[Talk-GB] accurate GPS

Warin 61sundowner at gmail.com
Sat Oct 12 10:26:11 UTC 2019

On 10/10/19 00:40, Simon Ritchie wrote:
>     The real question, really, is why you're aiming for that level of
>     precision
> That's what the emerging equipment does.

For 'precision' i.e. repeatability then simply monitor the indicated 
position over a short time frame - where the satellites used remain 
constant. The change in position is your 'precision'.

This does not take into account the corrections that are needed for each 
satellite that change with time, and corrections for the atmosphere that 
changes with time.
So I expect the data you collect will be worse that the actual equipment 
performance due to the lack of these corrections. Unless you pay real 
money then your stuck with that.

> I'm just planning on showing how it can be put together, but I'd like 
> to be able to say with confidence that it works properly.
> As to who will use it, there's the readers of this forum, or some of 
> them, and surveyors and architects, of course. There's also 
> archaeologists, because they are required to log where they find 
> objects, and they use GPS trackers to do it.  They often leave objects 
> in the ground to protect them, and then come back a few years later to 
> have another look using new techniques.  It would be nice if they knew 
> precisely where their target is. They would only have to dig a small 
> hole to find it.

Usually there are 'bench marks' that professional surveyors used (past 
tense in the UK) as a stating point to do their surveys. I know where 
some of the ones around me are .. but I have no UK source as I don't 
know the local terminology there. These are far more numerous that the 
trig points of OS.
> I note your point about plate techtonics. My local archaeology group 
> recently re-excavated a site that was first excavated a hundred years 
> earlier.  The records they had turned out to be quite misleading.  
> That was due to poor record keeping, but I guess over that time, the 
> UK might have moved around a bit.  I recall that one end is rising and 
> the other is sinking.
> However, when new equipment comes along, people find new uses for it.  
> We moved house a few years ago and I saw our Land Registry documents.  
> I was quite surprised at the rudimentary map that is the legal 
> definition of our property.  I'm supposed to resolve a boundary 
> dispute with this? Now that land is so valuable, I can see people 
> demanding better, so the estate agent will walk around the boundary 
> with a GPS device and the result will be logged with your land 
> registry records.
> In the future I can also see architects putting GPS coordinates on 
> plans, and builders using accurate GPS devices to do the initial  
> layout of the site.  At 2 cm accuracy, they will probably have to 
> tweak the positions using better instruments, but if GPS speeds up the 
> process or makes it more reliable, they will use it.
> .

An architect will work off plans, title deeds etc. They won't specify 
lat/lon but the position on the land parcel, usually a distance from a 

A  builder will employ a surveyor to get the thing located correctly, 
unless they are certain of a boundary.
Builders work with tape measures not GPSes. They use tape measures now. 
There is no improvement by going to a GPS and increased cost of 
equipment and paying for the correctional data. Why would they add costs 
to their business for no benefit???

> Given the inaccuracy of the trig point locations, I can't even do that :(

The trig points are accurate. But need corrections applied that are time 
variable. A good old school surveyor could do it...

Your equipment without corrections will not be as accurate as the trig 

Without corrections what you could measure is precision and that is that.

Without spending the money on the corrections you may as well buy the 
lower performance level equipment .. the overall result will be the same 
and it should cost less.
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