Re: [Talk-GB] accurate GPS

[Warin]

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 
boundary.


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 
points.


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.
___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Dave F via Talk-GB]

Oh, is /that/ what he's doing?

On 10/10/2019 17:47, Jez Nicholson wrote:

*Ahem* no offence to Simon, obviouslyhe's just trying to check out a
manufacturer's claims and opening a can of worms in the process.

On Thu, Oct 10, 2019 at 5:18 PM Dave F via Talk-GB <
talk-gb@openstreetmap.org> wrote:


On 09/10/2019 23:12, Warin wrote:

I'd think to get that level of accuracy you 'd need readings over some
considerable time... days?

Otherwise you get bias from, as you hint, the atmospheric conditions,
the satellites in view - their bias, angles ..
Unless you have access to correction data, say from a local fixed GPS.


Indeed.
For Simon to assume he got a single 2cm "accuracy" let alone
consistently is naive. To believe it usurped OS's reading is silly.

DaveF

___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb




___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Jez Nicholson]

*Ahem* no offence to Simon, obviouslyhe's just trying to check out a
manufacturer's claims and opening a can of worms in the process.

On Thu, Oct 10, 2019 at 5:18 PM Dave F via Talk-GB <
talk-gb@openstreetmap.org> wrote:

> On 09/10/2019 23:12, Warin wrote:
> >
> > I'd think to get that level of accuracy you 'd need readings over some
> > considerable time... days?
> >
> > Otherwise you get bias from, as you hint, the atmospheric conditions,
> > the satellites in view - their bias, angles ..
> > Unless you have access to correction data, say from a local fixed GPS.
> >
>
> Indeed.
> For Simon to assume he got a single 2cm "accuracy" let alone
> consistently is naive. To believe it usurped OS's reading is silly.
>
> DaveF
>
> ___
> Talk-GB mailing list
> Talk-GB@openstreetmap.org
> https://lists.openstreetmap.org/listinfo/talk-gb
>
___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Dave F via Talk-GB]

On 09/10/2019 23:12, Warin wrote:


I'd think to get that level of accuracy you 'd need readings over some 
considerable time... days?


Otherwise you get bias from, as you hint, the atmospheric conditions, 
the satellites in view - their bias, angles ..

Unless you have access to correction data, say from a local fixed GPS.



Indeed.
For Simon to assume he got a single 2cm "accuracy" let alone 
consistently is naive. To believe it usurped OS's reading is silly.


DaveF

___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Warin]

On 09/10/19 22:57, Simon Ritchie wrote:
> You'll need a GPS receiver with the capability of outputting carrier 
phase data (u-blox receivers will do this) and ideally a 
well-characterised external antenna (these are quite expensive).


That's very useful.  Thanks.  I am indeed using a uBlox device as my 
base station.  I ran it for 48 hours and got accuracy to maybe half a 
metre, but I haven't tried post-processing thedata.


What do you mean by the term 'accuracy'?

Ubox use the term 'precision' which in metorology means repeatability in 
common language.


Something can be highly repeatable but be a long way from the true value.

In this instance ubox are only evaluating their chip.
In order to obtain the best measurement you need an expensive antenna, 
pay for the correction data at the time of measurement and apply that 
correction data.


Claiming a 2 cm accuracy without stating the need to apply the 
corrections is misleading.

Ubox claim a 'precision' - not the same thing as 'accuracy'.

Be very careful in your use of terms, some people will become very upset 
if you claim a 2 cm 'accuracy' when all you are giving them is 'precision'.

I would suggest you research metrology terms.
You might start with https://www.npl.co.uk/resources/gpgs ?

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?


True 'accuracy' can only be determined when you know the 'true value'.

Ultimately no one can really determine the 'true value' they can only 
estimate it and, if they are metorologists, give an uncertainty 
statement on that value.




___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Warin]

On 09/10/19 22:03, Dave F via Talk-GB wrote:

2cm? I'm intrigued, what model are you using?
What were the atmospheric conditions on the day you took your reading?


I'd think to get that level of accuracy you 'd need readings over some 
considerable time... days?


Otherwise you get bias from, as you hint, the atmospheric conditions, 
the satellites in view - their bias, angles ..

Unless you have access to correction data, say from a local fixed GPS.

___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Warin]

On 09/10/19 22:42, Peter Neale via Talk-GB wrote:
... and if you had 2 devices, how would you know which is right? You 
would need at least 3 devices, so that you could take a majority vote.


