[Talk-GB] accurate GPS

[Russ Garrett]

On Wed, 9 Oct 2019 at 12:57, Simon Ritchie <simonritchie.uk at gmail.com> wrote:
> However, that still leaves the fundamental problem:   I can (and will) publish the kit of parts for making your own base station.  You could use something similar to build a rover or you could buy one off the shelf.  According to the ads this will give you an accuracy of 2 cm, but how will you check that you really are getting that accuracy?

At its heart, that's quite a complex question of metrology, geodesy,
and maths. But really I'm not sure you need to worry about it too much
with GPS. GPS receivers don't really exhibit constant errors (as I
mentioned before, the antenna may introduce some but I suspect they'll
be in the order of centimetres at worst), and the variable errors they
exhibit are well-characterised. Any GPS fix you get will have error
values provided with it, and you should be able to broadly trust those
numbers.

I'm sure there are commercial services which will give you a
calibration result against a known receiver, but they will likely be
expensive. Centimetre-level precision is close to the state of the art
in GNSS/GIS and so services will be priced accordingly.

The real question, really, is why you're aiming for that level of
precision. Relative accuracy (i.e. consistency of measured points
within a reasonably sized area) is much easier to achieve than
absolute accuracy (which is not even an especially well-defined
concept in this case).

If you're just making these measurements to put into OSM, you have to
realise that it's pointless to aim for accuracy better than 1m or so
in OSM, as it will degrade over time due to the use of the WGS84
coordinate system which doesn't take plate tectonics into account.

(Of course precision for precision's sake is a completely valid
endeavour in my opinion. But, as I mentioned before, that rabbit hole
can go extremely deep and is probably off topic here. I hope I've
given you a flavour of that though!)

Cheers,

-- 
Russ Garrett
russ at garrett.co.uk