[GRASS-user] Question about units and points of reference
Glynn Clements
glynn at gclements.plus.com
Wed May 23 19:35:44 EDT 2007
Wayde Allen wrote:
> > I am using 6.2.1, and g.region -p gives the info:
> >
> > projection: 1 (UTM)
> > zone: 10
> > datum: nad83
> > ellipsoid: grs80
> > north: 4269999
> > south: 4180119
> > west: 598614
> > east: 651978
> > nsres: 3
> > ewres: 3
> > rows: 29960
> > cols: 17788
> > cells: 532928480
>
> I've very new to all of this, so please forgive what may be a very obvious
> question, but can someone explain the units for these coordinates? I'm
> thinking that "north: 4269999" must indicate the distance North of some
> reference location, perhaps the equator? It this in feet, meters, miles,
> kilometers, or something else entirely?
In most cases, coordinates are in metres. Some coordinate systems use
feet (normally US survey feet; any countries which historically used
Imperial feet have long since gone metric for cartographic purposes),
but UTM is always in metres.
Sometimes the zero value is meaningful (e.g. a specific parallel or
meridian), sometimes it isn't. Many coordinate systems add a fixed
constant (false easting/northing) to ensure that coordinates are
always positive.
For UTM, the "base" origin is the intersection of the zone's central
meridian (127W for zone 10) with the equator, but a false easting of
500,000m is added. For the southern hemisphere a false northing of
10,000,000m is added.
So, in the above case, the north-west corner (4269999, 598614) is
~4,270 km north of the equator, and ~98.6 km east of the 127W
meridian.
However, the process of mapping an ellipsoid to a plane introduces
distortion, so the coordinates don't exactly correspond to distances.
E.g. the transverse Mercator projection exaggerates north-south
distances as you move further from the central meridian. To
compensate, a scale factor of 0.9996 is applied to northings, so that
the overall scale factor varies from 0.9996 at the central meridian to
~1.0010 at the boundaries.
--
Glynn Clements <glynn at gclements.plus.com>
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