One of the features Star*Net offers is being able to preanalyze survey designs. One need only enter the approximate coordinates of network points (sufficiently accurately to represent the approximate network geometry and distances between points) and specify the observations that are to be included in the adjustment, such as which angles and distances will be measured and which bearings will be either determined or held to some record value.
This is a diagram of the survey that Bowtie Surveyor posted in an earlier thread:
and this is how the design of that survey as originally posted would be represented for the purposes of preanalysis in Star*Net in case anyone would like to experiment with the most efficient way to satisfy the relative positional accuracy requirements that Bowtie was shooing for.
The exclamation marks indicate fixed coordinates. The other "C" values merely approximately define the size and configuration of the survey.
The "A" lines indicate which angles will be measured in From-At-To format.
The "D" lines are the distances to be measured.
[pre]
C 101 4982.03900 5037.07700 ! !
C 109 4743.02935 5056.34458
C 103 4979.02416 5524.66468
C 102 4951.35739 5181.78878
C 105 5024.97915 5000.99832
C 106 4966.74447 5207.68883
C 107 4973.26955 5042.13586
C 108 4975.03713 4998.98416
C 177 4706.67948 5040.78253
C 178 4737.58253 5178.40789
C 219 4705.93120 5206.21916
B 101-109
A 102-101-103
D 101-103
D 101-102
A 102-101-105
D 101-105
A 102-101-106
D 101-106
A 102-101-107
D 101-107
A 102-101-108
D 101-108
A 102-101-109
D 101-109
A 101-109-177
D 109-177
A 101-109-178
D 109-178
A 109-178-102
D 178-102
A 109-178-102
D 178-102
A 109-178-219
D 178-219
[/pre]
The professional skill to this lies in figuring out how to efficiently meet the specification.
I thought that the standard errors that Bowtie was using for some elements of the survey looked a bit loose, so the first thing I did was to see what substituting values that can be routinely obtained with some simple changes in methods and procedures would accomplish. Specifically, I was interested in the effect of improving target and instrument centering to the standard errors listed below:
[pre]
Instrument Standard Error Settings
Project Default Instrument
Distances (Constant) : 0.005000 FeetUS
Distances (PPM) : 2.000000
Angles : 4.000000 Seconds
Directions : 2.800000 Seconds
Azimuths & Bearings : 1.000000 Seconds
Centering Error Instrument : 0.001000 FeetUS
Centering Error Target : 0.003000 FeetUS
[/pre]
When preanalysis of the network was run, Star*Net produced the following estimates of relative positional uncertainties.
[pre]
Relative Error Ellipses (FeetUS)
Confidence Region = 95%
Stations Semi-Major Semi-Minor Azimuth of
From To Axis Axis Major Axis
101 102 0.013282 0.010982 111-14
101 103 0.050337 0.016548 0-21
101 105 0.014714 0.009836 139-58
101 106 0.018874 0.015196 5-07
101 107 0.014523 0.007775 150-01
101 108 0.014642 0.008870 79-35
101 109 0.011781 0.008247 175-23
102 178 0.016999 0.009553 86-40
109 177 0.014645 0.008286 23-11
109 178 0.013501 0.008207 71-11
178 219 0.014656 0.009150 138-42
[/pre]
All of those relative positional uncertainties meet the ALTA specification since no semi-major axis of the 95% relative error ellipses is larger than 0.07 ft.
Another way of verifying compliance with the relative positional uncertainty specification is by inspection. If no point positioned by a survey has a station coordinate error ellipse with a semi-diameter greater than 0.07 / SQRT(2) = 0.05 ft., then no pair of points that satisfies that requirement will have a relative error ellipse (at 95% confidence) with a semi-diameter greater than 0.07. In the case of the Bowtie survey, that would be true if the methods and procedures known to have the standard errors listed in the "Instrument Standard Error Settings" are used.
[pre]
Station Coordinate Error Ellipses (FeetUS)
Confidence Region = 95%
Station Semi-Major Semi-Minor Azimuth of
Axis Axis Major Axis
101 0.000000 0.000000 0-00
109 0.011781 0.008247 175-23
103 0.050337 0.016548 0-21
102 0.013282 0.010982 111-14
105 0.014714 0.009836 139-58
106 0.018874 0.015196 5-07
107 0.014523 0.007775 150-01
108 0.014642 0.008870 79-35
177 0.018474 0.012981 15-07
178 0.016357 0.011780 65-03
219 0.020729 0.018443 79-36
[/pre]
Preanalysis Adding Just One Angle
Point 103 is the dangler. Considering that the bearing from 101 to 109 is fixed, measuring the angle 103-101-109 is the obvious way to greatly improve the relative positional uncertainty of 103 with respect to the rest of the points connected by the survey.
To model the effect of that, all that is needed is to insert one line into the Star*Net input file to represent the fact that the angle will be measured.
[pre]
C 101 4982.03900 5037.07700 ! !
