@ Grant Slater
Thank you for spotting this and making it known.
One thing you need to know when using an RTK base station, is what is the
reference (datum). Unfortunately there is no convention for how this
information is given, and it is often not given at all. The stations listed by
Grant are on the Eurasian tectonic plate so the reference will be either ETRS
or ITRS ("WGS 84"). WGS 84 is the datum used in OSM.
I have previously connected to several of the stations on the list: DARE, HERS,
HERT and SHOE. All of these are on ETRS although HERS has a small position
error. I tried connecting to LICC and it is on ITRS. (I estimate there is a
position error, relative to ITRS, around 12cm too far north, 8cm too far west
and 31cm too high.) I think a mistake has been made in configuring the LICC
station. Incidentally, LICC is at Imperial College in South Kensington. There
is a site information page at
http://epncb.oma.be/_networkdata/siteinfo4onestation.php?station=LICC00GBR If
you click on Data Provided you can see any warnings that have been logged. It
was the warning here about a position error that prompted me to check it out.
The website I just referred to evidently expects the reference to be ETRS.
The stations on Grant's list are of a type known as Continuously Operating
Reference Stations (CORS). Stations of that type would be expected to produce
results consistent to the millimetre. The ITRS position of LICC shifts by a
millimetre every two weeks, so I hope they have an automated system, or at
least a simple system, for updating the position it is broadcasting.
If you have a consumer SatNav you probably can't tell the difference between
ETRS and ITRS. But if you are using RTK you certainly can tell the difference.
In south-east England, at the time of writing, ITRS gives a position 52.0cm
further north and 53.5cm further east than ETRS. This gives a horizontal
distance of 74.6cm. The horizontal distance increases by 2.4cm per year. The
altitude difference is less than 0.2cm.
If you record a tracklog using RTK from seven of the eight stations in the
list, the tracklog will be in ETRS. You will need to convert it to ITRS for use
in OSM. If the tracklog covers a small area, you can just apply a fixed offset
to the latitudes and longitudes. Unfortunately I don't know of any tool which
makes this simple to do.
Someone in France has organised funding for an independent network of open RTK
base stations. (The availability of free RTK base stations was even worse in
France than in the UK.) See https://centipede.fr/ They have produced detailed
instructions for setting up a base station, including a shopping list, how to
assemble the equipment, preconfigured software, and how to obtain the position
of the base station to within a couple of centimetres. It also covers setting
up a receiver for RTK. They have set up a server to broadcast all the streams
and there are already several dozen stations in operation. They will soon be
prevented from setting up a VRS, only by the cost of the software for doing
that. The documentation is all at https://jancelin.github.io/docs-centipedeRTK/
It is in French, but for those who don't speak French, I expect a well-known
online service would produce an adequate translation. There is a subscription
RTK stream service covering many countries, which professionals use. They no
longer quote prices on their web site, but when they did, the entry-level
subscription for real-time access was around £4000 per year. IIRC, that gave
the subscriber access to a maximum of five simultaneous VRS streams and
included access to a two-way satellite link, for areas where there was no
mobile phone signal.
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