Checking GPS Accuracy

[+Wood Smoke]

Hi All,

 

When I was at the top of Snowdon the other day, someone came up to the trigpoint with a GPS and reset it to the correct location.

 

Is there anywhere I can get this information (not Snowdon, I'm not going up there again) for the UK so that I can find a local spot and set the accuracy of my GPS?

 

WoodSmoke

 

Yours Roy

[+Slytherin]

Try www.gps.gov.uk

 

You will find details of the passive and active gps reference points throughout the country.

 

HTH

 

-

[+Wood Smoke]

Thanx for that, perfect :-)

 

WoodSmoke

 

Yours Roy

[Morseman]

quote:

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Originally posted by Wood Smoke:

Hi All,

 

When I was at the top of Snowdon the other day, someone came up to the trigpoint with a GPS and reset it to the correct location.

 

---

 

Does the trig point on Snowdon have a plaque on it saying that it's part of the Passive Station network maintained by Ordnance Survey? If not, then it's one of the thousands that OS have abandoned and are no longer maintained.

 

They will, in the main, still be within tollerance of most GPS units. However, did the person check their GPS using OSGB-36 or WGS84? It can make a difference.

 

--... ...--

Morseman

[+Lance Ambu]

This is all new to me.

 

What exactly is going on here?

 

Isn't true that some trig points are not where they thought they were since the advent of GPS systems? - is this what you mean by active and passive trig points.

 

Is one resetting enough or would you really need 2 or three?

 

Oh, and since the GPS we use have inherent level of accuracy is recalibrating actually possible? I mean if you stand still for a while you will still get different readings.

 

By the way if it invoves climbing snowdon then I'really not that keen.

 

Lance

It's dark and we're wearing sunglasses.

[+Wood Smoke]

quote:

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By the way if it invoves climbing snowdon then I'really not that keen.

---

 

But there's a cache there :-)

 

WoodSmoke

 

Yours Roy

[+jeremyp]

I don't understand this. How do you set the accuracy of a GPS receiver? mine has no functionality to say "I am at this known location".

 

Further, the OS trigpoints are set based on the OS GB36 datum. It's impossible to convert between this and WGS84 (used by the GPS system) accurately without access to lots of sophisticated transformational data, probably not in your consumer GPSr. "Accurately" means within 5 metres or so. This is only slightly better than you can get from a GPS receiver anyway.

 

-------

jeremyp

The second ten million caches were the worst too.

http://www.jeremyp.net/geocaching

[+Wood Smoke]

quote:

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Originally posted by jeremyp:

I don't understand this. How do you set the accuracy of a GPS receiver? mine has no functionality to say "I am at this known location".

---

 

Well it seems that some do and some don't????

 

I use an iPaq with a GPS attached and I have an option 'Correct GPS Position'

 

So that's what I intend to do :-)

 

If you look at that web site above, not all are trigpoints, some are poles which have been set for GPS checking and are checked at regular intervals (it tells you when it's position was last checked)

 

WoodSmoke

 

Yours Roy

[Morseman]

quote:

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Originally posted by Lance Ambu:

 

Isn't true that some trig points are not where they thought they were since the advent of GPS systems? - is this what you mean by active and passive trig points.

 

 

By the way if it invoves climbing snowdon then I'really not that keen.

---

 

If you compare the location of a trigpoint with the display on your GPS receiver there are a variety of reasons why it might not be where you thought it should be.

 

One reason might be the accuracy of your GPSr - two or three metres out is not uncommon, more in areas of poor reception such as wooded area, deep valleys, built up areas where the reception from satellites on the horizon is obstructed.

 

Another is due to map making errors, and errors by the Ordnance Survey in specifying where the Trig point was. I 'found' one recently where the location given was on a road parallel to where the trig point actually was!

 

The original work to retriangulate the UK would have accurately place the trig point in the system, but human error has probably crept into the records of where it actually is. A fact I have used in the cache Precisely Bolsover.

 

You don't have to climb Snodon to visit a trig point, there are over 6,000 of them across the UK. Some are right by footpaths and roads, others are a bit more of a walk. Not all are on publically accessable land. Which makes the 'hunt' a little more interesting.

 

If you have the OS Landranger maps, look for a blue triangle with a blue dot in the middle. These are sites where a trig point pillar is likely to be located.

 

Oh, and professional GPS receivers can be adjusted, using either passive (a place you can actually stand on, GPSr in hand) or active (these transmit a correction signal a bit like WAAS in the USA) which allow the more expensive, professional systems to be adjusted for errors caused by the problems of the satellite system mentioned above.

 

Hope that helps?

 

--... ...--

Morseman

[Sel]

quote:

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Originally posted by Morseman:

Another is due to map making errors, and errors by the Ordnance Survey in specifying where the Trig point was. I 'found' one recently where the location given was on a road parallel to where the trig point actually was!

---

 

I hate to be pedantic, but how can a point have a road parallel to it?

