purpose of map datum?

Hi!

What is the purpose of using sphere, ellipsoid or geoid shapes, globally
or locally optimized, in coordinate systems? Ok, I understand it affects
height. But how does it affect latitude and longitude? Can we not simply
pick Earths center of mass as origin of the coordinate system, the
equator as 0 latitude and Greenwich as 0 longitude? Then when GPS or
whatever determines lat/lon a ray starting at the center of Earth will
point to these coordinates and pierce through earths surface at some
point and that is then at these coordinates. Sounds simple, precise and
unambigious to me. Why complicate matters with WGS84, UTM etc?

Bernhard

WGS84 is a datum. UTM is a coordinate format, just like D M.MM or D M' S".
NAD27 is a map datum, normally displayed in UTM format.

In the simplest terms If you and I started mapping the same area from
different locations. I laid out my coordinates in one meter measurements
from my position. You laid out measurements in one foot increments from
your position. We would have to know whose datum was used on the map we
were looking at to pinpoint a coordinate location.

Different organizations created different map Datums.

Always use the datum that the map uses.

wrote:



Also note that the various tectonic plates on this earth are moving -
see, for example, the diagram in the Summary section of
http://en.wikipedia.org/wiki/Plate_tectonics

This shows Australia moving NNE at about 7 cms per year. So in 100
years, Australia will have moved 7 metres. Thus tectonic plate datums
vary with time with respect to any global datum. Therefore it is
convenient to define datums according to a particular epoch e.g.
"GDA94" see
http://www.geoproject.com.au/gda.faq.html

Alan Murphy

On 04/02/2011 15:45, Bernhard Kuemel wrote:

That would be OK if the Earth was a uniform spheroid, but it isn't! Any
datum is an approximation and a 'best fit' for the area covered.
WGS84 gives a best fit for the whole earth, other datums give a better
fit over smaller areas.

Jeff

On 02/04/2011 06:15 PM, Jeff wrote:

What's the difference? Don't they both have the origin in the center of
mass of Earth? Isn't in both the angle between a location, the origin
and the equatorial plane the latitude? And Longitude the angle between
the horizontal projection of the location onto the equatorial plane, the
origin and the 0 meridian?

Is the difference just in elevation above the reference shape (sphere,
ellipsoid, geoid)? I thought the coordinates would differ, too.

I read about UTM now. That's a wholly different concept, of course.

Bernhard