An ellipsoid is a mathematically defined approximation of the earth's surface.
An ellipsoid is defined by two parameters:
- the semi-major axis a (equatorial radius)
- the semi-minor axis b (polar radius)
a and b together define the flattening of the ellipsoid f:
f = (a-b) / a
NOTE: phpgeo's ellipsoids are defined by a and 1/f instead of a and b. That's not a problem because each of the three values can be calculated from the other two.
phpgeo supports arbitrary ellipsoids. WGS-84 is used as default when no other ellipsoid is given. For day-to-day calculations it's not needed to care about ellipsoids in the most cases.
It's possible to create an instance of the Ellipsoid class either by specifing a name or by providing the three parameters name, a, and 1/f.
use Location\Ellipsoid; $ellipsoid = Ellipsoid::createDefault('WGS-84'); printf( "%s: a=%f; b=%f; 1/f=%f\n", $ellipsoid->getName(), $ellipsoid->getA(), $ellipsoid->getB(), $ellipsoid->getF() ); $ellipsoid = new Ellipsoid('GRS-80', 6378137, 298.257222); printf( "%s: a=%f; b=%f; 1/f=%f\n", $ellipsoid->getName(), $ellipsoid->getA(), $ellipsoid->getB(), $ellipsoid->getF() );
The first ellipsoid is created from one the the default configurations. The second one is created by providing a name and the values of a and 1/f.
The code above will produce the output below:
WGS-84: a=6378137.000000; b=6356752.314245; 1/f=298.257224 GRS-80: a=6378137.000000; b=6356752.314133; 1/f=298.257222
Please take a look into the
Ellipsoid source file
for a list of pre-defined ellipsoids.