[Proj] How to convert a sphere to ellipsoid with correct datum?
j.l.h.hartmann at uva.nl
Sat Sep 4 07:15:32 EST 2010
On 09/03/10 18:22, Hermann Peifer wrote:
> I think this story with the Dutch church towers already circulated
> through the proj mailing list, in 2008, as far as I can see:
> I see that you started it, so it seems to be an ongoing issue? I am
> sorry, but I do not have any special expert knowledge in this specific
Thanks Hermann, that was really great help. To explain why I keep coming
back on the issue: I am georeferencing the oldest cadastral maps of the
Netherland from 1830. There was no nation-wide triangulation then, but
for the zero point of the local grids the surveyors chose in most cases
the same church towers that were twenty years later used for the
national triangulation. If I can put that point exactly on the modern
map, I can also put the whole cadastral map of a particular community on
the modern cadastral map, with accurucies up to a few meters.
That accuracy is important, since there are lots of legal issues about
old buildings, parcels and landscapes that influence their price and
what you are legally allowed to do with them. A difference of 50 meters
in a very parcellated city like Amsterdam can have severe consequences
for prices, subsidies, "protectability" etc and could affect lots of
real people at this actual moment.
So the 60 meters accuracy I can get by just using the conventional PROJ
parameters in transforming from the 19th to the 20th century system just
isn't enough. I can transfer the zero point of the 19th century
triangulation by 60 m., but as long as I can't underpin that in a
rigorous mathematical way, the results will always be disputable. I
really expect to open a big can of worms when people overlay their
historic properties over Google satellite with an apparent meter-like
accuracy, that turns out to be disputable when they have a closer look
at the methods I have used.
Again thanks for your help: I am now confident that converting latlon
values on different ellipsoids give correct results when using
+towgs84=0,0,0 to both parameter lists. I still have to hunt for the
last 60 meters of accuracy, though. The devil is, as always, in the
latest en finest detail.
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