Actually 5 would better


6 is general taken as a minimum number to get a good student's T

And they should be independent as far as possible.

So chose times when there are different satellites in use.

And chose the different systems, GPS then GLONASS



Or 7, or 9


More is better - improves student's T but diminishing improvement.
___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[David Woolley]

On 09/10/2019 14:40, Simon Ritchie wrote:
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.


For that, you really need to record one or more reference points that 
are likely to stay there and keep their relative position in the long 
term, as what you are interested is the location relative to the 
surrounding, not an absolute position on the globe.



___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Jez Nicholson]

BTW if you have any details to add on the relevance of trig points to OSM
then please do add to
https://wiki.openstreetmap.org/wiki/Ordnance_Survey_triangulation_stations

On Wed, Oct 9, 2019 at 2:41 PM Simon Ritchie 
wrote:

> The real question, really, is why you're aiming for that level of precision
>
>
> That's what the emerging equipment does.  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.
>
> 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.
> .
>
>> 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).
>>
>
> Given the inaccuracy of the trig point locations, I can't even do that :(
>
> Regards, Simon
> ___
> Talk-GB mailing list
> Talk-GB@openstreetmap.org
> https://lists.openstreetmap.org/listinfo/talk-gb
>
___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Simon Ritchie]

>
> The real question, really, is why you're aiming for that level of precision


That's what the emerging equipment does.  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.

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.
.

> 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).
>

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

Regards, Simon
___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Russ Garrett]

On Wed, 9 Oct 2019 at 12:57, Simon Ritchie  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
r...@garrett.co.uk

___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Simon Ritchie]

> You'll need a GPS receiver with the capability of outputting carrier
phase data (u-blox receivers will do this) and ideally a well-characterised
external antenna (these are quite expensive).

That's very useful.  Thanks.  I am indeed using a uBlox device as my base
station.  I ran it for 48 hours and got accuracy to maybe half a metre, but
I haven't tried post-processing thedata.
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?

That question was the driver for my original posting.

Regards, Simon
___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Andy Robinson]

I guess you could do that anyway. Most folks are only interested in relative 
position and not whether one point on one side of the globe has the same 
positional accuracy as a point on the opposite side. Surveyors have for 
centuries been relying on a local arbitrary reference point and still do today, 
even if the measurements nowadays are generally completed using GPS.

 

Cheers

Andy

 

From: Simon Ritchie [mailto:simonritchie...@gmail.com] 
Sent: 09 October 2019 12:32
To: Russ Garrett
Cc: Talk GB
Subject: Re: [Talk-GB] accurate GPS

 

> You're not going to find a (publically-accessible) physical location which 
> has better location error than 1m or so.  

 

That was the kind of conclusion that I was coming to.  There's the meridian 
line at Greenwich, but that only gives one coordinate.  It's a pity that they 
don't have a crosshair with a published position.

 

I think that the only way forward may be to get hold of another accurate GPS 
device and compare results.  Unfortunately, the others tend to be quite 
expensive.  Trimble have a cheap deice called the Catalyst, but you still have 
to buy their correction service at £300 per month.

 

If I did have such a device I could to my local trig points, get  accurate 
positions and publish them.  I could use those data to test and calibrate my 
"budget" solution. 

 

Regards

 

Simon

 

___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Russ Garrett]

On Wed, 9 Oct 2019 at 12:31, Simon Ritchie  wrote:
> > You're not going to find a (publically-accessible) physical location which 
> > has better location error than 1m or so.
> That was the kind of conclusion that I was coming to.  There's the meridian 
> line at Greenwich, but that only gives one coordinate.  It's a pity that they 
> don't have a crosshair with a published position.

Sadly I have to be the bearer of bad news there as well: the marked
meridian line at Greenwich no longer corresponds to a fixed coordinate
in any modern coordinate system! The WGS84 zero degree meridian is
approximately 100m to the east but that too will move due to
continental drift and other esoteric gravitational factors.

> I think that the only way forward may be to get hold of another accurate GPS 
> device and compare results.  Unfortunately, the others tend to be quite 
> expensive.  Trimble have a cheap deice called the Catalyst, but you still 
> have to buy their correction service at £300 per month.

