[Gvsig_english] BIG problems with reprojecting AGD66 to GDA94
Simon Cropper (Botanicus Australia Pty Ltd)
scropper at botanicusaustralia.com.au
Mon Nov 30 01:36:52 CET 2009
Juan Lucas,
Thanks for the feedback. So that others are able to follow the thread
and have the outcome at their disposal I will repeat what has been
stated so far both in this email and other that you have sent me.
*OBSERVATION A*
If you are using the reprojection tool and converting between systems in
the same datum then the tool will work.
This means if you are converting between AGD66 GEO <=> AGD66 AMG55 <=>
AGD66 VicGrid there are no problems.
This also applies for GDA94 GEO <=> GDA94 MGA55 <=> GDA94 VicGrid.
*OBSERVATION B*
It is necessary to 'transform' a dataset if you are changing between
datums. That is, moving between AGD66 to GDA94 or back the other way.
The regional transformation parameters for Victoria are published by the
Intergovernmental Committee of Surveying and Mapping in the GDA94
Technical manual Version 2.3, Amendment 1
(http://www.icsm.gov.au/icsm/gda/gdatm/gdav2.3.pdf).
The parameters *DO NOT WORK IF PLUGGED INTO THE REPROJECTION TOOL AS IS,
YOU NEED TO CHANGE THE SIGN (+/-) OF VARIOUS VALUES DEPENDING ON YOUR
DESIRED OUTCOME !* This is done by manually transforming the vector
files by typing in the following values under the various situation
specified.
The following values can be used to _correctly reproject one file to
another file_. Note you need to ensure the signs before the various
values are correct.
_*File2File: AGD66 => GDA94*_
Dx = 119.353
Dy = 48.301
Dz = -139.484
Rx = -0.415
Ry = -0.26
Rx = -0.437
Sc = 0.613
_*File2File: GDA94 => AGD66*_
Dx = -119.353
Dy = -48.301
Dz = 139.484
Rx = 0.415
Ry = 0.26
Rx = 0.437
Sc = -0.613
If you are _reprojecting of the fly_. The values are reversed.
*_View Projection=AGD66; File Projection=GDA94_*
Dx = 119.353
Dy = 48.301
Dz = -139.484
Rx = -0.415
Ry = -0.26
Rx = -0.437
Sc = 0.613
_*View Projection=GDA94; File Projection=AGD66*_
Dx = -119.353
Dy = -48.301
Dz = 139.484
Rx = 0.415
Ry = 0.26
Rx = 0.437
Sc = -0.613
What is frustrating is that these values need to be entered every time
you make a change. Very laborious and also prone to typographical errors.
The GDA Technical Manual indicates that this 'rough' transformation has
an accuracy of about 1 metre. The observations made by Juan Lucas
supports this claim. I also found that in points I checked the values
were <1m from the reference points created using the transformation grid
and that the transformation work for latitude-longitude as well as
northing-easting.
*OBSERVATION C*
Although a transformation grid is available, apparently in the correct
format, this grid does not work with gvSIG.
*OBSERVATION D*
Although I am not happy that the transformation grid does not work as my
reprojected points are not exactly the same as the reference points, the
question is whether I am in any better position if I use ArcView (the
pseudo-reference point at the moment). The answer is NO. I reprojected
the same source using ArcView and found that points in the two
reprojected files were <1cm apart. Not exactly the same but close enough
that the gvSIG measure tool did not register the difference (although it
did plot them in different locations if you zoomed in enough). I suspect
that the two points differ because the ArcView procedure uses an earlier
version of the Regional Transformation Parameters.
*MY SUGGESTIONS*
*Suggestion 1.* The reprojection tool should produce and error, or at
least a warning, if the source file and destination files are not in the
same datum letting the user know that it is necessary to select an
appropriate transformation. The impression you get when selecting a
source DATUM-COORDINATE combo and a destination DATUM-COORDINATE combo
that the system is automatically sorting this issue out.
*Suggestion 2.* Someone should look at the transformation grid produced
by the Intergovernmental Committee of Surveying and Mapping and figure
out why it does not work. gvSIG can't truly become an international
package if files apparently in the correct format don't work, and that
values (in this case rotation values published in the literature) have
to have their signs reversed in gvSIG to get the transformation to work
properly.
*Suggestion 3.* OK, a work around has been found but it is not simple or
quick. Manual values need to be entered, but these vary depending on the
situation (see above), and they need to be entered every time you
conduct a reprojection. It would be advantageous if manually entered
data could be saved and subsequently picked from a list. This would
speed up the process and avoid typographical errors resulting in the
incorrect reprojection of data.
Thanks Juan Lucas for your feedback on this issue.
Cheers Simon
Simon Cropper
Botanicus Australia Pty Ltd
PO Box 160, Sunshine, Victoria 3020.
P: 9311 5822. M: 041 830 3437.
mailto: scropper at botanicusaustralia.com.au
<mailto:scropper at botanicusaustralia.com.au>
web: www.botanicusaustralia.com.au <http://www.botanicusaustralia.com.au>
Juan Lucas Dominguez Rubio wrote:
> Hello, Simon.
