# map projections vs. coordinate systems

Admin. scarab J.M. ROQUES 5322 scarab at cisi.cnes.fr
Fri Jul 7 08:00:00 EDT 1995

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Hi Sharif,

Jerry couldn't have said it better but just for fun, before i grab my surfboard
and run to the ocean on holidays...
There is no need to be puzzled:
When you are on our planet you are somewhere in space say x,y,z in the geocentr
ic frame located in the earth center and rotating with it.
Then, as you are on the earth surface you are near the surface of some ellipsoi
d (several are used...) as close as possible as our potato looking like dear ol
d planet; Then you need a first non trivial transform from x, y, z to compute y
our lat, long height position which is a new system of coordinates.
why not use lat long as a coordinate system on a sheet? you can if you want to.
.. but you will have some metric definition problem to compute distances...
Though now, from lat ,long you make a second transform to a new analytical X,Y
coordinate system best suited for map representation, with a metric tensor near
ly euclidian what helps for distance computation. Notice you forgot the height
term in between...
This second transform toward the third coordinate system is what is refered to
geographic projection (no flames if it is not quite true...).
People very often use the same terms to designate these two thinks, why bother?
it is just a shortcut isnt'it?
If i say this point is x1, x2 in lambert III projection, (one of the french coo
rdinate systems in use with an official definition), every body will understand
that i am refering to a point which can be located on the earth surface using
the coordinate system defined by the lambert III projection, and it is much sho
rter :-).
Ok, then, have a nice summer

JM Roques

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