Getting to 2cm accuracy will be tricky by any mechanism, but there are
cheaper solutions if you're willing to do some work. Broadly:

You'll need a GPS receiver with the capability of outputting carrier
phase data (u-blox receivers will do this) and ideally a
well-characterised external antenna (these are quite expensive).
You can then fix this antenna somewhere and record several days worth of data.
This data can be post-processed with RTKLIB (http://www.rtklib.com/)
using the RINEX atmospheric correction data from OS
(https://www.ordnancesurvey.co.uk/gps/os-net-rinex-data/) - this data
is free for historical use, it's the realtime atmospheric corrections
which cost the money.

The combination of averaging, phase measurement, and atmospheric
correction should at least get you sub-20cm.

-- 
Russ Garrett
r...@garrett.co.uk

___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Peter Neale via Talk-GB]

... and if you had 2 devices, how would you know which is right? You would need 
at least 3 devices, so that you could take a majority vote. 
Actually 5 would better
Or 7, or 9 


Peter

Sent from Yahoo Mail on Android 
 
  On Wed, 9 Oct 2019 at 12:34, Simon Ritchie wrote:  
 ___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb
  
___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb
___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Simon Ritchie]

> You're not going to find a (publically-accessible) physical location
which has better location error than 1m or so.

That was the kind of conclusion that I was coming to.  There's the meridian
line at Greenwich, but that only gives one coordinate.  It's a pity that
they don't have a crosshair with a published position.

I think that the only way forward may be to get hold of another accurate
GPS device and compare results.  Unfortunately, the others tend to be quite
expensive.  Trimble have a cheap deice called the Catalyst, but you still
have to buy their correction service at £300 per month.

If I did have such a device I could to my local trig points, get  accurate
positions and publish them.  I could use those data to test and calibrate
my "budget" solution.

Regards

Simon

>
>
___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[ael]

On Wed, Oct 09, 2019 at 11:49:35AM +0100, Russ Garrett wrote:
> On Wed, 9 Oct 2019 at 11:40, Andy Robinson  wrote:
> > Are you using trig points that are also OS Net station locations? 
> > https://www.ordnancesurvey.co.uk/legacy/docs/gps/OSNet_GPSWebSite_Coordinates_File.txt
> 
> Pretty sure that all those OS Net locations are actually fixed GPS
> receivers rather than trig points, and so not something you can go and
> plonk your GPS receiver on. The file definitely seems to list the
> model of receiver at each one.

I have visited and mapped a few of the passive stations which are
usually small pillars a few cm high. Most (all?) are now "legacy"
but they ought to be much more accurate than the much older trig
points.

ael


___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Simon Ritchie]

I believe that the original data from the OS is completetrigarchive.zip.
Ian Harris created the trigpointing .uk website from that.  The map
references for the three trig points I used are from that website, and yes,
you can walk up to those trig points and plonk your GPS device onto them.
The problem, as I said, is that you don't know quite where they are.

The OS no longer use most of the concrete trig points scattered around the
country, they use accurate GPS, in fact I believe that they sell some of
their data to companies like Trimble and Leica, who sell it on for a hefty
monthly fee.  I think you are referring to their list of GPS stations.

Regards

Simon

On Wed, Oct 9, 2019 at 11:49 AM Russ Garrett  wrote:

> On Wed, 9 Oct 2019 at 11:40, Andy Robinson  wrote:
> > Are you using trig points that are also OS Net station locations?
> https://www.ordnancesurvey.co.uk/legacy/docs/gps/OSNet_GPSWebSite_Coordinates_File.txt
>
> Pretty sure that all those OS Net locations are actually fixed GPS
> receivers rather than trig points, and so not something you can go and
> plonk your GPS receiver on. The file definitely seems to list the
> model of receiver at each one.
>
> Cheers,
>
> --
> Russ Garrett
> r...@garrett.co.uk
>
___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Dave F via Talk-GB]

2cm? I'm intrigued, what model are you using?
What were the atmospheric conditions on the day you took your reading?

DaveF

On 09/10/2019 11:05, Simon Ritchie wrote:

I've been working with some GPS equipment that claims to be accurate to
2cm.  To test it, I've been visiting local OS trig points, taking position
measurements and checking if they are correct.

Unfortunately I've discovered that the data I'm getting from the OS is not
nearly as accurate as my equipment claims to be, which is wrecking my
testing.

We tend to assume (well, I do anyway) that OS trig points are very accurate
position markers, but compared with modern equipment, that's no longer so.
I thought people might be interested in knowing how accurate they are.

A related issue is this:  GPS devices don't work in terms of OS map
references.  If your tracker device gives you a position in that form, it's
done a conversion.  How accurate is that?

