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

[OvV_HN]

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