Hi Jim
Several of our programs, such as Maps4Site contain
the conversion. I could take out the basic conversion into a DLL that could be
called from MapBasic, and provide this free of charge as a download on our
website and MUGUKI.
Let me know if this would be of use. Due to other
commitments I would not be able to undertake the small amount of work
needed for about a week. This would be a function that you could pass BNG
node and get back WGS84 node. Also the reverse. You would need to add your
own code to get the coords for each polygon, polyline
etc.
Patrick at Blue Marble talks about Transformation.
Unless he has the look up table from Ordnance Survey embedded into his code he
cannot undertake the OSTN02 correction to get accurate results. As detailed
below this is not just formulae. The correction includes "historic errors". This
is why MapInfo gets the Transformation wrong and you are seeing your eleven
metre errors. The look up tables contain a correction for X,Y and Z for each 1
kilometre grid square within Great Britain.
Regards
Bob
----- Original Message -----
Sent: Tuesday, August 15, 2006 5:58
PM
Subject: RE: [MI-L] OT - UK Trig
Points
Hi Bob and list,
Following on from the discussion below could I ask if
you or anyone else on the list knows how to convert polygons from BNG
into WGS84 using the definitive correction Bob mentioned
below?
The problem is that while we use GPS derived WGS 84
points, lines and polygons in field mapping, we occasionally also get OS data
in BNG.
As we are based in Scotland when I open a BNG and a
WGS 84 table in mapinfo the BNG data is offset by about 11 meters to the South
West due to the one transformation that Mapinfo uses and the offset that is
built in due to the historic features in BNG.
Does anyone know if there is any way to convert
a mapinfo table with the definitive correction mentioned
below?
TIA
-------------------------------------------------------------------------------------
Jim Wilson,
Hilton of Fern,
By Brechin,
Angus, Scotland. DD9 6SB
Phone
+44 (0)1356 650307
Fax +44 (0)1356
650445
Mobile +44 (0)7702 741516
-------------------------------------------------------------------------------------
Hi Tim
There is a convertor on the OS site at
GPS.GOV.UK.
If you have Landline or MasterMap, particularly
post PAI, this will give you very accurate Easting and Northing for any
feature identifiable in your car park. Stats on accuracy of post PAI also on
the website, but you are likely to be within a metre. You could use chainage
and offset between two identifiable points to pick up other non-identifiable
points.
The free convertor will correct this BNG
position to GPS lat long, for anywhere within the Country (GB).
The method for the conversion is also
available on the site, so its possible to program British National Grid
Easting,Northing to Lat Long WGS84 ( ie GPS) or WGS84 to British
National Grid Easting Northing.
Its a great resource, and the only definitive
correction, as only Ordnance Survey know where British National Grid is
accurately! The correction is not just formulae. It uses a "look up" table
that takes into account historic "features" of BNG which have to be taken
into account to get correlation with GPS ( eg Use of old County Series maps
and "features" introduced when these were stitched together to form
BNG).
My understanding is that Trig Points are no
longer used in any OS Survey work and are therefore not maintained. I would
therefore not recommend their use.
Regards
Bob Young
----- Original Message -----
Sent: Tuesday, August 15, 2006 2:19
PM
Subject: [MI-L] OT - UK Trig
Points
Hi List.
Does anyone know the best way to get an exact
arbitrary lat/long position?
e.g. I want to know the exact lat/long at a
point in our carpark without using GPS. Are there any free resources
that can give me a detailed aerial photo that gives accurate lat/long
readings?
How accurate are UK trig
points?
Kind regards
Tim
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