The GPS device in a typical tracker is accurate to maybe three metres, so
the position you see on the screen will always be a bit wrong.  If you get
it to display your position in OS map reference form, it will need to do a
conversion, which introduces an extra error, so the result will be even
more wrong.  Not good if you are trying to produce an accurate map.

The OS published a spreadsheet giving the positions of their trig points in
OS map references.  This is available from them as a spreadsheet and Ian
Harris has used that data to create the web site:http://trigpointing.uk

The OS also offer a web page that can convert this to other forms including
Cartesian, which is one of the forms that my GPS device gives me:
https://www.ordnancesurvey.co.uk/gps/transformation/

To test my equipment, I take the OS map ref of a trig point, convert it to
Cartesian form,  visit the trig point, get the position in Cartesian form
from my device and compare the two.

The results are typically out by at least half a metre.  Is my equipment
faulty, or is the OS data wrong.  How accurate is the published position of
the trig point and, when I use the OS web page to convert that to Cartesian
form, how accurate is thatt?

This OS document was very enlightening:
https://www.ordnancesurvey.co.uk/legacy/docs/gps/updated-transformations-uk-ireland-geoid-model.pdf
It explains how the Cartesian coordinates work, which is useful.  It
reminds me that OS maps pretend that the Earth is flat, which introduces an
error, but that's tiny, and for my purposes itcan be ignored.  It explains
how accurate you can expect the published measurements of trig point
positions to be - they can be out by as much as 60 cm!  In general, the
document stresses that there is no sure-fire way to convert a position from
one system to another.  The result will always be inaccurate.

So now I know that the published positions of the trig points are a bit
wrong, but how accurate is the conversion from OS map ref to Cartesian form?

OS map references plus height above sea level and Cartesian coordinates
both specify a position using a 3D coordinate system.  The origin and the
direction of the axes are different in each system so you can't compare thm
directly.  However, the distances between two points should be the same
regardless of which system you use.  If you have two points in the same
coordinate system (a1,b1,c1) and (a2,b2,c2) and the difference along each
axis is a,b and c then the distance between them is

 the square root of (a squared plus b squared plus c squared) by
Pythagoras

If you have two points in a different coordinate system representing the
same two positions, the distance between them should be the same.

So I can test the accuracy of the conversion from OS map references to
Cartesian.  In the table below, on the left, we have the trig points at Box
Hill and Leith Hill in OS map reference form, the difference along each
axis and below that the resulting distance.  On the right we have the same
calculation but using the Cartesian coordinates from the OS conversion page.

Below that I do the same comparison, this time using the trig point at
Mickleham Down and the one at Leith Hill.

In both cases, the distances are out by over two metres.

So, I'm trying to test equipment which is supposed to be accurate to two cm
using data that is out by at least two metres.  That's not going to work.
I need something more accurate to compare my results with.


  OS Map Ref
   Cartesian

 Box Hill Leith Hill   Difference   Box Hill  Leith Hill
Difference
easting517971.06  513949.28  4021.78   x 4000676.63  4006902.33
   -6225.70
northing   151163.16  143161.71  8001.45   y  -21724.35   -25963.72
4239.37
height above  171.97 307.00  -135.03   z 4950992.32  4946141.89
4850.43
sea level

distance8956.35 8958.70


 Mickleham  Leith  Hill  DifferenceMickleham  Leith
  Hill 

Re: [Talk-GB] accurate GPS

[Russ Garrett]

On Wed, 9 Oct 2019 at 11:47, Gareth L  wrote:
> Are you including the continental drift? That will make etrs89 gps coords be 
> about 60-70cm off by now

That's the total net error since ETRS was established, but the ETRS
reference frame moves with the Eurasian plate and corrections are
issued, so presumably the GPS receiver has some of those incorporated.
I think for the highest accuracy it's probably worth taking the
position as WGS84 rather than doing the coordinate system
transformation on-device, but I'm not 100% sure on this.

-- 
Russ Garrett
r...@garrett.co.uk

___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Russ Garrett]

You are entering an extremely deep rabbit hole here and there are
hundreds of extremely nerdy aspects you have to consider when aiming
for sub-metre precision.

As you correctly point out, the position of trig points is no longer
accurate nor canonical. Errors of half a metre sound absolutely
plausible, as the OS coordinate system has undergone significant
revision since trig points were phased out. You're not going to find a
(publically-accessible) physical location which has better location
error than 1m or so.

