
 dsd   IS DOUBLE-SECOND-DIFFERENCE, OF THE ALTITUDE Hc.
 err30 IS ERROR AT DECLINATION ??:30 DEG AFTER LINEAR INTERPOLATION.
 cor30 IS DOUBLE-SECOND-DIFFERENCE CORRECTION FOR Hc ALTITUDE, AT 30
          MINUTES DECLINATION, USING SINE FORMULA TO CALCULATE dsd.
          (CLOSE TO HO-229 DSD INTERPOLATION TABLE)

 DSD is usually needed if Lat is within 12 deg of Dec and
   LHA is between 1 to 45 deg or 315 to 359 deg and
   Hc is greater than 60 deg.
   Minimum of .25 minutes error befor DSD corr is used.

 NORTH LAT, LHA > 180,  Zn = Z
            LHA < 180,  Zn = 360 - Z

 SOUTH LAT, LHA > 180,  Zn = 180 - Z
            LHA < 180,  Zn = 180 + Z

 HO-229 has Lat 0 listed in both SAME and CONTRARY name as Dec.
 HO-249 has Lat 0 listed in only SAME name as Dec.
