British National Grids MI 10

[Jamie]

Whats the difference between british coordinate systems?

British National Grid [EPSG:27700]
British National Grid (1mm accuracy)
British OSGRS80 Grid

[Adrian Gray]

Jamie,

 

my take on it is:

 

* British National Grid [EPSG:27700] - MapInfo's default projection which is accurate to between 6 and 13mm

 

* British National Grid (1mm accuracy) - Add to MapInfo Pro more recently to cope with the 1mm accuracy level of OS MasterMap and the translators which were outputing to this level

 

* British OSGRS80 Grid - A British National Grid created in 1980 but not as accurate as the previous two. Don't know any more about it than than or anyone who uses it.

 

Hope this helps

 

Regards

Adrian Gray

Viewshed Limited

http://www.viewshed.co.uk/

t: 01392 861946 f: 01392 861947

Orchard House Church Road Silverton Exeter EX5 4PD

registered in England and Wales: 5661115 VAT# 876 6186 72

 

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[Tim Rideout]

I am never sure what the OSGRS80 Grid is, but the other two are easy:

They are both the Standard OSGB 1936 National Grid. The difference is
down to MapInfo only working to 8 significant digits, so on the 1mm grid
the bounds are much smaller, literally just enough to fit Great Britain.
This reduces the digits taken up by the number of metres and allows more
for the decimal places. Thus the standard grid is only to about the
nearest 10cm, while the latter is to 1mm.

Note there is a difference between these two grids in MapInfo 10.5 and
all older versions. Buried in 10.5 is the OS Translation Matrix which
will affect how co-ordinates are converted into and out of the National
Grid. This is because the OS co-ordinates are actually still in the
wrong positions despite the accuracy improvement programme so they have
a rubber sheeting matrix to apply a further correction. If you convert
your data from Lat / Long to NG in 10.5 your answer is therefore
strictly speaking wrong for the Transverse Mercator projection while it
would have been right in all older versions of MapInfo (but not fitting
the 'wrong' OS data so well). If you are not using OS data and want
great accuracy then you might remove the OS Translation Matrix so that
the rubber sheeting errors are not introduced to your data. I believe
you have to fiddle with the MapInfo .ini files or some such to disable
the setting and use the 10.0 method instead.

Tim

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[CDR Group]

Hi Jamie

 

After a couple of replies on the list, I can't help thinking why you want (need) to know.

 

Essentially, if you are using Ordnance Survey large scale mapping then you should be

using the 1mm accuracy projection and stick with it.

Set it as your default projection.

Problems will arise if you have tables in the different projections - so just stick to the one.

 

And no - Ordnance Survey data is not accurate to 1mm  !!!! (Adrian) ;-)

 

Regards

 

John Ievers

CDR Group
www.cdrgroup.co.uk
www.puredotnet.co.uk
Tel: 01433 621282
Fax: 01433 621292
Specialists in Geographic Information Systems

CDR Group is the trading name of Contract Data Research Ltd.
Registered address Eccles House, Eccles Lane, Hope, Hope Valley, S33 6RW.
Registration No. 1972326
VAT No. GB373 3117 67

----- Original Message -----

From: Jamie

To: MapInfo-L

Sent: Thursday, September 09, 2010 3:06 PM

Subject: [MI-L] British National Grids MI 10

[Adrian Gray]

John,

 

you are correct OS data is not accurate to 1mm, but the By Design OS MasterMap translator (MapGML) and later the DottedEyes translator  (InterpOSe) where converting the data to the 1mm projection before MapInfo Pro officially supported it. I personally think it was for this reason that we saw the inclusion of the 1mm projection in MapInfo Pro. Otherwise we'd still be on the default BNG.

 

Regards

Adrian Gray

Viewshed Limited

http://www.viewshed.co.uk/

t: 01392 861946 f: 01392 861947

Orchard House Church Road Silverton Exeter EX5 4PD

registered in England and Wales: 5661115 VAT# 876 6186 72

 

MapInfo Professional, MapBasic and MapXtreme training, off-the-shelf and bespoke application development

 

miFormz Lite - Generic forms application for MapInfo Professional and MapInfo Professional RunTime Manipulators from v6 to the latest. Your forms can include Drop Down Lists, Check Boxes, Radio Buttons, List and Multi-List Boxes populated from Value Lists or from table lookups. Plus data validations: Dates, Integers, Small Integers and Decimals. Download a shareware version from our website.

 

Got an LLPG? - try FuzzyLLPG for FREE. It builds a layer of your addresses, has a fuzzy search engine enabling you to search for any part of an address, it can merge tables together based on the UPRN. Version 2 now comes as a web service. Download shareware versions from our website!

 

[Jamie]

Thank you for your reply's its what i expected.

Has anyone had any problems when creating tables using the new 1mm
accuracy in MI 10 then opening these files in earlier versions ie. MI
7.5 where this projection wasn't an option?

[Andy Harfoot]

Hi James,

The 1mm version of the British National Grid is compatible with all
versions of MapInfo supporting the default projection and bounds, as the
difference between them is only in the bounds clause.

The 1mm version can be added to earlier versions of MapInfo by editing
the Mapinfo.prj file in the MapInfo program directory (make a copy
first!). Find the line

"--- British Coordinate Systems ---"

and then paste in the following line beneath (note this should all be on
one line, but may be split in the email).

"British National Grid (1 mm accuracy)", 2008, 79, 7, -2,
49,0.9996012717, 400000, -100000, 0, 0, 2000000, 2000000

Andy

--
Andy Harfoot

GeoData Institute
University of Southampton
Southampton
SO17 1BJ

Tel: +44 (0)23 8059 2719
Fax: +44 (0)23 8059 2849

www.geodata.soton.ac.uk

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