I suspect (I may be wrong) that the "conversion errors" you're see are
likely artefacts of the geoid difference between the ETRS and OSGB
coordinate systems. ETRS covers a wider area than OSGB so the OSGB
numbers are likely to be more accurate (even if you've converted them
from ETRS numbers output by your GPS). Different coordinate systems
serve different purposes so this is absolutely normal.

Once you start trying to map things to sub-50cm precision over 10+km
baselines you enter a world where OSGB is not accurate enough either.
You then need to expend a tremendous amount of effort establishing a
custom coordinate system - this is what Crossrail had to do across the
entire London area to provide enough precision for tunnelling
operations.

tl;dr: All maps are wrong.

Russ

On Wed, 9 Oct 2019 at 11:06, Simon Ritchie  wrote:
>
> I've been working with some GPS equipment that claims to be accurate to 2cm.  
> To test it, I've been visiting local OS trig points, taking position 
> measurements and checking if they are correct.
>
> Unfortunately I've discovered that the data I'm getting from the OS is not 
> nearly as accurate as my equipment claims to be, which is wrecking my testing.
>
> We tend to assume (well, I do anyway) that OS trig points are very accurate 
> position markers, but compared with modern equipment, that's no longer so.  I 
> thought people might be interested in knowing how accurate they are.
>
> A related issue is this:  GPS devices don't work in terms of OS map 
> references.  If your tracker device gives you a position in that form, it's 
> done a conversion.  How accurate is that?
>
> The GPS device in a typical tracker is accurate to maybe three metres, so the 
> position you see on the screen will always be a bit wrong.  If you get it to 
> display your position in OS map reference form, it will need to do a 
> conversion, which introduces an extra error, so the result will be even more 
> wrong.  Not good if you are trying to produce an accurate map.
>
> The OS published a spreadsheet giving the positions of their trig points in 
> OS map references.  This is available from them as a spreadsheet and Ian 
> Harris has used that data to create the web site:http://trigpointing.uk
>
> The OS also offer a web page that can convert this to other forms including 
> Cartesian, which is one of the forms that my GPS device gives me:  
> https://www.ordnancesurvey.co.uk/gps/transformation/
>
> To test my equipment, I take the OS map ref of a trig point, convert it to 
> Cartesian form,  visit the trig point, get the position in Cartesian form 
> from my device and compare the two.
>
> The results are typically out by at least half a metre.  Is my equipment 
> faulty, or is the OS data wrong.  How accurate is the published position of 
> the trig point and, when I use the OS web page to convert that to Cartesian 
> form, how accurate is thatt?
>
> This OS document was very enlightening:  
> https://www.ordnancesurvey.co.uk/legacy/docs/gps/updated-transformations-uk-ireland-geoid-model.pdf
> It explains how the Cartesian coordinates work, which is useful.  It reminds 
> me that OS maps pretend that the Earth is flat, which introduces an error, 
> but that's tiny, and for my purposes itcan be ignored.  It explains how 
> accurate you can expect the published measurements of trig point positions to 
> be - they can be out by as much as 60 cm!  In general, the document stresses 
> that there is no sure-fire way to convert a position from one system to 
> another.  The result will always be inaccurate.
>
> So now I know that the published positions of the trig points are a bit 
> wrong, but how accurate is the conversion from OS map ref to Cartesian form?
>
> OS map references plus height above sea level and Cartesian coordinates both 
> specify a position using a 3D coordinate system.  The origin and the 
> direction of the axes are different in each system so you can't compare thm 
> directly.  However, the distances between two points should be the same 
> regardless of which system you use.  If you have two points in the same 
> coordinate system (a1,b1,c1) and (a2,b2,c2) and the difference along each 
> axis is a,b and c then the distance between them is
>
> the square root of (a squared plus b squared plus c squared) by Pythagoras
>
> If you have two points in a different coordinate system representing the same 
> two positions, the distance between them should be the same.
>

Re: [Talk-GB] accurate GPS

[Russ Garrett]

On Wed, 9 Oct 2019 at 11:40, Andy Robinson  wrote:
> Are you using trig points that are also OS Net station locations? 
> https://www.ordnancesurvey.co.uk/legacy/docs/gps/OSNet_GPSWebSite_Coordinates_File.txt

Pretty sure that all those OS Net locations are actually fixed GPS
receivers rather than trig points, and so not something you can go and
plonk your GPS receiver on. The file definitely seems to list the
model of receiver at each one.

