[Qgis-user] Best practice when using UK national grid and GPS

[Greg Troxel]

Jim Jackson via QGIS-User <qgis-user at lists.osgeo.org> writes:

> QGIS version  3.40.6-Bratislava
> QGIS code branch Release 3.40

That's old, but probably not hurting you in this regard.

> As a member of a community Archaeology Group, I'm using QGIS for recording 
> archaeological features in the UK. I'm using Google Satellite, 
> OpenStreetMap and National Library of Scotland historic basemaps, which (as 
> I understand it) use EPSG:3857.

You'd have to ask NLS but surely those historic maps are not in 3857,
but something else, and they've transformed them for webmapping use.  In
the US, the USGS has done such transforms.  But sub-meter and historic
map don't go together.


> I have an RTK GPS receiver for collecting data points which are in 
> EPSG:4326. 

Really?  That sounds fishy.  What provider are you connecting to that is
sending reference data (via NTRIP and RTCM3 probably)?  How did they
establish base station coordinates in 4326?  Do they understand that
it's an ensemble, and that they are saying they don't know what datum
their base coordinates are really in?

If you set up your own base and did "survey in", you're probably pretty
far off.

> I also get UK National Grid data points from a colleague who has
> access to a total station.

You should ask your colleague to explain where their coordinates come
from, as total stations don't do absolute positioning.  Lots of
possibilities here.

Read this, and note that the grid is a projection from an underlying
datum, OSGB36:

  https://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid

I think you should be aiming to use "OSTN15" as a transform between
various modern datums and OSGB36 (well, it's for one modern datum, but
you will go through that datum from others).

> As I think I understand it, I need to set the project CRS to UK national 
> grid EPSG:27700 to get sensible distance measurements However I then get 
> occasional warnings "Used a ballpark transform from EPSG:27700 to 
> EPSG:3857" indicated less than accurate transformations, and then I get 
> lost!!!!

The measure tool should read in meters regardless.  3857 does not have a
single scale from coordinates to distance, but the tool should work.

If you are trying to measure distances between your RTK measurements and
the total station measurements, you're likely going to have errors.

However, using 27700 as a project CRS is sensible.  UKNG coordinates
should be close enough to meters - but use the measure tool.

> Can anyone point me to a tutorial on how to set up QGIS in the UK to get 
> accurate transformations in these circumstances (which can't be that 
> unusual).

There is no such thing as accurate data in 4326, if you care about a
meter.  It is an ensemble with an intrinsic uncertainty of 2m and it
makes zero sense to use it for RTK.  You will have to resolve that
problem.

Probably, the coordinates in your RTK system are in either some recent
ITRF or in some recent ETRF (<humor>but maybe the UK stopped using ETRF
due to Brexit?</humor>).

proj, and hence qgis, will use a "ballpark transformation" that is
typically low accuracy when one of the frames in the transform has
similarly low intrinsic accuracy.

If your RTK data meaasured in April is really "ITRF2020 epoch 2026.3",
then if you label it that way, and make sure you let proj load grids,
you should end up with accurate UKNG/OSGB36 coordinates.


I would recommend that you work with your total station colleague to
take your RTK setup to an OS passive control and measure its location
and see if you can get your values and the OS published values to line
up.

Or, establish some passive controls at the edges of your site and have
both of you measure each of them.

In the US (MA), I've measured points with RTK in "NAD83(2011) epoch
2010.0", our current national datum, and lined them up with imagery that
has been ground controlled to on the order of 10 cm and they match well.
So I'm pretty confident in my RTK-derived coordinates.