Hi Michael
>.. a precision of 1-2m (5m at worst), which is not that good.
>It is now recommended to use a grid to interpolate precise parameters which
> makes it possible to get a precision of a few centimeters..
I am aware of this. Since my original interest was the "geological maps", an
error of 2 metres is almost 0,4 mm on a 1:5.000 scale map. Of coarse other
pratical usage need mora accuracy than bursa-wolf or other can give.
>OJ warping tool : you did an interesting remark about using warping tool
>to calculate local parameters. In the cases I know, precise parameters
>are interpolated on a regular grid, which is enough to have a good
>precision. Using another type of interpolation (with warping tool) may
>be useful if BW parameters are given for some random places. Is this the
>case for Italia ? Are BW parameters associated to a precise place or to
regions ?
In the last years, by proceeding of catographic projects in Italy, many
Italian Regions defines local bursa-wolf parameters (South Tyrol,
Emilia-Romagna , etc). Other probabily simply "trusted on their ArcGIS" -
that's where I discover big gaps between ROME40 to WGS84 (Campania, for
example!).
Locally scientific projects had adopted modified Bursa Wolf (on Trasimeno
Lake-Umbria, for instance). It is a confusion
On my personal experience. When I had to convert some shapefiles from old to
new projections, I discovered more easy and accurate to use warp tool in
OpenJUMP, having at least 4 reference points. f coarse this method can work
only locally.
The situation in Italy is not so clear. There is a national grid joined by
the Italian government which defines a standar. This grid could be used to
define a NTV2 grid but it seems to have a sort of Copyright (still the effects
of Could War) despite of public access to geographic informations.
>I'm on the way of writing a new library for coordinate transformation,
>and I'm interested by your experience, as my goal is to manage properly
>3D and grid-based transformations, not only projections and BW
>transformations.
I am interested to this project of yours. I will be glad to help you in some
way.
>The error you oberved between WGS84 and ED50 is very important. I
>checked the BW parameters recommended in France between those
>systems, and I found values which are about half the error you observed. Did
> you make sure that you applied the transformation in the right direction
> with the right sign ?
I am not sure if I did a mistake. I will do other carefull tests and write
you down.
Regards
peppe
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