I’m sure I’ve mentioned here before that our approach to managing coordinates anywhere in the world is to use WGS84 coordinates to store locations in the database. Recently we changed the way we project them to OSGB coordinates. This post gives a bit more detail about how we do the coordinates projection.
tl;dr We’ve changed from using OSTN02 to using OSTN15 coordinate projection. It will have changed your coordinate projections by +/- 0.025m (2.5cm) but you probably didn’t notice.
OSTN Transformations
Both OSTN02 and OSTN15 are transformations between WGS84 coordinates in latitude & longitude and projected coordinates in Easting / Northings on the Ordnance Survey grid. Both transformations work through an Ordnance Survey published a datafile containing the transformation for formula each point within GB.
Our software includes these transformations for use in GB (as well as other projections for use in other regions).
The data files for OSTN02 conversions and OSTN15 conversions can be found on the Ordnance Survey website.
What does this mean?
The main result of this change is that we are now aligned with new OS model and this can be stated on all your processing. There are slight changes in horizontal transformation (table below) and slightly bigger (but still not significant) changes in vertical transformation (picture below).
Please note that we do not use the transformed GPS position to calculate height in the TrueViewVisuals app as the local DTM model gives a better result.
| East | North | |
| Min Change | -0.037 | -0.015 |
| Max Change | 0.019 | 0.018 |
| RMS | 0.009 | 0.007 |
None of these changes is greater than 0.3m and as such will result in an insignificant change in TrueViewVisuals software.
This has resulted in a small average shift in coordinates of less than 1cm, but the height has changed by more than this – about 2.5 cm on average on mainland Britain
Blog post on new OSGM geoid – source
The update to use OSTN15 has been released on TrueViewVisuals app and portal, so if you’ve been using our app in the last couple of months then you will have already been using this change (and probably not noticed).