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I have a question about the British National Grid (ESPG:27700) projection and how it's defined in both QGIS, ArcGIS and MapInfo. The go to GIS editor in my industry has been MapInfo. It seems however that some parts of the industry have been shifting to Arc and/or QGIS and I have been encountering shapefiles produced with either of these more and more frequently.

The problem lies in the fact that the hydraulic modelling software (Infoworks) that we use employs the MapXtreme engine in order to project model and GIS data in the Geoplan. The projection used for all data is the British National Grid (BNG). When a shapefile, saved in Arc or QGIS as BNG, is opened in Infoworks, the data doesn't line up with existing MapInfo TAB files which are also BNG but saved in MapInfo. I'll give an example to illustrate.

I have received a polygon shapefile, produced in ArcGIS, that I open in QGIS along with a background map TAB we already have, produced in MapInfo. All layers were saved in BNG originally. Upon opening in QGIS however, the software doesn't recognise either as being BNG and creates USER CRS for both.

The ArcGIS shapefile is imported with the following CRS:

USER:100000: +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

The MapInfo TAB file is imported with:

USER:100001: +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=375,-111,431,0,0,0,0 +units=m +no_defs

BNG is defined in QGIS as:

+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m +no_defs

While BNG in MapInfo is defined as:

"British National Grid\p27700", 8, 79, 7, -2, 49, 0.9996012717, 400000, -100000

I have tried saving the shapefile to BNG in QGIS as well as the USER:100001 CRS created when importing MapInfo TAB files, however when opened in Infoworks, it still doesn't line up. One option would be to reproject all the data (MapInfo files included) in QGIS. However, then the GIS wouldn't line up with the model in the Infoworks Geoplan, seeing as Infoworks still uses the MapXtreme engine.

As an additional note, after importing MapInfo TABs into QGIS and saving simply using the USER:100001 CRS that QGIS automatically assigns, the data won't line up any more. The only way I was able to get the ArcGIS shapefile to line up is by opening it in MapInfo and saving a copy with the projection set to BNG.

The problem with this is we are investigating moving to QGIS full time to avoid MapInfo or Arc licensing fees.

Is there a way that I can define a custom CRS that would enable me to save layers in QGIS that will line up with MapInfo data in a MapXtreme environment?

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    Sounds to me like errors in transformation. Do you know much about transformations? Are you using OSTN02? If not, google it now! – Matt Jul 14 '17 at 14:37
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    ... or look into gis.stackexchange.com/questions/40461/…. – AndreJ Jul 14 '17 at 14:54
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    What Matt and AndreJ said. See the +towgs84 parameters? See how they're zeroes in ArcGIS version, and have different values for the other two? It looks like Infoworks is actually using WGS84. – mkennedy Jul 14 '17 at 18:12
  • Thanks guys, especially AndreJ for linking to all that great info. One thing I still don't quite understand is the need for the correct transformation parameters when I am going from OSGB to OSGB. From what I can tell both MapInfo (and so MapXtreme I'm guessing) and QGIS define BNG with the same datum and ellipsoid. If that's the case, why would I need transformation parameters? And if I do need the correct transformation parameters, how can I find which ones MapInfo uses to go from OSGB to WGS? Let me know if I'm looking at this the wrong way. – Nicolas Stipo Jul 15 '17 at 14:20
  • The projection is the same, however the Datum is different. For an exact transformation of datum-shift you should use the NTv2 file. You can download it from here: ordnancesurvey.co.uk/business-and-government/help-and-support/… Check also these sites: osedok.wordpress.com/2012/01/17/… and digimap.edina.ac.uk/webhelp/digimapgis/…. – Wernfried Domscheit Jul 26 '17 at 8:05

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