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

[Greg Troxel]

Jim Jackson <jj at franjam.org.uk> writes:

> On Fri, 1 May 2026, Greg Troxel via QGIS-User wrote:
>
>> 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.
>
> Ok. I didn't set the CRS I just used the XYZ url they advertise in XYZ 
> tiles and I get the maps and the CRS shown for the layer says EPSG:3857

I meant to say that probably this is ok.

>> > 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)?  
>
> Yes yes, but the output to the app is WGS84 lat lon co-ordinates.

I remain skeptical.

>> 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?
>
> Ok so I load GPX data saved by my phone app using this external GPS 
> receiver into QGIS and as if by magic it has a CRS of EPSG:4326 WGS84
> I didn't do anything.

It's not magic but it's probably wrong.  The GPX specification says
coordinates are in WGS84.  But that doesn't make it so.  The
coordinates, if they are solutions from an RTK receiver, are in the
reference frame of the base station.  The RTK receiver has no idea what
this is.  They get put in the GPX, and it gets added with a default.

To get this right you need to understand how the base station is set up.

>> If you set up your own base and did "survey in", you're probably pretty
>> far off.
>
> Nah used a nearby "centipede-rtk.org" base station.

That actually looks pretty careful.  (I'm using survey-grade bases, run
by my state's transportation department, a number of which are also in
the NGS national network.  So yes, I'm spoiled.)

I looked at their site, and was able to click on various stations and
get coordinates and reference frames.  I saw NAD83 in the US, and
ETRF2000

I picked at semi-random BALL, just east of Inverness, and the data are
in ETRF2000, not WGS84.  This makes sense; ETRF2000 is a crust-fixed
frame.

  Geo Coordinates ETRF2000
  Latitude :    57.49530272
  Longitude : -4.11980587
  Height :     103.353


All the "declared" bases I checked were ETRF2000.  The non-declared
(blue) ones did not have a datum.

>> You should ask your colleague to explain where their coordinates come
>> from, as total stations don't do absolute positioning.  Lots of
>> possibilities here.
>
> Total station triangulation in from known UK OS grid points, then surveying
> and recording UK OS grid points.

That's probably good then.

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

You'll need to explain more what's going on.  There is a toggle for map
units vs something else, and I've had no trouble asking for meters.

>> 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.

I stand by and emphasize this advice.



I suggest labeling the data from your receiver as ETRF2000 and see how
that goes.