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ZemaLupin

Most accurate GPS tool?

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Hi,

 

The company I work for has given me the job of investigating GPS tools. The current requirements are that it needs to be within 5cm accurate, works all over England, measures X, Y and Z coordinates, and somehow someway can be converted into a DWG format (for AutoCad). The price of the device most likely won't be a problem as what we are currently looking at is Leica ATX1230GG Smart Rover. Any suggestions and advice would be great. If you need any more information please do say and I will find out ASAP.

 

Thanks.

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You are definitely looking for professional equipment, as any recreational or car GPS unit will be a couple orders of magnitude poorer than your requirement.

 

If your company needs only a moderate amount of data, it will be advantageous to contract a professional surveyor to perform the service for you. If you have an ongoing need for a lot of data, then you or someone in the company may want to get educated on it, but it will be a big commitment. Depending on what the data is used for (e.g., land boundaries and perhaps some kinds of construction), there may also be regulations governing who is licensed to perform the work.

 

You might try registering (free) on www.surveyorconnect.com and asking for advice from the professionals there.

 

The 5 cm specification is not quite complete, since any measurement has statistical error. Are you looking for a standard deviation of 5 cm, or 95% confidence (i.e. 19 times out of 20) within 5 cm, or what?

 

Is the requirement to find positions relative to a national or world coordinate system, or is the primary need to have accurate relative positions among certain points?

 

How long are you willing to have the equipment on a point to get a reading? With a professional receiver operating by itself, "static autonomous" measurements of 15 minutes to a few hours may be needed to get that accuracy, depending on satellite positions at the time, and how clear the view of the sky is.

 

Some people use a base and rover setup with a radio link. If the base is placed on a known point (Ordinance Survey trig point?) the rover can get positions relative to the base with very short occupation times. Of course, it is advisable to make the rounds of your unknown points twice to confirm that there was no anomalous reading.

 

There are Real Time Kinetic networks (RTK or RTN) in some areas that you can subscribe to, which eliminate the need for you to have a base unit. I don't know anything about RTN coverage in England.

 

Leica certainly sells equipment that will meet your needs, as do other manufacturers as well (Trimble, Javad, etc.) Besides equipment capability and price, pay a lot of attention to software compatibility with your CAD system and how easy it is to get training and service support from the company.

 

Professional equipment will require some training to get good results. And there is a lot to understand about what you are really measuring at the cm level. Are you familiar with horizontal datums and vertical datums? Map projections? Map grid versus ground scaling? If you use the native GPS coordinate system, do you know how to account for continental drift that will be significant over a few years? And many other issues.

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