FAQ: WHAT ARE MAP DATUMS AND PROJECTIONS?

                 "NON-TECHNICAL ANSWER"
                      (14 June 1999)

Different datums are based on different mathematical models of the
earth's shape and dimensions (ELLIPSOIDS) plus an additional factor of 
PROJECTION. In Japan, say, they used a projection point that isn't at 
the center of the earth, but somwhere under Japan.  This gives less 
distortion of projecting a sphere onto a "flat map" there, but using 
that projection for the US would result in a very wierd map!

Also some projections use diferent ideas of what a "flat map" IS.  
(I define a "flat map" as something that can be "unrolled" and laid 
on a table).  One of these is a CONE intersecting the earth at two
latitudes with the "point" above the pole.  Another is a CYLINDER
touching the earth at some latitude or longitude.  The famous
Mercator projection is a cylinder touching the earth at the equator
(LATITUDE 0 0' 0").

The UTM (Universal Transverse Mercator) PROJECTION touches the earth 
at various LONGITUDES called Central Meridans and uses a projection 
point at the center of the earth.  The mathematical model (datum) 
is WGS-84 which defines an ellipsoid.  The WGS-84 datum is a major 
departure from the Clarke 1866 datum used by most USGS maps.  WGS-84 
(and the virtually identical NAD-83) datums specify that the earth 
"bulges" more, so that a measure of the number of meters from the 
equator northward (northing) is some 200m greater than that measured 
with the Clarke 1866 model for points in the US. 

Most USGS maps in the US are NAD-27 CONUS datum which uses the Clarke
1866 mathematical model and a Polyconic (cone) projection. Later maps 
are NAD-83 datum and use the UTM projection from the center of the 
earth.  This projection from the center of the earth gives rise to 
the "Universal" part of UTM.

For a more detailed discussion of the UTM system, see: 
      http://gpsinformation.net/main/utm-faq

CAUTION:  The true shape of the earth is NOT any of the above ellipsoids!
From monitoring satellite orbits for several years, it has been 
determined that the earth is quite "lumpy" due to mass concentrations.

Therefore, elevation models of the earth are published every few years.
These are tables of the elevation "above-the-WGS-84-ellipsoid" sea level 
would be, if sea water could flow to anywhere on the earth.  (It's 
interesting that even at the coasts, this "elevation-model" table doesn't 
exactly agree with actual mean sea level due to the prevaling winds piling 
up (or lowering) the water.)

Elevation-model tables are stored in some GPS units (Garmin for one) in 
order to display true elevation.  The amount of the difference in your 
elevation above the WGS-84 ellipsoid and your actual elevation at your 
loaction is shown in the last item of NMEA sentence $GPGGA and is about 
30m in Atlanta.

--

Jack Yeazel