$Id: earth_boundary.pro 7092 2010-01-12 20:18:45Z jimm $ NAME: EARTH_BOUNDARY PURPOSE: Calculate spherical coordinates of a parameterized the earth boundary curve projected onto a viewing sphere centered at position V and with North the north pole (+z) of the sphere. The input coordinate system is assumed to be an orthogonal earth centered system. CATEGORY: Magenetosphere imaging CALLING SEQUENCE: Result = EARTH_BOUNDARY,V Result = EARTH_BOUNDARY,V,North INPUTS: V - Center of viewing sphere. A 3 element vector in an earth-centered coordinate frame and in units of earth radii. OPTIONAL INPUTS: North - +z direction of the spherical coordinates, i.e. the north pole of the viewing sphere. See the comments in view_coord.pro for a description of this system. In standard coordinates. Defaults to [0., 0., 1.]. KEYWORD PARAMETERS: NPOINTS - Number of points in parameterization. Defaults to 100. PRIME_MERIDIAN - A 3 element vector that lies in the plane of the sphere pole and the zero reference angle of the sphere equator (the +x axis). This has nothing to do with the prime meridian of the earth; it is just the most convenient description. DEGREES - If set then return angle values are in DEGREES. EARTH - The spherical coordinates of the center of the earth. HELP - Provide help (this information). No other action is performed. To invoke, dummy=earth_boundary(/help). OUTPUTS: Result - The spherical coordinates for the parameterized earth boundary in the reference system given by North. Result is a 3xN array. The columns are radius, angle from pole, and equatorial angle (rho, theta, phi). The theta, phi coordinates can be equated to lattitude, longitude coordinates for use with IDL mapping procedures. All angles are in radians unless the DEGREES keyword is set. PROCEDURE: Calculate spherical coordinates of a the earth boundary projected onto a viewing sphere centered at position v. The spherical coordinate system is specified in view_coord.pro which is based on an input North vector that is used for the +z azis of the sphere. The earth boundary is a parameterized curve on the projection sphere for a canonical spherical coordinate system that has NORTH pointing at the center of the earth. This parameterization is transformed to the new coordinates specified by the input NORTH. EXAMPLE: ;; Suppose we are using GSE coordinates. ;; View position 3 Re north GSE, View NORTH direction toward sun ;; Prime meridian (phase 0) is GSE north. en=earth_boundary([0.,0,3.],[1.,0.,0],prime=[0,0,1.],/deg,n=1000) ;; View position at 3Re, 60 deg lattitude, dawn. ee=earth_boundary([0.,-1.5,2.6],/deg) ;; View position at 3Re, 60 deg lattitude, dawn, North toward sun es=earth_boundary([0.,-1.5,2.6],[1.,0.,0],/deg) ;; Azimuthal projection centered at north pole with lattitude ;; Full sphere map_set,90.,/az,/grid,/lab,lonlab=0,latlab=45 ;; plotting the earth boundary plots,en(2,*),en(1,*) plots,ee(2,*),ee(1,*) plots,es(2,*),es(1,*) ;; display only a hemisphere. map_set,90.,/az,/grid,/lab,limit=[0.,0,180.,360.],lonlab=180./6,latlab=45 ;; plotting the earth boundary plots,en(2,*),en(1,*) plots,ee(2,*),ee(1,*) plots,es(2,*),es(1,*) Author: Chris Chase (chase@jackson), Feb, 1994 Modification $Author: jimm $ MODIFICATION HISTORY: Thu Mar 16 15:16:45 1995, Chris Chase S1A Added PRIME_MERIDIAN, HELP, DEGREES keywords and examples. Mon Feb 28 13:34:53 1994, Chris Chase S1A Created.