM
> too.

exactly 1852m in a nautical mile.
exactly 60 nautical miles in a degree latitude.

that gives a basic meter to degree conversion for the map, BUT
it is not guaranteed that the DEM data is in meters, not feet
or anything else. r.support/r.info units= was added to record
this, but it is a freeform field so you can make it micro-Einsteins
per m^2 per sec, \mu and all, or whatever, so can only be used
as a hint if it is there and matches a known list.

tcltk nviz has a hack to support that, but it isn't fully
implemented, e.g. the lighting is still quite wacky for that.

theoretically for regions smaller than say 3 degrees in EW extent
lat/lon we might look at skewing the aspect ratio for cells by
1:cos(lat_center) to reduce the distortion. very crude but fast
and passable as a first order correction for something which
is for visible purposes only. or just display an error message
saying you should try to use a projected location instead.


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