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Using GPS to find a point on a cadastre


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I would like to find out exactly where one of the boundaries on a cadastre is. It would seem the easiest way to do this is using a GPS device but that only measures latitude and longitude and I can't work out how to match the co-ordinates on the cadastre with latitude/longitude coordinates. Can anyone help?
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Now this could be a good one!!

The synergy between a change in Gédesique Datum, WGS84/ED50 to a Géeometre Expert with a cloth tape plotting on the Cadastre Napoléonique.

Small point the measurements on the screen and measurements between established points defining a parcelle and established by a Geometre expert will vary considerably. I have tried using my white plastic thingies; très drôle.

The other consideration is that GPS tools available to the general public only establish position relatively inaccurately due to only using 3/4 satellites. I doubt that you will get better than 5 metres or so; the USA only permits accurate positioning for its own military purposes.

The Europeans have been planning their own array of sats for years now; we await with interest.

Why not try flying a kite, meataphor; set your GPS to ED50 and check the geodesic coordinates for a cadastral point close to where you are trying to establish position; then on your gouv.fr cadastral display enable outils avancées and click on fichier de points option.

You will then be able to establish an array of X, Y coordinates corresponding to the cadastral projection; then do the same on the ground with your GPS set to ED 50 Datum for the same array of points.

Bon Voyage.

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[quote user="pachapapa"]

Why not try flying a kite, meataphor; set your GPS to ED50 and check the geodesic coordinates for a cadastral point close to where you are trying to establish position; then on your gouv.fr cadastral display enable outils avancées and click on fichier de points option.

You will then be able to establish an array of X, Y coordinates corresponding to the cadastral projection; then do the same on the ground with your GPS set to ED 50 Datum for the same array of points.

[/quote]

I'll pass this on to my French neighbour, who is currently involved in a boundary dispute (not with me - I'm on his side).  I can already feel the grey mist coming down over this part of the Midi !  Will also provide numerous brown paper bags for the use of him, the numerous geometres who have been involved and the guys at the Mairie.

Sorry - not having a go at you Pachapapa.  Just a little [6]

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@ gardien[:)]Not sure if brown paper bags will be strong enough for the emetic results of colonic irrigation on géeeeomètres.[:D]

Shall we start a new thread dripping with sarcastic irony entitled:

USING GEOMETRE TO FIND A POINT ON A CADASTRE!!!![:P]

By the way I consider SPANC inspectors to be more gifted than GEEEEOS.

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[quote user="pachapapa"]

The other consideration is that GPS tools available to the general public only establish position relatively inaccurately due to only using 3/4 satellites. I doubt that you will get better than 5 metres or so; the USA only permits accurate positioning for its own military purposes.[/quote]

Not quite correct pachapapa. Modern GPS instruments obtain fixes from more than 3 or 4 satellites, coupled with WAAS capability can produce a fix within 1 metre.

The US control the GPS satellite network but they had the decency to switch off the 'wobble factor' some years ago. This was responsible for the inability to obtain accurate fixes within a 3 to 5 metre range. Today, even the most basic GPS receiver with SERF technology will recieve and compute the information from at least 5 satellites.

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[quote user="Salty Sam"][quote user="pachapapa"]

The other consideration is that GPS tools available to the general public only establish position relatively inaccurately due to only using 3/4 satellites. I doubt that you will get better than 5 metres or so; the USA only permits accurate positioning for its own military purposes.[/quote]

Not quite correct pachapapa. Modern GPS instruments obtain fixes from more than 3 or 4 satellites, coupled with WAAS capability can produce a fix within 1 metre.

The US control the GPS satellite network but they had the decency to switch off the 'wobble factor' some years ago. This was responsible for the inability to obtain accurate fixes within a 3 to 5 metre range. Today, even the most basic GPS receiver with SERF technology will recieve and compute the information from at least 5 satellites.

[/quote]

Thanks for update on accuracy to 1 metre.

Can you point me to a useful reference for next time involved with a Géomètre.

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[quote user="pachapapa"][quote user="Salty Sam"][quote user="pachapapa"]

The other consideration is that GPS tools available to the general public only establish position relatively inaccurately due to only using 3/4 satellites. I doubt that you will get better than 5 metres or so; the USA only permits accurate positioning for its own military purposes.[/quote]

Not quite correct pachapapa. Modern GPS instruments obtain fixes from more than 3 or 4 satellites, coupled with WAAS capability can produce a fix within 1 metre.

The US control the GPS satellite network but they had the decency to switch off the 'wobble factor' some years ago. This was responsible for the inability to obtain accurate fixes within a 3 to 5 metre range. Today, even the most basic GPS receiver with SERF technology will recieve and compute the information from at least 5 satellites.

[/quote]

Thanks for update on accuracy to 1 metre.

Can you point me to a useful reference for next time involved with a Géomètre.

[/quote]

I presume you mean SiRF technology rather than SERF technology.

I have had a quick nosey round the site at

http://www.sirf.com/Default.aspx

But still lost on the one metre specification.

A lot of 2.5 metre products at

http://www.google.com/custom?hl=fr&cof=S%3Ahttp%3A%2F%2Fwww.sirf.com%3BL%3Ahttp%3A%2F%2Fwww.sirf.com%2Fimages%2Fhomelogo2.gif%3BLH%3A103%3BLW%3A104%3B&domains=www.sirf.com&q=Position+Accuracy&btnG=Rechercher&sitesearch=www.sirf.com

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Sorry for the error, it should indeed have read as SIRF (or even SiRF).

It's very complicated to explain to the uninitiated but basically, SiRF technology where fitted, makes the GPS unit more sensitive. This in turn will increase accuracy as more satellite signals are received and analysed.
The increase in sensitivity also allows signals to be better interpreted under poor conditions.

Now couple SiRF with WAAS (or more correctly for Northern Europe - EGNOS) technology and you have a much more accurate system.

100 metres: Accuracy of the original GPS system, which was subject to accuracy degradation under the US government-imposed Selective Availability (SA) program - (Affectionately known as the 'Wobble Factor'!)

15 metres: Typical GPS position accuracy once Selective Availability was switched off.

3-5 metres: Typical differential GPS (DGPS) position accuracy.

However, a WAAS / EGNOS capable receiver can give you a position accuracy of better than 3 metres, 95% of the time, which will be fine tuned further if the receiver is SiRF capable. This combination also allows for a more accurate reading of velocity and altitude.

The accuracy of a device is also governed somewhat by the mapping software installed. With street mapping found in vehicle mounted receivers, the accuracy is around 95% due to a factor known as GPS shift otherwise every time you changed lane or meandered from side to side, the direction indicator would resemble a drunken cursor. Whereas topographic software where the prime use is for foot navigation, the accuracy will be greater as the two legged user is likely to change position quickly and frequently.

However if you wish for centimetre accuracy, you are going to need to dig deep as such receivers will cost quite a bit more than your average TomTom or Navman!

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