An Easy Way to Improve Survey Accuracy

Only one question here Kent; is the grass always that green in early June down there?

paden cash, post: 431799, member: 20 wrote: Only one question here Kent; is the grass always that green in early June down there?

Actually, we've had enough rain recently that it does look greener right about now. The rain brought the wild flowers, too, so things are looking good.

Cool. Why were the PPK measurements so sloppy between points?

Kent McMillan, post: 431796, member: 3 wrote: [Trigger Alert: This post contains material related to the adjustment of survey measurements by least squares.]

From To Grid Bearing Grid Dist 95% RelConfidence
Grnd Dist Brg Dist PPM
2 3 S14-45-02W 157.833 74 0.056 360
157.846
3 4 S05-27-36W 232.034 37 0.049 214
232.053

Hello Kent. Nice analysis.
In the Adjusted Grid Bearing and Distance output, how do the PPM calculations work? They're not related to the PPM values that are set in Project Settings for the instrument are they? Are they evaluating the total standard error as a proportion of the distance measured?

naw054, post: 431804, member: 10013 wrote: Cool. Why were the PPK measurements so sloppy between points?

As the standard errors indicate, the PPK accuracy was not unusually bad. It is probably what you'd expect from RTK in that setting. The situation is that the sloppy relative accuracy directly follows from that level of uncertainty when you evaluate the point-to-point uncertainty at 95% confidence.

rfc, post: 431807, member: 8882 wrote: In the Adjusted Grid Bearing and Distance output, how do the PPM calculations work? They're not related to the PPM values that are set in Project Settings for the instrument are they? Are they evaluating the total standard error as a proportion of the distance measured?

Yes, the distance uncertainties are expressed as a fraction of the distance between the specific pair of points in Parts Per Million or PPM.

The routes of the proposed easements follow existing roads and jeep trails. Here is what the vicinity of Point 14 looks like as a fairly typical example:

PPK is probably the most underutilized GPS tool.

MightyMoe, post: 431816, member: 700 wrote: PPK is probably the most underutilized GPS tool.

I use it extensively to boost productivity with my vintage L1 GPS equipment (the rover is a Trimble 4600LS unit) that just keeps going and going. Obviously, one critical element is examining the uncertainties in points positioned and identifying weak points that need some additional work, but generally it's extremely useful.

MightyMoe, post: 431816, member: 700 wrote: PPK is probably the most underutilized GPS tool.

It's underutilized for a reason: you don't know what you've got until you're no longer on site. I agree that it's a good alternative to RTK if you don't have RTK, but having real-time quality indicators that can save a return trip is worth a lot.

Combining GPS and conventional measurements is also a very good way to test the estimated uncertainties of each. For example, when the point-to-point distance uncertainty of EDM measurements is on the order of about 1mm, that is a fairly strong condition that the adjusted GPS vectors must meet.

Just the simple expedient of measuring the distances back and forward is an easy way to demonstrate the absence of blunders (as well as to test the estimates of instrument and prism centering errors and the EDM range uncertaintes).

For example, here are the adjusted back and forward EDM slope distances after adjustment. The "Residual" column lists the corrections that were applied in the adjustment to the measured distances.

[PRE]
Adjusted Measured Distance Observations (FeetUS)
Slope
From To Distance Residual StdErr StdRes
3 2 158.4866 0.0016 0.0040 0.4
3 2 158.4479 -0.0021 0.0040 0.5

3 4 233.5401 0.0001 0.0041 0.0
4 3 233.4770 -0.0005 0.0041 0.1

4 5 331.1106 0.0006 0.0043 0.1
5 4 331.0473 -0.0035 0.0043 0.8

5 6 187.5061 0.0011 0.0041 0.3
6 5 187.3890 -0.0010 0.0041 0.3
6 5 187.3890 -0.0022 0.0041 0.6

6 7 285.8459 -0.0041 0.0042 1.0
7 6 285.7725 -0.0006 0.0042 0.1

7 8 130.4199 0.0012 0.0040 0.3
8 7 130.4555 -0.0045 0.0040 1.1

8 9 140.0298 -0.0015 0.0040 0.4
9 8 140.0585 -0.0015 0.0040 0.4

9 10 140.5712 -0.0025 0.0040 0.6
10 9 140.5774 0.0024 0.0040 0.6

10 11 241.8861 0.0011 0.0041 0.3
11 10 241.8496 -0.0004 0.0041 0.1

11 12 299.8987 -0.0013 0.0042 0.3
12 11 299.8672 0.0009 0.0042 0.2

12 13 97.4800 0.0050 0.0039 1.3
13 12 97.4820 -0.0030 0.0039 0.8

13 14 112.8102 0.0002 0.0039 0.1
14 13 112.8643 -0.0007 0.0039 0.2

14 15 286.2996 -0.0004 0.0042 0.1
15 14 286.2967 0.0042 0.0042 1.0

15 16 79.0719 0.0019 0.0039 0.5
16 15 79.0683 -0.0017 0.0039 0.4

16 17 184.2305 -0.0008 0.0040 0.2
17 16 184.1853 0.0003 0.0040 0.1

17 18 209.0029 0.0004 0.0041 0.1
18 17 209.0146 -0.0012 0.0041 0.3

18 19 59.2530 0.0030 0.0038 0.8
19 18 59.2811 -0.0039 0.0038 1.0

19 20 97.2095 -0.0043 0.0039 1.1
20 19 97.2467 0.0042 0.0039 1.1

20 21 77.6272 -0.0028 0.0039 0.7
21 20 77.6492 0.0017 0.0039 0.4

21 22 119.7134 0.0034 0.0039 0.9
22 21 119.7405 -0.0045 0.0039 1.2

22 23 102.9445 -0.0005 0.0039 0.1
23 22 102.9701 0.0001 0.0039 0.0
23 22 102.9701 0.0001 0.0039 0.0

23 24 161.4059 0.0009 0.0040 0.2
[/PRE]

Jim Frame, post: 431818, member: 10 wrote: It's underutilized for a reason: you don't know what you've got until you're no longer on site. I agree that it's a good alternative to RTK if you don't have RTK, but having real-time quality indicators that can save a return trip is worth a lot.