 

You need two lines for parallel conditions, not a line and a point...?

 

--

Why'd ya have to go and make things so complicated?

[+The Good Shepherds]

quote:

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Originally posted by Sel:

I hate to be pedantic, but how can a point have a road parallel to it?

---

 

If the trigpoint was in the dead centre of a roundabout it would meet this criteria [:-)]

 

(though I suspect this was not the case...)

[Morseman]

quote:

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Originally posted by Sel:

quote:

---

Originally posted by Morseman:

Another is due to map making errors, and errors by the Ordnance Survey in specifying where the Trig point was. I 'found' one recently where the location given was on a road parallel to where the trig point actually was!

---

 

I hate to be pedantic, but how can a point have a road parallel to it?

 

You need two lines for parallel conditions, not a line and a point...?

 

---

 

Appologies for the poor use of the English language.

 

There were two roads, running nearly parallel to one another. The Trig Point database, which is I belive based upon information gathered from a variety of sources but mainly from the Odrnance Survey records, put it in a location just off one of the roads. The pillar was, however, actually just off the other road and about a half mile south of the location given A fact I could work out as the name of the point was (I'm going to make up a name here, as I can't be bothered to log off and go and get the real name ) something like "Morsemans Reservoir" - The reservoir was off one road, the location of the trigpoint was wrongly given as off another road and was in the grounds of an observatory.

 

It was, however, parallel to the point given, so human error had crept in making the possition parallel to where it should be, but offset to the north of its actual place.

 

Phew!

 

--... ...--

Morseman

[Sel]

quote:

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Originally posted by Morseman:

 

It was, however, parallel to the point given, so human error had crept in making the possition parallel to where it should be, but offset to the north of its actual place.

---

 

Oh, you were doing so well expalining yourself right up to the end there.

 

A position cannot be parallel to anything, it's a matter of geometry. But I see what you mean, I'm just nitpicking your semantics now

 

--

Why'd ya have to go and make things so complicated?

[+Bill D (wwh)]

Ok, if we're going to nitpick...

 

quote:

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Sel wrote:

 

Oh, you were doing so well expalining yourself right up to the end there.

---

 

I don't know what expalining is, but it sounds extremely painful!

 

Bill

[Morseman]

quote:

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Originally posted by Sel:

quote:

---

Originally posted by Morseman:

 

It was, however, parallel to the point given, so human error had crept in making the possition parallel to where it should be, but offset to the north of its actual place.

---

 

Oh, you were doing so well expalining yourself right up to the end there.

 

A position cannot be parallel to anything, it's a matter of geometry. But I see what you mean, I'm just nitpicking your semantics now

 

--

Why'd ya have to go and make things so complicated?

---

 

Sorry, I couldn't resist it.

 

The roads were parallel to one another, which is what I meant in the original post. I did consider making the last paragraph, on parallels, longer and even more incorrect but decided to spare everyone more pain.

 

--... ...--

Morseman

[+Team Hydro]

quote:

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Oh, and professional GPS receivers can be adjusted, using either passive (a place you can actually stand on, GPSr in hand) or active (these transmit a correction signal a bit like WAAS in the USA) which allow the more expensive, professional systems to be adjusted for errors caused by the problems of the satellite system mentioned above.

---

 

Hey Morseman,

 

I believe Garmin do sell differential receivers that, as you suggest above, correct in real time, the signals received by your GPSr and can be plugged direct into the little etrex. I believe that the cost is somewhere areound the 500-600 pound mark. However, I am not too sure what they lock in to for the diff corrections - at one stage, Virgin Radio were looking into transmitting corrections around the UK (following the demise of the Decca Navigation Signals run by Trinity House probably resulting from the increasing interest in use of GPS for commercial navigation) - and all you would have to do would have been to tune your differential receiver into Virgin's radio frequency (music and corrections at the same time!).

[+Pharisee]

A friend of mine worked for a company that specialised in carrying out geological surveys, both marine and airborn all over the world. This was in the days before 'selective access' was turned off.

They set up their own base station at an accurately known locaction close by and had some clever software that used the GPS signal recieved by the base station to correct the GPS data received by the ship or 'plane. It wasn't corrected in real time but 'post processed' at the end of each days surveying.

Guess it must be a lot easier now.

 

John

 

Age and treachery will always triumph over youth and ability.

[+Teasel]

quote:

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Originally posted by Pharisee:

Guess it must be a lot easier now.

---

Not really! If you want super-accurate positioning, you still need to install your own base station on top of one of the OS's passive stations (that's what they're there for!).

 

Or you can use the OS's active stations. These are not actually transmitters, they're merely base stations which continuously save their corrections onto a website so that people don't need to buy their own kit. They cover the UK such that you're never more than 100Km from one.

 

There used to be transmitted Differential GPS around the entrances to some harbours but I imagine this, and the Virgin radio corrections, probably died a death once SA was switched off.

 

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