>
> I have done a test with a view in EPSG:4202 and two shapefiles (a
> target shapefile in EPSG:4202 and a source shapefile in EPSG:4283)
> extracted from your DBF. After adding the target shapefile (same
> projection as the view) I have added the source shp trying a variety
> of transformations. When you add it to the view, gvSIG lets you choose
> among eleven available "EPSG transformations" between those two EPSG
> codes. The one that gives me the best result is the first one:
>
> Transformation code: 1278
> Name: AGD66 to GDA94 (1)
> (we are actually doing the inverse transformation (GDA94 to AGD66) but
> the algorithm seems to be aware of this and does it well)
>
> I have measured the distance between the obtained coordinates
> (transformed source shp) and the expected coordinates (target
> shp) for 5 random points and I get these errors:
>
> 1.2 m
> 0.9 m
> 1.57 m
> 0.88 m
> 0.96 m
>
> I have also looked into some pdf files and I have found these
> parameters (AGD66 to GDA94) for Victoria and NSW:
>
> http://www.icsm.gov.au/icsm/gda/gdatm/gdav2.3.pdf
>
> Dx = -119.353
> Dy = -48.301
> Dz = 139.484
> Rx = -0.415
> Ry = -0.26
> Rx = -0.437
> Sc = -0.613
>
> Since we are doing the inverse transfomation, we should reverse the
> sign for each parameter, but I don't get good results.
>
> I have also read somewhere that in Australia and the US, the rotation
> parameters have the inverse meaning than in Europe (big ???), so I
> tried these parameters (reversing the ones above except for the
> rotation parameters):
>
> Dx = 119.353
> Dy = 48.301
> Dz = -139.484
> Rx = -0.415
> Ry = -0.26
> Rx = -0.437
> Sc = 0.613
>
> And indeed, I get significantly smaller errors for those 5 points:
>
> 0.7 m
> 0.15 m
> 0.75 m
> 0.8 m
> 0.25 m
>
> Those 7 parameters can be entered with the "manual transformation"
> option. I didn't get the grid to work either, sorry, although the
> format seems to be one supported by gvSIG. And I have not done any
> tests with non-geographical SRS.
>
> Regards,
>
> Juan Lucas Domínguez Rubio
> ---
> Prodevelop SL, Valencia (España)
> Tlf.: 96.351.06.12 -- Fax: 96.351.09.68
> http://www.prodevelop.es <http://www.prodevelop.es/>
> ---
>
> ------------------------------------------------------------------------
> *De:* gvsig_internacional-bounces at listserv.gva.es en nombre de Simon
> Cropper (Botanicus Australia Pty Ltd)
> *Enviado el:* sáb 28/11/2009 7:36
> *Para:* Users and Developers mailing list
> *Asunto:* [Gvsig_english] BIG problems with reprojecting AGD66 to GDA94
>
> Hi Guys,
>
> I am having problems getting the reproject tool to work as expected with
> Australian Projections/Datums.
>
> If I create a shapefile in AGD66 then reproject the file to GDA94 using
> the reproject tool, the points are moved but only ~20m rather than 204
> metres northeast of the original position. If I use the resulting GDA94
> file in the AGD66 View and tell the program, it reprojects on the fly
> and the points overlap.
>
> This shows (a) that the reproject on the fly facility works, and (b) I
> appear to have chosen the wrong projection.
>
> The problem is I have tried reprojecting northing-eastings,
> latitude-longitude, in EPSG and ESRI format but can't get the correct
> reprojection. I have been using EPSG:4202 for AGD66 geographicals,
> EPSG:4283 for GDA94 geographicals, EPSG:20255 for AGD/ AMG zone 55 and
> EPSG:28355 for GDA94/MGA zone 55. I have also tried ESRI: 28355 and
> ESRI:20255 with no luck.
>
> I have even downloaded the transformation grid but not been able to get
> this to work either.
>
> Can anyone help?
>
> Attached is a list of points in a table that are released by the local
> government apparently created by FME. This has the comparable
> coordinates in 4 commonly used projections for 160 distinct points
> across Victoria.
>
> Can anyone get the correct setting and procedure to get these points to
> superimpose when reprojecting on the fly and for the reproject facility
> to work correctly?
>
> I have also included some useful links to help the more industrious and
> knowledgeable to figure out what is wrong.
>
> http://www.land.vic.gov.au/Land/lcnlc2.nsf/LinkView/9B33746B3D77040F4A256A250008E70A5B923E73477D6D65CA256E5F0013D73B
>
> http://www.icsm.gov.au/icsm/gda/gdatm/
>
> http://www.icsm.gov.au/icsm/gda/gdatm/gdav2.3.pdf
>
> Any help would be appreciated as I am trying to tick this off my list of
> things gvSIG can do, and was surprised to see when resulting reprojected
> files were compared to external reference documents they did not align.
>
> Any help would be greatly appreciated.
> --
>
> Cheers Simon
>
> Simon Cropper
> Botanicus Australia Pty Ltd
> PO Box 160, Sunshine, Victoria 3020.
> P: 9311 5822. M: 041 830 3437.
> mailto: scropper at botanicusaustralia.com.au
> <mailto:scropper at botanicusaustralia.com.au>
> web: www.botanicusaustralia.com.au
> <http://www.botanicusaustralia.com.au
> <http://www.botanicusaustralia.com.au/>>
>
> ------------------------------------------------------------------------
>
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