[GRASS-dev] /lib/gis/area_poly1.c: algorithm poorly described, new comments for consideration please

[Richard Roger]

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: Richard.Roger at water.nsw.gov.au
W: www.water.nsw.gov.au ( http://www.trade.nsw.gov.au/ )



>>> Glynn Clements <glynn at gclements.plus.com> 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 <glynn at gclements.plus.com>
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