Hi Peppe, As you mention, bursa-wolf transformation is not always enough to make precise transformation between old local reference system and new spatial reference system. The advantage is that it relies on a set of only 7 parameters and a well known algorithm. To do more precise transformations, countries often use grids to "adapt" the BW parameters locally or to apply corrections on the results. In France, if old system coordinates are transformed towards the new system with a BW transformation (indeed a simple translation), one get 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 don't know what is the system used in USA, but I know that Canada and Australia also use grid based transformations (or corrections?) based on NTV2 grid format. 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.
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 ? 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 ? Michaël Giuseppe Aruta a écrit : > Hi, > I did a test with the CTS plugin. > As control I used Taspunto, a software for Italian coordinate > transformation edited by Italian government. This software doen't use > formulas for transformation but on a grid of points (1500) in the > Italian territory and surroundings. It is considered to give an > accuracy of less than 1 metre. > Finally, opening all files with OJ and using Edgar plugin, I was able > toI define that EPSG parameters for helmet transformation gave an > error between 0.5-2 metres on mainland (continental Italy). Close to > easter side of Italy (Puglia) the error is almost of 8 metres. > Now 0.5-2 metres is more than acceptable, since EPSG spec gave an > accuracy of 4 metres. > I think that Edgar plugin works, at least between WGS84 and ROMA40. > So I wiull prepare an (un)official config file for Italian > transformation with the BW par. 1) Italy mainland, 2) Italy Puglia, 3) > Italy Sardinia and 4) Italy Sicily. > Nothing to do between WGS84 and ED50, the error is still of 400-600 > metres (any idea?) > > Regards > > Peppe > */Giuseppe Aruta <[EMAIL PROTECTED]>/* ha scritto: > > Hi all, > working around Edgar Soldin's Coordinate transformation plugin I > finally reached the (un)-famous problem of setting Bursa-Wolf > parameters (BW) for the tranformation between different > datum/projections. > I realize that the problem is not so simple as it is shows: > 1) despite the EPSG official archive on net > (http://www.epsg-registry.org/) and other informations, there is > no unique BW parameters even between the same transformation. > For instance, searching on EPSG register, EPSG1660 (transformation > from Italian ROMA40 to WGS84) I had the official parameters for > Italy mainland with a maximum error of 4 metres. > Well, I tested these parameters in some places "mainland" in > Italy, I had an errror even of 60 metres, for instance, in > Southern Italy > More than this I discover that some regions/areas defined their > own parameters. > This happens becaue the transformation between ROMA40, ED50 e > WGS84 is not correlated by geometric relations or simple > math but by comparing coordinates of some points within different . > > > The resault is this: > > Edgar's plugin works fine but people need to know the BW > parameters for the area of their interest. And sometimes this is > not easy to do..... (in respect to EPSG, geotools, etc). I image > this is a common problem in Europe > > ************************************************************ > > I will go anyway to write the tutorial page about projections on > wiki since this could be useful for who is interested. I will also > put on wiki some modified cs.conf (configurations) files > optrimized for local transformation (Umbria-Tuscany, South Tyrol > and Campania-Basilicata). > Note that there is no problem with transformation between crs that > don't BW parameters (eg. WGS84 to WGS84/UTM..) to be used for GPS > fanatics! > > ********************************************************** > The bulk of my letter is the follow notes: > Since OpenJUMP has some warping tools (Warp and even New affine > transformation). Is it possible to calculate local Bursa-Wolf > parameters using these tools on some reference points? Does > somebody thought about before? > This probabily could be an interesting area of research (for > pratical usage) of OpenJUMP: A way or a plugin that calculate > local Bursa Wolf parameters. > > I pass these questions expecially to Europian collugues since this > maybe is acommon pronblem: Gauss (Kruge, Boaga, etc) vs WGS84 or > ETRS89 > > *************************************** > > Regards and thanks for the answers > > Peppe > > ------------------------------------------------------------------------ > Tante idee per la salvaguardia del pianeta su > Yahoo! for good > > <http://us.rd.yahoo.com/mailuk/taglines/isp/control/*http://us.rd.yahoo.com/evt=52436/*http://it.promotions.yahoo.com/forgood/environment.html>.------------------------------------------------------------------------- > This SF.net email is sponsored by the 2008 JavaOne(SM) Conference > Don't miss this year's exciting event. There's still time to save > $100. > Use priority code J8TL2D2. > > http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone_______________________________________________ > Jump-pilot-devel mailing list > Jump-pilot-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel > > > ------------------------------------------------------------------------ > Tante idee per la salvaguardia del pianeta su > Yahoo! for good > <http://us.rd.yahoo.com/mailuk/taglines/isp/control/*http://us.rd.yahoo.com/evt=52436/*http://it.promotions.yahoo.com/forgood/environment.html>. > > > ------------------------------------------------------------------------ > > ------------------------------------------------------------------------- > This SF.net email is sponsored by the 2008 JavaOne(SM) Conference > Don't miss this year's exciting event. 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