Hi All

I need to pick up some survey points using a Trimble Geo7X GPS Rover, and want to know what would be best practice to confirm/demonstrate survey accuracy. My plan was to record the horizontal/vertical variances compared to known checkpoints, using nearby permanent survey control point/marks. 

A few related questions:

- Firstly, is this approach sufficient?

- The job is approx. 3 weeks work across a residential neighbourhood - how often should I conduct a checkpoint comparison: start of job only? Or more often - weekly/ daily etc?

- It is proposed to post process survey points, as this requires only 1 day hire of a VRS-RINEX licence on job completion - versus - daily hire for 3 weeks of a VRS-RTK licence. However, this approach means I won't understand my checkpoint accuracy until the finish which could be disastrous! So assume I will also need a VRS-RTK licence for those checkpoint comparison times as well?

- I believe GPS Pathfinder will correctly handle the mix of RTK-corrected and uncorrected data in the field when it comes to post processing.

Any assistance would be greatly appreciated.

Cheers Jon

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Dear Mr. Buddz,

Have you ever used satellite surveying before? If not, engage someone to initiate you in the processes.

The Geo7X has a very nice spec-sheet. Like all GNSS receivers, its stand alone accuracy is estimated at several meters. Code level processing provides a precision of sub-meter. Static post-processed results it claims to be centimeter level. For real-time kinematic, the claim is 4cm with the built-in antenna and 1.5 for an external antenna. Your first criteria will be the method you use in the field.

GNSS errors are not usually of the systematic type that will be discovered by occasional checks into a point. Those checks will alert one if the base has been disturbed or the initialization has gone bad. Interference on one side of the street is often different from that on the other so checking into the first as a check on the second is irrelevant.

Best wishes and remember a prudent surveyor can defend his work. That IS why he is paid.


Thanks gents (Anthony and Steven) for the comments - very much appreciated.

- The work is for mapping purposes - so sub 5cm is the spec.

- I have an external antenna and can choose to either post process, or operate in RTK mode.

- Terrasync does give me estimates on accuracy, it sounds like these can be trusted without needing to check against a known point. I was thinking this might serve as an independent check against the reported accuracy. 

- If I am post processing - I will expect to see Terrasync's accuracy in the field in the order of several meters until I post process to bring to centimeter level accuracy. I'm just apprehensive initially about doing 3 wks of field work before confirming that I'm down to centimeter accurate.  

- Alternatively I can operate in RTK mode and be able to confirm I'm within the necessary tolerance as I go - however that requires hiring a licence for every day of the survey which is expensive.

Nice units for MAPPING grade standards.  My advice: build your needs from the end result back, i.e, what sort of accuracies are you hoping to achieve?  You can measure and post-process, but what do you hope to gain with this data?  RTK (real-time kinematic) surveying would help you achieve true sub-1.5cm accuracy, as Anthony stated.  Again, build from the end product needed..... then decide what equipment/technology supports this.  Speak with your local Trimble/Leica/Topcon or whatever survey vendor is available in your area.  They usually know how to help....

My two cents...



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