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I'm historically a user of MapInfo Professional but have recently started using FME and have been talked into now testing out a copy of QGIS.

The main problem I've come across is that the coordinate systems used in QGIS do not appear to match what is produced by the other two?

For example, setting both MapInfo Professional and FME to BNG seems to generate a CRS of: +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

This appears to differ form QGIS's interpretation of BNG which is; +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

I'm going to admit that I have no idea what the towgs84 parameter relates to but the differences appear to cause a few meters of discrepancy when looking at features on the ground. Much as I like the simplicity of QGIS this could affect any decision to move forward with adopting the software in any capacity.

Could anybody provide me with a layman's explanation of what is going on?

Many thanks

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    You can add your own custom projection in FME in (for example) C:\apps\FME\2013_SP4\Reproject >edit MyCoordSysDefs
    – Mapperz ♦
    Apr 7, 2014 at 14:21
  • Can you (or someone) provide the correct EPSG code for BNG?
    – til_b
    Apr 7, 2014 at 14:34
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    The correct EPSG code is 27700.
    – mkennedy
    Apr 7, 2014 at 21:50

1 Answer 1

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towgs parameter is actually a 7 parameter datum shift from the CRS in question to WGS84 (EPSG:4326), because WGS84 is commonly used as a kind of a "base datum", to which all other CRSs have known 7 parameters transformations. Why the definition in QGIS is different is hard to say, but in QGIS you could easily create a custom CRS which would be defined exactly like the one in FME/Mapinfo.

The parameters are 3 translations in meters, 3 rotations usually in angle seconds and third is scale change given in ppm (in 10^-6 meter).

The parameters from software you are used to seem a bit generic (no decimal parts for translations, angles are all 0, no scale change. This system feels like a translated WGS84, and nothing else) and i would say that the ones from QGIS would probably be correct.

But these parameters only come into play if you would for example be loading GPS data to the project, which is WGS84 based of course, then the data transformed with the given 7param transformation would of course be incorrect, or better... it would be different.

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  • Many thanks for this, think that I'll also post on the FME forum as looking at the CRS details within the software it does seem to indicate parameters more in line with those in QGIS. It's entirely possible that I'm not applying these correctly to my output files?
    – neoDaz
    Apr 7, 2014 at 14:39
  • What kind of data are you trying to load into a QGIS project, (what kind of CRS does it have) and what is your current project CRS?
    – U2ros
    Apr 7, 2014 at 15:08
  • MapInfo TAB files either generated by MapInfo Professional internally or sent to us by 3rd parties, also TAB files generated by FME in translating from Ordnance Survey provided GML. I've looked into this a bit further the the TAB files in question do appear to have decimals, angles and scale defined when looking through the FME data viewer but QGIS appears to indicate otherwise. Not sure which one is telling the truth!
    – neoDaz
    Apr 7, 2014 at 15:19
  • I would perhaps try translating the TAB file into something more simple... shapfile (with FME), then i would erase the .prj file that is created along (contains CRS data), and set the CRS manually when added to QGIS (to BNG). Btw, are you sure this is the right CRS, there may be more version of it available in QGIS?
    – U2ros
    Apr 7, 2014 at 15:22
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    The FME/MapInfo one is OSGB 1936 to WGS 84 (1), defined by US military (DMA now NGA), EPSG:1195, roughly 21 m accuracy. The QGIS one is EPSG:1314, OSGB 1936 to WGS 84 (6), defined by UKOOA, around 2-4 m accuracy.
    – mkennedy
    Apr 7, 2014 at 22:06

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