nvidia a écrit :
> (...snip...) How do these codes like EPSG:4326 relate to the projection of a
> shapefile?
In order to position a geographic coordinate on the Earth, we need at least an
approximation of the Earth shape. This approximation is the ellipsoid. There is
many approximations in use, because we got more accuracy as technology
progress,
and because different approximations are used by different country.
The ellipsoid is made of semi-major and semi-minor axis length (or an inverse
flattening). So there is at least two numbers that we need here, and those
numbers are in the EPSG database. How the ellipsoid is attached to the Earth
may
also vary for different country - this information is determined by the datum.
The relationship between different datum is also stored in the database
(typically as Bursa-Wolf parameters, but this is not the only way).
In the case of projected CRS, we need other numerical informations like the
standard parallel, central meridian, latitude of origin, scale factor, false
easting, false northing, etc. Again this is all numbers that are stored in the
EPSG database and are required for projections.
Projections like "Mercator" are just formulas. If we ask for a Mercator
projection, actually it doesn't mean much. We need to said what are the values
for the above-cited parameters. There is only a few tens of projections (I
means
a few tens of different formulas), but thousands of projected CRS. They are
always the same tens projection formulas, but applied with different parameters
(different semi-major/semi-minor axis length, different standard parallels,
etc.).
Martin
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