[GRASS-user] (kein Betreff)

[Hamish]

Nikos Alexandris wrote:
> Just another piece of information (although don't remember where I
> read it): In general every 0.0001° of latitude equals ~ 11m.

1852m in a nautical mile. 60 nautical miles in a degree latitude.
so 1 degree latitude = 111.12km

> Longitude I think is the tough one... ;-)

dist*cos(lat)


Jerry:
> > I'm curious about the statement that "Lat-Long is not good to do
> > distance measurements" Someone else made a similar observation in
> > a different conversation recently too. I'm not a geographer so
> > I'm probably missing something but doesn't lat long just give you
> > a point on the surface of the earth and if you have two of these
> > don't you more or less automatically know the distance between
> > them?

For distance measurements in Lat/Lon GRASS uses the
G_geodesic_distance() function found in lib/gis/geodist.c
to calculate the great-circle distance.

This is widely used by the raster modules and libs (which are generally
very mature [sometimes to the point of geriatrics]) as well as the
display modules (d.measure, d.what.vect, ...)

The vector and DB modules are brand new for GRASS 6 and so many/most
vector modules only consider Cartesian space when doing there
calculations. These will be slowly updated, but it takes time.


So modules like r.buffer will make correct pear shaped circles in a lat
lon location, but v.buffer will just make an incorrect lat=lon even at
high latitudes circle.


> > Lat-Long is not good to do distance measurements!
> > 
> > Why don't you reproject your lines in a "metric" projection system
> > and check the distances again.

If working with vector modules this is probably your best bet, work in
a projected location then reproject the results back.


for v.to.points, dmax= dist should be made to work in meters for
lat/lon locations and calculate with G_geodesic_distance() as it makes
its way between verticies. (e.g. the d.what.vect Tcl popup window does
this for line length, AFAICT)   Feel free to file wish/bug reports as
needed.


Hamish



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