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I was talking with a potential client, and they requested that we plot some points with GPS, with a maximum (or should that be minimum?) accuracy of 2 m.

This is an area with no WAAS, and I was under the impression that even in the best of conditions, a single gps point can be accurate up-to only 15 meters(Horizontal field). Is this correct?

What is the maximum theoretical accuracy of GPS without using WAAS or differential GPS?

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What type of receivers will you be using? –  R.K. Dec 10 '12 at 13:05
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It is receiver dependent of course, see what a low end gps provides in this thread gis.stackexchange.com/questions/12011/… –  Dan Patterson Dec 10 '12 at 13:27
    
I'm not interested in any specific instrument. I'm concerned with the Theoretical Accuracy in Typical Environmental conditions. –  Devdatta Tengshe Dec 10 '12 at 13:42
    
@DanPatterson I think that particular instrument and readings are using WAAS, and are not pure Satellite based GPS positioning. –  Devdatta Tengshe Dec 10 '12 at 13:45
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Not WAAS enabled @Devdatta, 7 year old Garmin Etrexs. Sometimes stationarity at a site yields better results for any GPS. The link was just food for thought. –  Dan Patterson Dec 10 '12 at 14:28
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The United States government currently claims 4 meter RMS (7.8 meter 95% Confidence Interval) horizontal accuracy for civilian (SPS) GPS. Vertical accuracy is worse. Mind you, that's the minimum. Some devices/locations reliably (95% of the time or better) can get 3 meter accuracy. For a technical document on that specification you can go here.

For more general GPS accuracy information, head to GPS.gov's website. That website also includes data and information on WAAS-enabled systems and accuracy levels depending on location. It's a great resource.

Basically, you can't get 2 meter accuracy reliably without some form of correction.

Edit: Something else to contemplate is using a device that can communicate with both GPS and GLONASS satellites. I'm not aware of any accuracy articles or studies that combine both systems to improve accuracy, but at the very least, it increases the potential satellites that may be available at one particular location/time, especially near the poles.

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I had looked at the 2008, SPS standard before, but it is very dense and a difficult read. I can see that they have mentioned 4m as the 95 percentile for the error; But I was rather hoping for some kind of formula or equation that proved it. –  Devdatta Tengshe Dec 11 '12 at 3:21
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why do you insist on "theoretical" best performance, or want to see the math behind the answer above (esp after you say that the standard that lays it out is "dense and a difficult read")? Your potential client has a budget in mind which will determine what grade of GPS units you can take to the field. Your readings will not be any better than the vendor claims for the unit, and probably will be worse. As several answers/comments have noted, a consumer grade GPS will get you 10-15M. Without WAAS or differential GPS, you won't achieve your goal, regardless of the equations. –  Llaves Dec 11 '12 at 5:01
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in European countries, out in the field (not inside a city with buildings), the best accuracy without any aid is 5 meters. I have also witnessed a 2 meter accuracy but that is extremely rare and I would not take it into account. The average best would be 15 meters and the average worse close to 30-40 meters.

The results stated above are from my own field work and come from using various types of smartphones. GPS accuracy greatly varies depending on surroundings, devices used, weather and many other factors. The accuracy results are derived from compairing my actual position with the GPS position.

I hope this helps.

Cheers, A

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And how do you define accuracy? –  johanvdw Dec 10 '12 at 20:59
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I use Arcmap to set my fixed test positions and then measure the distance from the GPS positions. The results that I stated above are average and rounded accuracy values for European countries (mostly UK, Spain and Germany ) –  Antony Dec 11 '12 at 11:14
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Ionospheric delay effects are the largest source of error in a single-frequency GPS receiver. WAAS and CORS are able to correct for this better than a receiver's almanac, so the best you can do with uncorrected GPS is typically about 15 meters. Survey-grade GPS using RTK is able to achieve centimeter accuracy.

Differential correction methods Image source: http://www.spatial-ed.com/gps/gps-basics/135-differential-correction-methods.html

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