In my experience, PPK results are fairly predictable. The exceptions tend to be in locations where GPS positioning probably shouldn't be used anyway, such as in highly obstructed environments, and at times when the surveyor isn't paying attention to the quality of satellite geometry. Avoiding those pitfalls should not be a major problem.

The question that I'd have is: how many RTK users are simply relying upon the uncertainty estimates generated by the RTK controller, with no further validation or assessment of the actual results?

Jim Frame, post: 431818, member: 10 wrote: It's underutilized for a reason: you don't know what you've got until you're no longer on site. I agree that it's a good alternative to RTK if you don't have RTK, but having real-time quality indicators that can save a return trip is worth a lot.

That's always been the major complaint for PPK but there are ways to control for that real time, it can be a huge time saver when done correctly and it can also allow for a increase in your project's accuracy.

I've not had the issue of missing data for PPK locations for a long time, but then I was lucky to jump into it very early and learned the pitfalls. Now with so many extra satellites that issue has waned, and if you want static locations on your rover points (after all PPK is actually a static session) then I always use my PPK style on the rover for that. Plus you can tie into whatever broadcasting station is in the area to firm up your project using the PPK sessions. Also PPK will work with very short sessions and very long vectors which is another big plus.

Nice work! do you do your traverses solo??

Andy J, post: 431836, member: 44 wrote: Nice work! do you do your traverses solo??

Yes, that was just me and some prism pole tripods. That and 8-minute vials on the prism poles did the trick.

You mentioned doing this as efficiently as possible. Is it possible that hitting all the control points 2 - 24 with the PPK GPS is overly redundant? Would your results be sufficiently good if you had only observed say 1, 5, 10, 19, & 24? How long did you spend observing each point?

Peter Lothian - MA ME, post: 431844, member: 4512 wrote: You mentioned doing this as efficiently as possible. Is it possible that hitting all the control points 2 - 24 with the PPK GPS is overly redundant? Would your results be sufficiently good if you had only observed say 1, 5, 10, 19, & 24? How long did you spend observing each point?

The observation times were five minutes on each point, which didn't seem like much of an investment of time to me. That particular series of control points has the feature of mixing legs of different lengths, the short legs making weak points in the conventional connections which the GPS vectors strengthened.

I suppose that I could demonstrate this, but I think its fairly straight-forward that as the spacing of the GPS-derived positions gets wider, the quality of the combination of conventional and GPS drops off significantly in a way that does not happen with a tighter spacing of GPS-derived positions. That is, if you have GPS-derived positions on 2, 3 and 4 that are directly connected by conventional angles, distances and elevation differences with an uncertainty on the order of about 2mm, the adjusted results of 2, 3, and 4 approach what three occupations of all would yield. I may work the numbers on this example just to demonstrate the drop in quality with a sparser set of GPS vectors.

Peter Lothian - MA ME, post: 431844, member: 4512 wrote: You mentioned doing this as efficiently as possible. Is it possible that hitting all the control points 2 - 24 with the PPK GPS is overly redundant? Would your results be sufficiently good if you had only observed say 1, 5, 10, 19, & 24? How long did you spend observing each point?

Here's the first answer. This set of standard errors of control points is what the network would yield if only roughly every other one had a GPS vector to it. There is an increase in uncertainty, but it isn't much. These are the sparser set of GPS vectors used in the adjustment of the same conventional traverse through the entire set of control points:

And the resulting uncertainties of the control points to compare to those listed in my post above:

[PRE]
Error Propagation
=================

Station Coordinate Standard Deviations (FeetUS)

Station N E Elev
1 0.010 0.007 0.024
2 0.013 0.015 0.031
3 0.012 0.013 0.031
4 0.012 0.011 0.030
5 0.012 0.010 0.030
6 0.012 0.010 0.030
7 0.012 0.009 0.030
8 0.011 0.009 0.030
9 0.011 0.009 0.030
10 0.011 0.009 0.030
11 0.012 0.009 0.030
12 0.012 0.010 0.030
13 0.012 0.010 0.030
14 0.012 0.010 0.030
15 0.014 0.011 0.030
16 0.014 0.011 0.030
17 0.014 0.010 0.030
18 0.015 0.010 0.030
19 0.014 0.010 0.030
20 0.014 0.010 0.030
21 0.014 0.010 0.030
22 0.014 0.010 0.030
23 0.015 0.010 0.031
24 0.014 0.011 0.031
[/PRE]

I think it would be fair to say that locating every other control point is a relatively efficient scheme since the effort is roughly half and the increase in uncertainty is only about 10% or less.

[SARCASM]I used to use old receivers like that..... until my dad got a job.....[/SARCASM] - ( actually, thanks for the info- Definitely something to consider in our networks here)