[Proj] How to convert a sphere to ellipsoid with correct datum?

Tue Sep 7 11:13:36 PDT 2010

This has been an interesting thread.  Consider taking a look at: 

G. Timár, and C.J. Mugnier, GIS Integration of the 1:75,000 Romanian Topographic Map Series from the World War I, Geophysical Research Abstracts, Vol. 11, EGU2009-1-3, 2009.  

If you look up more of Dr. Timár's publications, you will find a number of helpful insights in georeferencing ancient maps of Europe.

Cliff

Clifford J. Mugnier, C.P., C.M.S.
Chief of Geodesy,
Center for GeoInformatics
Department of Civil Engineering 
Patrick F. Taylor Hall 3223A
LOUISIANA STATE UNIVERSITY 
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic] 
Voice and Facsimile:  (225) 578-4578 [Research] 
Cell: (225) 238-8975 [Academic & Research]
Honorary Life Member of the 
Louisiana Society of Professional Surveyors 
Fellow Emeritus of the ASPRS 
Member of the Americas Petroleum Survey Group

________________________________

From: proj-bounces at lists.maptools.org on behalf of OvV_HN
Sent: Tue 07-Sep-10 12:35
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] How to convert a sphere to ellipsoid with correct datum?

In the mean time I've found a contemporary reference about the modified
Flamsteed projection.

Traité de topographique, d'arpentage et de nivellement, L. Puissant, 2ème
edition, Paris, 1820.
Chapitre III: Théorie analytique de la projection modifiée de Flamsteed, pp.
114 f.f.

It's on google books.

This chapter gives a mathematical description of said projection. It's
probably worth checking this projection against the (current interpretation
of the) Bonne projection you used.

Oscar van Vlijmen

IN REPLY TO:

From: Jan Hartmann <j.l.h.hartmann <at> uva.nl>
Subject: Re: How to convert a sphere to ellipsoid with correct datum?
Date: 2010-09-07 10:10:24 GMT

Thanks Mikkael, I'll follow that road too. I corresponded over this with a
Dutch geodesist who programmed a datum conversion tool (Jan Hendrikse,
http://members.home.nl/hendrikse/), but the results were not as good as
transforming the triangulation points using the original PROJ formula, and
afterwards rubbersheeting them to their exact modern position. There was a
problem wit GDAL, though: it's not possible to rubbersheet an already
georeferenced map, due to a limitation in the Geotiff format. I think now
that it can be done by using the VRT format.

And there remains the question of proof. I can (and will) compute the datum
shift, and can get a map that is within 10m accuracy, about the theoretical
maximum, but is a datum shift really the reason for the deviation? Did they
really use an ellipsoid with a different center and location in 1850? I find
it hard to believe, at least I never found an indication for that, and I
read the handbooks used in 1850. And Cliff Mugnier doesn't think it either
(http://lists.maptools.org/pipermail/proj/2008-November/004042.html).

So, while I know now how to solve the problem computationally  and get a map
that is as exact as can be (thanks to all you input, thanks!), I am still
wondering about the reasons for the deviation. There could be some legal
issues (although I am not much afraid of those, it's too long ago), but for
the most I don't wont to be right for the wrong reasons. I'm not a great
believer in statistical "proofs" where the underlying law or model is
unknown.

Thanks again for all your responses,

Jan

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