cleanup conversion to acres (merge from devbr6)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@36745 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.what/v.what.html

@@ -6,6 +6,10 @@ in user-specified vector map layer(s).  This module was derived from the
 <em>d.what.vect</em> module by removing all interactive code and
 modification of the output for easy parsing. Using the <em>-g</em> flag permits to
 generate script style output which is easily parsable.
+<P>
+This module always reports standard acres, even when the location uses
+US Survey feet as the map unit.
+
 
 <h2>EXAMPLE</h2>
 
@@ -14,13 +18,16 @@ Spearfish example:
 v.what fields east_north=599121.75,4922981.125
 </pre></div>
 
+
 <h2>SEE ALSO</h2>
 
 <em><a HREF="d.what.vect.html">d.what.vect</a></em>
 
+
 <h2>AUTHOR</h2>
 
 Trevor Wiens<br>
 Edmonton, Alberta, Canada
 
-<p><i>Last changed: $Date$</i>
+<p>
+<i>Last changed: $Date$</i>

vector/v.what/what.c

@@ -107,7 +107,7 @@ void what(double east, double north, double maxdist, int topo, int showextra, in
 {
     int type;
     char east_buf[40], north_buf[40];
-    double sq_meters;
+    double sq_meters, sqm_to_sqft, acres, hectares, sq_miles;
     double z = 0, l = 0;
     int notty = 0;
     int getz = 0;
@@ -124,6 +124,12 @@ void what(double east, double north, double maxdist, int topo, int showextra, in
     Cats = Vect_new_cats_struct();
     db_init_string(&html);
 
+    /* always use plain feet not US survey ft */
+    /*  if you really want USfeet, try G_database_units_to_meters_factor()
+        here, but then watch that sq_miles is not affected too */
+    sqm_to_sqft = 1 / ( 0.0254 * 0.0254 * 12 * 12 );
+
+
     for (i = 0; i < nvects; i++) {
 
 	Vect_reset_cats(Cats);
@@ -134,8 +140,7 @@ void what(double east, double north, double maxdist, int topo, int showextra, in
 			   maxdist, 0, 0);
 	if (line == 0) {
 	    line = Vect_find_line(&Map[i], east, north, 0.0,
-				  GV_LINE | GV_BOUNDARY | GV_FACE, maxdist, 0,
-				  0);
+				  GV_LINE | GV_BOUNDARY | GV_FACE, maxdist, 0, 0);
 	}
 
 	if (line == 0) {
@@ -364,7 +369,16 @@ void what(double east, double north, double maxdist, int topo, int showextra, in
 		    fprintf(stdout, _("Type: Area\n"));
 		}
 	    }
+
+
 	    sq_meters = Vect_get_area_area(&Map[i], area);
+	    hectares  = sq_meters / 10000.;
+	    /* 1 acre = 1 chain(66') * 1 furlong(10 chains),
+		or if you prefer ( 5280 ft/mi ^2 / 640 acre/sq mi ) */
+	    acres = (sq_meters * sqm_to_sqft) / (66 * 660);
+	    sq_miles = acres / 640.;
+
+
 	    if (topo) {
 		int nisles, isleidx, isle, isle_area;
 
@@ -405,26 +419,23 @@ void what(double east, double north, double maxdist, int topo, int showextra, in
 	    else {
 		if (script) {
 		    fprintf(stdout, "Sq_Meters=%.3f\nHectares=%.3f\n",
-			    sq_meters, (sq_meters / 10000.));
+			    sq_meters, hectares);
 		    fprintf(stdout, "Acres=%.3f\nSq_Miles=%.4f\n",
-			    ((sq_meters * 10.763649) / 43560.),
-			    ((sq_meters * 10.763649) / 43560.) / 640.);
+			    acres, sq_miles);
 		}
 		else {
 		    fprintf(stdout, _("Sq Meters: %.3f\nHectares: %.3f\n"),
-			    sq_meters, (sq_meters / 10000.));
+			    sq_meters, hectares);
 		    fprintf(stdout, _("Acres: %.3f\nSq Miles: %.4f\n"),
-			    ((sq_meters * 10.763649) / 43560.),
-			    ((sq_meters * 10.763649) / 43560.) / 640.);
+			    acres, sq_miles);
 		}
 		if (notty) {
 		    fprintf(stderr,
 			    _("Sq Meters: %.3f\nHectares: %.3f\n"),
-			    sq_meters, (sq_meters / 10000.));
+			    sq_meters, hectares);
 		    fprintf(stderr,
 			    _("Acres: %.3f\nSq Miles: %.4f\n"),
-			    ((sq_meters * 10.763649) / 43560.),
-			    ((sq_meters * 10.763649) / 43560.) / 640.);
+			    acres, sq_miles);
 		}
 		nlines += 3;
 	    }