[PROJ] How does proj deal with ellipsoid with respect to reprojection

[Lesparre, Jochem]

Pierre,

For my analysis, I assumed someone without much geodetic knowlegde doing a point in polygon in projected coordinates would treat these coordates as unprojected cartesian coordinates. Are you saying that is an incorrect assumption?

I'm not sure if I understand your question correctly. I think the answer might be the RD column of my table. This is the deviation between a conformally projected geodesic of an ellipsoid with a line y=ax+b in +proj=sterea.

Regards, Jochem

Get Outlook for Android<https://aka.ms/ghei36>

________________________________
From: PROJ <proj-bounces at lists.osgeo.org> on behalf of Pierre Abbat <phma at bezitopo.org>
Sent: Sunday, March 29, 2020 9:00:01 AM
To: proj at lists.osgeo.org <proj at lists.osgeo.org>
Subject: Re: [PROJ] How does proj deal with ellipsoid with respect to reprojection

On Sunday, 29 March 2020 02:28:57 EDT Lesparre, Jochem wrote:
> Doing point in polygon in a projection will result in occasional wrong
> conclusions. A point near de edge can seem to be inside the polygon while
> it's outside, or the other way around, since a straight line in the
> projection deviates from the geodesic.

> I analysed this problem for the Netherlands (51 - 55 degrees north) in the
> azimuthal projection (+proj=sterea) of the national coordinate reference
> system RD (epsg:28992) and in plate-caree projection (+proj=lonlat). The
> deviation depends on the length, orientation and location of a polygon
> segment. I computed the maximum possible deviation in the Netherlands for
> both projections to advise the Dutch government not to allow any segments
> longer than 200 m in a new digital storage system for policy and zoning
> borders.

When doing point in polygon in the spherical stereographic projection, you are
checking circular arcs, not straight lines, in the plane. A geodesic projects
to a circle. How much does a geodesic of an ellipsoid, projected conformally
onto a sphere, differ from a geodesic of a sphere?

Pierre
--
When a barnacle settles down, its brain disintegrates.
Já não percebe nada, já não percebe nada.



_______________________________________________
PROJ mailing list
PROJ at lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/proj


Disclaimer:
De inhoud van dit bericht is uitsluitend bestemd voor geadresseerde.
Gebruik van de inhoud van dit bericht door anderen zonder toestemming van het Kadaster 
is onrechtmatig. Mocht dit bericht ten onrechte bij u terecht komen, dan verzoeken wij u 
dit direct te melden aan de verzender en het bericht te vernietigen. 
Aan de inhoud van dit bericht kunnen geen rechten worden ontleend.

Disclaimer:
The content of this message is meant to be received by the addressee only.
Use of the content of this message by anyone other than the addressee without the consent 
of the Kadaster is unlawful. If you have received this message, but are not the addressee, 
please contact the sender immediately and destroy the message.
No rights can be derived from the content of this message
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/proj/attachments/20200329/e7ccc7ee/attachment.html>