NAME:
LL_NARCS_DISTANCES
PURPOSE:
This function returns the longitude and latitude [lon, lat] of
a point a given arc distance (-pi <= Arc_Dist <= pi), and azimuth (Az),
from a specified location Lon0, lat0.
Same as LL_ARC_DISTANCE but for n points without do loop.
CATEGORY:
Mapping, geography.
CALLING SEQUENCE:
Result = LL_NARCS_DISTANCES(Lon, lat0, Arc_Dist, Az)
INPUTS:
Lon0: An array containing the longitude of the starting point.
Values are assumed to be in radians unless the keyword
DEGREES is set.
Lat0: An array containing the latitude of the starting point.
Values are assumed to be in radians unless the keyword
DEGREES is set.
Arc_Dist: The arc distance from Lon_lat0. The value must be between
-!PI and +!PI. To express distances in arc units, divide
by the radius of the globe expressed in the original units.
For example, if the radius of the earth is 6371 km, divide
the distance in km by 6371 to obtain the arc distance.
Az: The azimuth from Lon_lat0. The value is assumed to be in
radians unless the keyword DEGREES is set.
KEYWORD PARAMETERS:
DEGREES: Set this keyword to express all measurements and
results in degrees.
OUTPUTS:
This function returns a (2, n) array containing the
longitude / latitude of the resultings points. Values are in radians
unless the keyword DEGREES is set.
PROCEDURE:
Formula from Map Projections - a working manual. USGS paper
1395. Equations (5-5) and (5-6).
EXAMPLE:
Lon_lat0 = [1.0, 2.0] ; Initial point specified in radians
Arc_Dist = 2.0 ; Arc distance in radians
Az = 1.0 ; Azimuth in radians
Result = LL_ARC_DISTANCE(Lon_lat0, Arc_Dist, Az)
PRINT, Result
2.91415 -0.622234
IDL> lon0 = [-10, 20, 100]
IDL> lat0 = [0, -10, 45]
IDL> lon1 = [10, 60, 280]
IDL> lat1 = [0, 10, 45]
IDL> dist = map_npoints(lon0, lat0, lon1, lat1, azimuth = azi, /two_by_two)
IDL> earthradius = 6378206.4d0
IDL> res = ll_narcs_distances(lon0, lat0, dist/earthradius, azi, /degrees)
IDL> print, reform(res[0, *])
10.000000 60.000000 280.00000
IDL> print, reform(res[1, *])
1.1999280e-15 10.000000 45.000000
MODIFICATION HISTORY:
Based on the IDL function ll_arc_distance.pro,v 1.11 2003/02/03