Hi Patrick
Sorry to have posted incorrect information on your product. When I last
looked at your transformation I did not notice it could deal with shifts
based on a grid.
At least Jims aware of the issue, and has several choices now - which is one
of the great things about the list.
Thanks for letting me know.
Best Wishes
Bob
----- Original Message -----
From: "Patrick Cunningham" <[EMAIL PROTECTED]>
To: "Bob Young" <[EMAIL PROTECTED]>; "Jim Wilson"
<[EMAIL PROTECTED]>; <[email protected]>
Sent: Wednesday, August 16, 2006 1:22 PM
Subject: re[2]: [MI-L] OT - UK Trig Points
Bob,
It is supported by the Geographic Calculator. To use the grid shift in
the Calculator, there is an extra download that contains the OSTN02 grid
file needed. Both an evaluation version of the Calculator and the OSTN02
grid file can be downloaded from our website at:
http://www.bluemarblegeo.com/products/calculator.php?op=download
If you have any other data conversion issues let us know.
Patrick
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 -----
From: Jim Wilson
To: 'Bob Young' [email protected]
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 weare 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
email [EMAIL PROTECTED]
web www.soilessentials.com
www.cropcircle.eu.com
-------------------------------------------------------------------------------------
From: Bob Young [mailto:[EMAIL PROTECTED]
Sent: 15 August 2006 14:48
To: Tim Smith; [email protected]
Subject: Re: [MI-L] OT - UK Trig Points
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, andthe 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
www.MapsByDesign.co.uk
----- Original Message -----
From: Tim Smith
To: [email protected]
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 ourcarpark 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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