### SLA_DTP2S

Tangent Plane to Spherical

ACTION:
Transform tangent plane coordinates into spherical coordinates (double precision)
CALL:
CALL sla_DTP2S (XI, ETA, RAZ, DECZ, RA, DEC)
##### GIVEN:
 XI,ETA D tangent plane rectangular coordinates (radians) RAZ,DECZ D spherical coordinates of tangent point (radians)

##### RETURNED:
The projection is called the gnomonic projection; the Cartesian coordinates $\left[\phantom{\rule{0.3em}{0ex}}\xi ,\eta \phantom{\rule{0.3em}{0ex}}\right]$ are called standard coordinates. The latter are in units of the distance from the tangent plane to the projection point, i.e. radians near the origin.
When working in $\left[\phantom{\rule{0.3em}{0ex}}x,y,z\phantom{\rule{0.3em}{0ex}}\right]$ rather than spherical coordinates, the equivalent Cartesian routine sla_DTP2V is available.