[GRASS-user] 1 degree = 1 km*1000 ?

[Hamish]

achim wrote:
> Its in grass 6.5:
> v.to.db map=rivers at achim option=length units=d columns=a_lenght
> same as
> v.to.db map=rivers at achim option=length units=degrees columns=a_lenght
> 
> (there is no degree-option in grass 6.4; I am interested in
> length relative to an degree-raster)

a degree is a unit of angle, not a unit of length. you may consider that
a degree latitude has a certain number of meters in it (~1852*60) depending
on the radius of the sphere, and perhaps modify that if you are measuring
on an ellipsoid not a true sphere- but then it is only good for measuring
exactly in the north-south direction, as scale in east-west varies with
the cos(lat).

For v.to.db, radian and degree units are only useful with option=azimuth.
For option=anything_else it should probably exit with an error.
Also option=azimuth should potentially refuse to work if the location is
LL. (in a geodesic/great circle line there is a departure angle and an
arrival angle, but between those it necessarily slowly changes)


Hamish