### SLA_DPAV

Position-Angle Between Two Directions

ACTION:
Returns the bearing (position angle) of one celestial direction with respect to another (double precision).
CALL:
D = sla_DPAV (V1, V2)
##### GIVEN:
 V1 D(3) vector to one point V2 D(3) vector to the other point

##### RETURNED:
 sla_DPAV D position-angle of 2nd point with respect to 1st

NOTES:
(1)
The coordinate frames correspond to $\left[\phantom{\rule{0.3em}{0ex}}\alpha ,\delta \phantom{\rule{0.3em}{0ex}}\right]$, $\left[\lambda ,\varphi \right]$ etc..
(2)
The result is the bearing (position angle), in radians, of point V2 as seen from point V1. It is in the range $±\pi$. The sense is such that if V2 is a small distance due east of V1 the result is about $+\pi /2$. Zero is returned if the two points are coincident.
(3)
There is no requirement for either vector to be of unit length.
(4)
The routine sla_DBEAR performs an equivalent function except that the points are specified in the form of spherical coordinates.