Cheers,

-- 
Russ Garrett
r...@garrett.co.uk

___
Talk-GB mailing list
Talk-GB@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-gb

Re: [Talk-GB] accurate GPS

[Gareth L]

Are you including the continental drift? That will make etrs89 gps coords be 
about 60-70cm off by now


On 9 Oct 2019, at 11:06, Simon Ritchie 
mailto:simonritchie...@gmail.com>> wrote:

I've been working with some GPS equipment that claims to be accurate to 2cm.  
To test it, I've been visiting local OS trig points, taking position 
measurements and checking if they are correct.

Unfortunately I've discovered that the data I'm getting from the OS is not 
nearly as accurate as my equipment claims to be, which is wrecking my testing.

We tend to assume (well, I do anyway) that OS trig points are very accurate 
position markers, but compared with modern equipment, that's no longer so.  I 
thought people might be interested in knowing how accurate they are.

A related issue is this:  GPS devices don't work in terms of OS map references. 
 If your tracker device gives you a position in that form, it's done a 
conversion.  How accurate is that?

The GPS device in a typical tracker is accurate to maybe three metres, so the 
position you see on the screen will always be a bit wrong.  If you get it to 
display your position in OS map reference form, it will need to do a 
conversion, which introduces an extra error, so the result will be even more 
wrong.  Not good if you are trying to produce an accurate map.

The OS published a spreadsheet giving the positions of their trig points in OS 
map references.  This is available from them as a spreadsheet and Ian Harris 
has used that data to create the web 
site:http://trigpointing.uk

The OS also offer a web page that can convert this to other forms including 
Cartesian, which is one of the forms that my GPS device gives me:  
https://www.ordnancesurvey.co.uk/gps/transformation/

To test my equipment, I take the OS map ref of a trig point, convert it to 
Cartesian form,  visit the trig point, get the position in Cartesian form from 
my device and compare the two.

The results are typically out by at least half a metre.  Is my equipment 
faulty, or is the OS data wrong.  How accurate is the published position of the 
trig point and, when I use the OS web page to convert that to Cartesian form, 
how accurate is thatt?

This OS document was very enlightening:  
https://www.ordnancesurvey.co.uk/legacy/docs/gps/updated-transformations-uk-ireland-geoid-model.pdf
It explains how the Cartesian coordinates work, which is useful.  It reminds me 
that OS maps pretend that the Earth is flat, which introduces an error, but 
that's tiny, and for my purposes itcan be ignored.  It explains how accurate 
you can expect the published measurements of trig point positions to be - they 
can be out by as much as 60 cm!  In general, the document stresses that there 
is no sure-fire way to convert a position from one system to another.  The 
result will always be inaccurate.

So now I know that the published positions of the trig points are a bit wrong, 
but how accurate is the conversion from OS map ref to Cartesian form?

OS map references plus height above sea level and Cartesian coordinates both 
specify a position using a 3D coordinate system.  The origin and the direction 
of the axes are different in each system so you can't compare thm directly.  
However, the distances between two points should be the same regardless of 
which system you use.  If you have two points in the same coordinate system 
(a1,b1,c1) and (a2,b2,c2) and the difference along each axis is a,b and c then 
the distance between them is

the square root of (a squared plus b squared plus c squared) by Pythagoras

If you have two points in a different coordinate system representing the same 
two positions, the distance between them should be the same.

So I can test the accuracy of the conversion from OS map references to 
Cartesian.  In the table below, on the left, we have the trig points at Box 
Hill and Leith Hill in OS map reference form, the difference along each axis 
and below that the resulting distance.  On the right we have the same 
calculation but using the Cartesian coordinates from the OS conversion page.

Below that I do the same comparison, this time using the trig point at 
Mickleham Down and the one at Leith Hill.

In both cases, the distances are out by over two metres.

So, I'm trying to test equipment which is supposed to be accurate to two cm 
using data that is out by at least two metres.  That's not going to work.  I 
need something more accurate to compare my results with.


