[Proj] Re: Datum shift for Dutch maps 1850-2000
strebe at aol.com
Wed Nov 26 12:57:05 EST 2008
I think you can assume the projection is the Bonne. Flamsteed, as you note, is the sinusoidal. The sinusoidal is the Bonne with its standard parallel at the equator. Topographic mapping in France does refer to the Bonne as "modified Flamsteed". (See Hunt & Schott 1854, p. 98: "Tables for Projecting Maps, with Notes on Map Projection", from "Report to the Superintendent of the Coast Survey... 1853", Appendix 39. Washington D.C.: Robert Armstrong, Public Printer.)
-- daan Strebe
On Nov 26, 2008, at 9:06:05 AM, "Jan Hartmann" <j.l.h.hartmann at uva.nl> wrote:
This certainly makes a difference! Adding +nadgrids=@null to the
original definition gives a map with a 120m South shift, still too much
but nearing a solution. Isn't this a bug in PROJ? I did not request a
datum shift, and neither the from-projection, nor the to-projection had
a datum defined, only ellipsoids. I am using Proj 4.6.1.
I looked again at the 19th century documentation, and it says the map is
in the "modified Flamsteed or French" projection. All later
documentation refers to "Bonne" as the projection for this map and its
successors; they are actually still called "Bonne maps" here. The
Flamsteed projection is the classic sinusoidal projection, which is
certainly not the one for this map, and the "modified Flamsteed
projection" seems to be some sort of conic projection, but I haven't
found any information on this. It could be the same as the Bonne
projection, which is conic too. Does anyone have any information on this?
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