Dear friends,
a few questions on some things I do not understand well about datum:
1) what is a datum tranformation parameter?
2) why there are 2 datum transformation paramenters for osgb36?
3) why does the system ask for a datum transformation parameter when I create
a location with EPSG code 27700? The code should be enough to create
everything!
Regards
--
Corrado Topi
Global Climate Change & Biodiversity Indicators
Area 18,Department of Biology
University of York, York, YO10 5YW, UK
Phone: + 44 (0) 1904 328645, E-mail: ct529@york.ac.uk
Hi Corrado,
On Fri, 2008-10-24 at 17:11 +0100, Corrado wrote:
Dear friends,
a few questions on some things I do not understand well about datum:
1) what is a datum tranformation parameter?
You might find the following pages interesting:
http://en.wikipedia.org/wiki/Datum_(geodesy) (under
http://en.wikipedia.org/wiki/Geographic_information_system)
and http://grass.osgeo.org/wiki/GIS_Concepts
[...]
3) why does the system ask for a datum transformation parameter when I create
a location with EPSG code 27700? The code should be enough to create
everything!
Be carefull with the codes. Not all epsg codes correspond to one
projection system. Some refer for example to the ellipsoid, others to
the units and more.
Maybe this document explains all!
http://www.epsg.org/guides/docs/G7-1.pdf
Regards
Kind regards, Nikos
On Fri, 24 Oct 2008, Corrado wrote:
Dear friends,
a few questions on some things I do not understand well about datum:
1) what is a datum tranformation parameter?
An extra calculation is needed when re-projecting maps and data between two different co-ordinate systems (in addition to the re-projection) if the two co-ordinate systems are based on different models of the curvature of the earth. E.g. OSGB36 uses the Airy ellipsoid and WGS84 uses the WGS84 ellipsoid, which have slightly different sizes and shapes. The error is not large - generally a few hundred metres at most on the ground. The datum transformation parameters describe this adjustment mathematically.
2) why there are 2 datum transformation paramenters for osgb36?
No particular reason - one of them has been in GRASS for a long time and one is a little bit more accurate and officially published by the OSGB on their website. Several countries have more sets of parameters than this - as the transformation between any two datums is approximate and varies by location, different sets of parameters are often offered to give improved accuracy in different regions of a country. In general there is no one "correct" set of transformation parameters - indeed the accuracy changes over time due to tectonic movements in the earth (an answer that you will get if you ask about this on the PROJ.4 mailing list!). OSGB is actually a special case (though not treated as such in GRASS) - see below.
3) why does the system ask for a datum transformation parameter when I create
a location with EPSG code 27700? The code should be enough to create
everything!
Why do you say that? In general, different datum transformation parameters are appropriate according to the region your data is in and the accuracy you require in a reprojection. However, OSGB36 is a special case as it has recently (in the least few years) been redefined by the Ordnance Survey of Great Britain in terms of WGS84 rather than the original 1936 definition:
http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide5.html
GRASS doesn't (yet) support the OSTN02 gridshift transformation defined there though. (EPSG code 27700 doesn't specify it either; as far as I'm aware the EPSG database has no way of specifying gridshift-based datum transformations.) But you can get a about 5m reprojection accuracy (depending on the area) using the 7-parameter OSGB36 transform provided in GRASS.
[Note to self: 5 metres is quite a low accuracy really compared to that available directly in GRASS for other European countries. We should look at converting OSTN02 into the NTv2 format so it can be used with GRASS and PROJ.4 to give improved accuracy for users in Great Britain.]
Of course if you don't want to reproject data between datums then you don't need to worry about the datum transformation settings when creating a new location.
Paul