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I have just come across something bizarre and I can't explain it. I have programs for creating grid squares and for converting points to polygons based on an OS grid reference [GB BNG]. Recently we upgraded to version 10 having previously been on version 7. Some data needed updating based on a new protocol for the disclosure of a protected species. The data, using a new easting and northing was given a point and then run through the same polygon generating routine which creates a polygon based on the accuracy of the grid reference.

When projected on current data and grids the newly created data is not aligned correctly. On further investigation all our data appears west 5 meters and 222 metres south of where it should be in MapInfo 10 but is in the correct position in MapInfo 7.

I cannot see what it is that could be different or changed. All the data created in MapInfo 7 appears in the correct position but if it is generated in MapInfo 10 it is not. We've checked alignment against on-line OS data and what we seen in MapInfo 7 is correct!

Some have suggested that MapInfo 10 is more accurate but a metre is still a metre and 10 metres from the projection origin is still the same 10 metres.

Any thoughts?

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2 Answers 2

Coordinate System Transformation Improvements for the UK This release [10.0.1] significantly improves the accuracy of the coordinate system conversion between WGS 84 (and EUREF89) based coordinate systems and British National Grid. This improvement is achieved by using the NTv2 datum conversion algorithm between OSGB36 and WGS 84(and EUREF89) datum. If you need to disable this NTv2 datum conversion, open the ntv2.xml file in the MapInfo application folder and make the following entries "false".

Source: PDF "MapInfo Professional 10.0.1 Maintenance Release Notes" http://reference.mapinfo.com/software/mapinfo_pro/english/10.0.1/MapInfoProfessionalReleaseNotes_1001.pdf bottom of page 5

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I regularly do the same operation - take a list of alphanumeric BNG references with varying levels of precision (eg SU4310, SD251670) and convert them into squares with dimensions appropriate to the precision of the grid reference. I have done this on most versions of MapInfo from 6.0 to 10.5 without noticing any offsets between versions of the type that you have seen. The MIv10+ datum conversion has not been disabled, but as the process should use BNG throughout, I wouldn't expect that to make a difference. All I would suggest is that you look at your tool source code if you have access to it - could you make it available?

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