 OS Map Ref 
 Cartesian

Box Hill Leith Hill   Difference   Box Hill  Leith Hill 
Difference
easting517971.06  513949.28  4021.78   x 4000676.63  4006902.33   
-6225.70
northing   151163.16  143161.71  8001.45   y  -21724.35   -25963.72
4239.37
height above  171.97 307.00  -135.03   z 4950992.32  4946141.89
4850.43
sea level

distance

Re: [Talk-GB] accurate GPS

[Andy Robinson]

Are you using trig points that are also OS Net station locations? 
https://www.ordnancesurvey.co.uk/legacy/docs/gps/OSNet_GPSWebSite_Coordinates_File.txt

I don’t believe many traditional trig stations were ever properly co-ordinated 
to OS Net.

 

Cheers

Andy

 

From: Simon Ritchie [mailto:simonritchie...@gmail.com] 
Sent: 09 October 2019 11:06
To: talk-gb@openstreetmap.org
Subject: [Talk-GB] accurate GPS

 

I've been working with some GPS equipment that claims to be accurate to 2cm.  
To test it, I've been visiting local OS trig points, taking position 
measurements and checking if they are correct.

 

Unfortunately I've discovered that the data I'm getting from the OS is not 
nearly as accurate as my equipment claims to be, which is wrecking my testing.

 

We tend to assume (well, I do anyway) that OS trig points are very accurate 
position markers, but compared with modern equipment, that's no longer so.  I 
thought people might be interested in knowing how accurate they are.

 

A related issue is this:  GPS devices don't work in terms of OS map references. 
 If your tracker device gives you a position in that form, it's done a 
conversion.  How accurate is that?

 

The GPS device in a typical tracker is accurate to maybe three metres, so the 
position you see on the screen will always be a bit wrong.  If you get it to 
display your position in OS map reference form, it will need to do a 
conversion, which introduces an extra error, so the result will be even more 
wrong.  Not good if you are trying to produce an accurate map.

 

The OS published a spreadsheet giving the positions of their trig points in OS 
map references.  This is available from them as a spreadsheet and Ian Harris 
has used that data to create the web site:http://trigpointing.uk 
 

 

The OS also offer a web page that can convert this to other forms including 
Cartesian, which is one of the forms that my GPS device gives me:  
https://www.ordnancesurvey.co.uk/gps/transformation/

 

To test my equipment, I take the OS map ref of a trig point, convert it to 
Cartesian form,  visit the trig point, get the position in Cartesian form from 
my device and compare the two.

 

The results are typically out by at least half a metre.  Is my equipment 
faulty, or is the OS data wrong.  How accurate is the published position of the 
trig point and, when I use the OS web page to convert that to Cartesian form, 
how accurate is thatt?

 

This OS document was very enlightening:  
https://www.ordnancesurvey.co.uk/legacy/docs/gps/updated-transformations-uk-ireland-geoid-model.pdf

It explains how the Cartesian coordinates work, which is useful.  It reminds me 
that OS maps pretend that the Earth is flat, which introduces an error, but 
that's tiny, and for my purposes itcan be ignored.  It explains how accurate 
you can expect the published measurements of trig point positions to be - they 
can be out by as much as 60 cm!  In general, the document stresses that there 
is no sure-fire way to convert a position from one system to another.  The 
result will always be inaccurate.

 

So now I know that the published positions of the trig points are a bit wrong, 
but how accurate is the conversion from OS map ref to Cartesian form?

 

OS map references plus height above sea level and Cartesian coordinates both 
specify a position using a 3D coordinate system.  The origin and the direction 
of the axes are different in each system so you can't compare thm directly.  
However, the distances between two points should be the same regardless of 
which system you use.  If you have two points in the same coordinate system 
(a1,b1,c1) and (a2,b2,c2) and the difference along each axis is a,b and c then 
the distance between them is

 

the square root of (a squared plus b squared plus c squared) by Pythagoras

 

If you have two points in a different coordinate system representing the same 
two positions, the distance between them should be the same.

 

So I can test the accuracy of the conversion from OS map references to 
Cartesian.  In the table below, on the left, we have the trig points at Box 
Hill and Leith Hill in OS map reference form, the difference along each axis 
and below that the resulting distance.  On the right we have the same 
calculation but using the Cartesian coordinates from the OS conversion page.

 

Below that I do the same comparison, this time using the trig point at 
Mickleham Down and the one at Leith Hill.

 

In both cases, the distances are out by over two metres.

 

So, I'm trying to test equipment which is supposed to be accurate to two cm 
using data that is out by at least two metres.  That's not going to work.  I 
need something more accurate to compare my results with.

 

 

 OS Map Ref 
 Cartesian  
   
Box Hill Leith Hill   Difference   Box Hill  Leith Hill 
Difference