In my proposed comment, I said "This is not a straight line on the ellipsoid." which begs the question what else did I mean by "straight" on an ellipsoid since I'd already said the sides weren't geodesics? Since the side of a polygon is defined by a linear relationship between latitude and longitude, it is a rhumb line/loxodrome, isn't it? On reflection, I'd change the second sentence of my proposed comment to: The sides of the polygon are rhumb lines and, in general, not geodesics. and delete the sentence about the "straight line on the ellipsoid". Richard
Dr. R. E. Roger| Senior Spatial Analyst NSW Department of Primary Industries |NSW Office of Water 181 - 183 Anson Street| Orange NSW 2800 | PO Box 53 | Orange NSW 2800 T: 02 6363 8729 |F: 02 6361 3839 |E: [email protected] W: www.water.nsw.gov.au ( http://www.trade.nsw.gov.au/ ) >>> Glynn Clements <[email protected]> 4/11/2013 12:57 pm >>> Markus Neteler wrote: > > I hope this is the correct place to post a request for an update to two > > comments in /lib/gis/area_poly1.c, and replace it with something more > > informative. > > > > My remarks are based on reading /lib/gis/area_poly1.c from > > grass-7.0.svn_src_snapshot_2013_07_13. > > ... I think that his suggestions deserve a feedback, anyone of the > grass devs knowledgeable with G_begin_ellipsoid_polygon_area()? It's been a while since I looked at it, but I'm fairly sure that he's correct. ISTR that the calculation is essentially the area integral of the scale factor (which is a function of latitude) over a polygon in the Euclidean plane. -- Glynn Clements <[email protected]>
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