I believe it's a matter of precision, here's an except from some content we've issued to a couple of our customers:
British National Grid
In the default FME reprojection
engine datum transformations to WGS84/ETRS89
are tied to the coordinate systems. The old BritishNationalGrid coordinate
system was tied to a transformation that is no longer current and has been
replaced with:
OSGB-GPS-1997
--> uses OSTN97 parameters to/from ETRS89 or WGS84
OSGB-GPS-2002
--> uses OSTN02 parameters to/from ETRS89 or WGS84
OSGB-GPS-2015
--> uses OSTN15 parameters to/from ETRS89 or WGS84 (the default choice new
in FME 2017.0)
Which you use is dependent on
when the data was captured and if you will be reprojecting in the workspace. Safe highlight that the only
difference between the OSGB-GPS coordinate systems is the inherent datum
transformation from OSGB36 to WGS84.
The reasons for the multiple
transformations:
- Plate
Tectonics - The OSGB36 datum is tied to the British Isles, while WGS84 is
a worldwide datum, with tectonic plate movements the transformation
between OSGB and WGS84 will change slightly.
- Quest
for Greater Accuracy - When the best GPS resolution was in the meter
range, the older less accurate conversion (used by BritishNationalGrid)
between its WGS84 coordinates and OSGB was sufficient. Now that GPS
resolution is much higher, a more accurate transformation is needed.
The advice is…
- If you are not reprojecting the data within FME, you can use EPSG:27700 without worrying about the other coordinate systems.
- If
you are reprojecting the data to WGS84 or ETRS89, Safe would
recommend using the GridInquestIIReprojector since it has the
highest accuracy transformation (new in FME 2017.0). (OSGB-GPS-2015
--> uses OSTN15 parameters to/from ETRS89 or WGS84 uses the NTv2
approximation very
close (<1mm) to the full OSTN15 transformation. The full OSTN15
transformation is available in the new GridInQuestIIReprojector
transformer).
There was a slight issue…
In
addition to these coordinate systems, FME 2017.0 introduced one other
approximation which was inadvertently
applied to any conversion between WGS84 and OSTN15. This was fixed in FME 2017.0 hotfix build 17275. The
issue was that approximation in the seven-parameter transform described here - https://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf
- is only good to a 3.5m accuracy. The intent was to use this instead of the
BritishNatGrid approximation (which is slightly worse) in one narrow edge case:
Workspaces authored prior to FME 2013, which reference British National Grid
via "EPSG:27700" or some other exception mapping, and which have not
been updated to set "Workspace Parameters -> Translation -> Implicit
CS-MAP Reprojection Mode" to "Auto".
I hope that helps.