C 109 4743.02935 5056.34458
C 103 4979.02416 5524.66468
C 102 4951.35739 5181.78878
C 105 5024.97915 5000.99832
C 106 4966.74447 5207.68883
C 107 4973.26955 5042.13586
C 108 4975.03713 4998.98416
C 177 4706.67948 5040.78253
C 178 4737.58253 5178.40789
C 219 4705.93120 5206.21916
B 101-109
A 102-101-103
D 101-103
D 101-102
A 102-101-105
D 101-105
A 102-101-106
D 101-106
A 102-101-107
D 101-107
A 102-101-108
D 101-108
A 102-101-109
A 103-101-109
D 101-109
A 101-109-177
D 109-177
A 101-109-178
D 109-178
A 109-178-102
D 178-102
A 109-178-102
D 178-102
A 109-178-219
D 178-219
[/pre]
and these are the improvements in 95% confidence station coordinate error ellipses. None are close to 0.05 ft. in semi-major axis length, so by inspection one should expect that the ALTA relative positional accuracy specification will be met.
[pre]
Station Coordinate Error Ellipses (FeetUS)
Confidence Region = 95%
Station Semi-Major Semi-Minor Azimuth of
Axis Axis Major Axis
101 0.000000 0.000000 0-00
109 0.011285 0.008247 175-23
103 0.029896 0.016548 0-21
102 0.013232 0.009443 105-54
105 0.014714 0.009595 139-58
106 0.017683 0.015196 5-07
107 0.014523 0.007765 150-01
108 0.014642 0.008743 79-35
177 0.018200 0.012928 16-24
178 0.016192 0.011309 68-24
219 0.020668 0.018027 83-18
[/pre]
Preanalysis Adding Just One Angle
> and these are the improvements in 95% confidence station coordinate error ellipses. None are close to 0.05 ft. in semi-major axis length, so by inspection one should expect that the ALTA relative positional accuracy specification will be met.
Does the Positional Tolerance Checking option and/or the PTOLERANCE inline option work with a preanalysis run?
Preanalysis Adding Just One Angle
> Does the Positional Tolerance Checking option and/or the PTOLERANCE inline option work with a preanalysis run?
Yes, .PTOL seems to work fine in preanalysis. Here's a specimen output report with 0 PPM as the distance-dependent error component (my choice):
[pre]
Positional Tolerance Check (FeetUS)
Allowable Tolerance = 0.0700 + 0 PPM
Tolerance Check Confidence Region = 95%
Listing Failures Only
Stations Horizontal Semi-Major-Axis Ratio
From To Distance Actual Allowed Actual/Allowed
Connections Checked = 11
Number of Failures = 0
[/pre]
Preanalysis Adding Closing Angle 178-102-101
Oh, and this is what preanalysis indicates would be expected of the station coordinate error ellipses if the closing angle 178-102-101 were observed:
[pre]
Station Coordinate Error Ellipses (FeetUS)
Confidence Region = 95%
Station Semi-Major Semi-Minor Azimuth of
Axis Axis Major Axis
101 0.000000 0.000000 0-00
109 0.010843 0.008247 175-23
103 0.047482 0.016548 0-21
102 0.013068 0.009237 123-01
105 0.014714 0.009646 139-58
106 0.017938 0.015196 5-07
107 0.014523 0.007767 150-01
108 0.014642 0.008770 79-35
177 0.017965 0.012876 17-35
178 0.016188 0.011213 61-23
219 0.020360 0.018263 72-51
[/pre]
Those may be compared to these 95% confidence error ellipses without the closing angle:
[pre]
Confidence Region = 95%
Station Semi-Major Semi-Minor Azimuth of
Axis Axis Major Axis
101 0.000000 0.000000 0-00
109 0.011781 0.008247 175-23
103 0.050337 0.016548 0-21
102 0.013282 0.010982 111-14
105 0.014714 0.009836 139-58
106 0.018874 0.015196 5-07
107 0.014523 0.007775 150-01
108 0.014642 0.008870 79-35
177 0.018474 0.012981 15-07
178 0.016357 0.011780 65-03
219 0.020729 0.018443 79-36
[/pre]
The differences are quite minor. The closing angle functions more as a field check in the case of this four-sided figure.
Preanalysis Adding Closing Angle 178-102-101
> Oh, and this is what preanalysis indicates would be expected of the station coordinate error ellipses if the closing angle 178-102-101 were observed:
>
> Station Coordinate Error Ellipses (FeetUS)
> Confidence Region = 95%
>
> Station Semi-Major Semi-Minor Azimuth of
> Axis Axis Major Axis
> 103 0.047482 0.016548 0-21
Why would only the Semi Major axis of point 103 go UP as a result of adding more measuremnts? For that matter, why would ANY RPA's go up (I mean the number up, the accuracy down). ? Prior to the added measurment, it was:
103 0.029896 0.016548 0-21
Preanalysis Adding Closing Angle 178-102-101
I gave two alternate scenarios in which an angle could be added.
Adding angle 103-101-109 OR adding angle 178-102-101.
The first scenario reduced the semi-major axis of the Station Coordinate Error Ellipse. The second didn't change much of anything in the way of the error ellipses.
