Hi Gdal-ers,
This is cross-posted on the proj list, but my query also concerns
whether gdalwarp notices the NTv2 grid or not, as my results vary from
the control data (point BM1) I was given.
I'm using gdal/ogr v1.9.1 on ubuntu 12.04.
Please read on, and comment:
Hi All,
This issue goes on, slower than a snail.
I have finally managed to get some source and target points from the
original surveyor.
BM1
2872274.540m
49717.410m
7128571.659m
450273.237m
4275103.524m
I load my source image that is on Lo33 Clarke 1880 into QGIS and hover
the mouse over BM1, a nicely painted white target.
I get: -49717.33 -2872272.46 which is good on the eastings, but my
source image is 2m north of this value from the surveyor. For the
moment, so what.
I then run:
ogr2ogr -f "DGN" -dsco "3D=YES" -s_srs "+proj=tmerc +lon_0=33
+a=6378249.145 +b=6356514.966398753 +nadgrids=./SA.gsb" -t_srs
"+init=epsg:32736" Cl_Revised_v7_36s.dgn Cl_Revised_v7.dgn
gdalwarp -s_srs "+proj=tmerc +lon_0=33
+a=6378249.145 +b=6356514.966398753 +nadgrids=./SA.gsb" -t_srs
"+init=epsg:32736" images.vrt images_36s.tif
My UTM 36S WGS84 coordinate for BM1 is now:
450288.88 and 7128578.76 and this puts my transformed image 15m to
the east and 7m to the north of the Surveyors value.
My question is:
Did I specify the s_srs and t_srs parameters correctly?
Can I be certain that the SA.gsb grid was used?
I have no idea how the original surveyor got his source and target
values for BM1, as I have no access to this person.
Regards,
Zoltan
I have no idea how big, or what extents are in these NTv2 grid files, as
I have never used them before.
To this end, my area of interest is in Maputo (Mozambique), and
Mozambique apparently uses Clarke 1866 (not 1880) - but the surveyor
"swears blind" that the coords are on "1880" and not "1866".
This begs the question:
Do the extents of this Clarke 1880 NTv2 grid stop at the South African
border (some 50Km west of my area of interest), or do they cover Maputo
sufficiently?
Yes, they go up to 33E which just covers Maputo, and when I load the
grid plus my data into QGIS, my data is inside the grid extents.
On 2012/07/26 21:42, Mikael Rittri wrote:
> Hello Zoltan.
> Yes, as you say, the traditional transverse mercator projections
for Southern Africa have
> axes positive west and south. This can be handled in Proj 4.8.0
(see http://trac.osgeo.org/proj/wiki/TMSO ).
> However, since both your example coordinates are negative, I think
you have data in a less traditional
> but more GIS-friendly variant, where the axes go east and north and
the traditional coordinate values
> have been negated.
>
> Another problem is caused by the Cape datum having distortions that
are not handled well
> by a simple Helmert datum shift ( +towgs84 in Proj.4). Such a datum
shift will have errors up to 15 meters,
> perhaps more. You can get much better accuracy from a grid shift
file. So I think your source CRS
> is best constructed as
>
> +proj=tmerc +lon_0=33 +a=6378249.145 +b=6356514.966398753
+nadgrids=SA.gsb
>
> where SA.gsb is an NTv2 grid shift file from Cape to
Hartebeesthoek94 (or to WGS84),
> covering South Africa. To find out where you can download it, see
>
>
http://eepublishers.co.za/article/datum-transformations-using-the-ntv2-grid.html
>
> About your target CRS, I don't think there is an EPSG code for the
combination of Hartebeesthoek94 and
> a UTM projection. But since H94 is the same as WGS84 except for
tectonic motion, I think you
> can use WGS84 / UTM zone 36S, which can be defined as
>
>Â Â Â Â +init=epsg:32736
>
> Hope this helps,
>
> Mikael Rittri
> Carmenta
> Sweden
> http://www.carmenta.com
>
> 26 jul 2012 kl. 17:14 skrev "Zoltan Szecsei"
<[email protected] <mailto:[email protected]>>:
>
>> Hi Everyone,
>> I have some orthos and some 3D vector data that I need to
reproject from
>> Clarke 1880 Cape Datum, Lo33 to WGS84 Hartebeeshoek94 datum UTM 36S.
>>
>>
--
===========================================
Zoltan Szecsei PrGISc [PGP0031]
Geograph (Pty) Ltd.
P.O. Box 7, Muizenberg 7950, South Africa.
65 Main Road, Muizenberg 7945
Western Cape, South Africa.
34° 6'16.35"S 18°28'5.62"E
Tel: +27-21-7884897 Mobile: +27-83-6004028
Fax: +27-86-6115323www.geograph.co.za
===========================================
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