[GRASS-SVN] r58163 - grass/trunk/lib/gis
svn_grass at osgeo.org
svn_grass at osgeo.org
Thu Nov 7 04:11:07 PST 2013
Author: neteler
Date: 2013-11-07 04:11:06 -0800 (Thu, 07 Nov 2013)
New Revision: 58163
Modified:
grass/trunk/lib/gis/area_poly1.c
Log:
area_poly1.c function documentation improved (contributed by Richard Roger richard.roger at water dot nsw dot gov dot au)
Modified: grass/trunk/lib/gis/area_poly1.c
===================================================================
--- grass/trunk/lib/gis/area_poly1.c 2013-11-07 06:14:31 UTC (rev 58162)
+++ grass/trunk/lib/gis/area_poly1.c 2013-11-07 12:11:06 UTC (rev 58163)
@@ -3,7 +3,7 @@
*
* \brief GIS Library - Polygon area calculation routines.
*
- * (C) 2001-2009 by the GRASS Development Team
+ * (C) 2001-2013 by the GRASS Development Team
*
* This program is free software under the GNU General Public License
* (>=v2). Read the file COPYING that comes with GRASS for details.
@@ -55,7 +55,7 @@
* eccentricity squared <i>e2</i>.
*
* \param a semi-major axis
- * \param e2 ellipsoid eccentricity
+ * \param e2 ellipsoid eccentricity squared
*/
void G_begin_ellipsoid_polygon_area(double a, double e2)
@@ -89,9 +89,31 @@
* <i>n</i> pairs of <i>lat,long</i> vertices for latitude-longitude
* grids.
*
- * <b>Note:</b> This routine assumes grid lines on the connecting the
- * vertices (as opposed to geodesics).
+ * <b>Note:</b> This routine computes the area of a polygon on the
+ * ellipsoid. The sides of the polygon are rhumb lines and, in general,
+ * not geodesics. Each side is actually defined by a linear relationship
+ * between latitude and longitude, i.e., on a rectangular/equidistant
+ * cylindrical/Plate Carr{'e}e grid, the side would appear as a
+ * straight line. For two consecutive vertices of the polygon,
+ * (lat_1, long1) and (lat_2,long_2), the line joining them (i.e., the
+ * polygon's side) is defined by:
+ * lat_2 - lat_1
+ * lat = lat_1 + (long - long_1) * ---------------
+ * long_2 - long_1
+ * where long_1 < long < long_2.
+ * The values of QbarA, etc., are determined by the integration of
+ * the Q function. Into www.integral-calculator.com, paste this
+ * expression :
*
+ * sin(x)+ (2/3)e^2(sin(x))^3 + (3/5)e^4(sin(x))^5 + (4/7)e^6(sin(x))^7
+ *
+ * and you'll get their values. (Last checked 30 Oct 2013).
+ *
+ * This function correctly computes (within the limits of the series
+ * approximation) the area of a quadrilateral on the ellipsoid when
+ * two of its sides run along meridians and the other two sides run
+ * along parallels of latitude.
+ *
* \param lon array of longitudes
* \param lat array of latitudes
* \param n number of lat,lon pairs
More information about the grass-commit
mailing list