[GRASS5] Lat conventions - r.proj

[Rich Shepard]

On Mon, 1 Jul 2002, Alessandro Frigeri wrote:

> I've found different definitions of latitude on different books, it seems
> to me that the problem is not the term 'latitude' but the extra term
> geocentric or geodetic that often is not specified in books and various
> documents.
 
> For a sphere two equal latitudes point to the same place but not in the
> case of ellipsoid.
> 
> Since at the moment I'm studying reference systems in planets, I wonder
> what lat convention is used in a GRASS lat/lon location?

Alessandro,

  Not a stupid question at all. The terms "datum", "projection" and
"coordinate system" are easily confused; to a large extent because we are
sloppy in our use of language when speaking (and writing) of them.

  I cannot address this issue for planets other than Earth becuse I've
neither mapped nor lived on them. However, on Earth, latitude and longitude
are coordinates. That is, they define a specific point on the Earth's
surface as measured from the equator and the Prime Meridian. Projection
systems are mathematical transforms that let us display the
three-dimensional Earth on a flat surface. Each projection is based on a
particular datum that defines the origins and takes into account a
mathematical description of the Earth's shape. The coordinate system of each
projection can be in metres, feet, rods, chains or other units of linear
measurement. Latitude and longitude ignore all this. The measurement units
are degrees, minutes and seconds (or decimal degrees) and are independent of
the datum or projection.

  Most spatial analyses are based on projections and coordinate systems
using feet or metres. Data in lat/lon format is generally converted before
use. However, it's a "lowest common denominator" kind of measurement easily
understood by the non-technical public and is widely used.

  FWIW, the inexactness of the Earth's representation by various ellipsoids
described by mathematical models is why elevation determinations by GPS are
2-3 times less accurate than are the horizontal position determinations at
the same spot. So, while my sub-survey grade GPS receiver provides +/- 30 cm
accuracy (differentially corrected) for northing and easting, it provides
only +/- 60-90 cm vertical accuracy at the same spot.

HTH,

Rich 

Dr. Richard B. Shepard, President

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