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GEODETIC CONTROL SURVEYS

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Title: GEODETIC CONTROL SURVEYS


1
GEODETIC CONTROL SURVEYS
  • Definition, Standards of Accuracy,
    Classification, Specifications, etc.

2
A control survey is a class of survey that
establishes positions of points with a high
degree of accuracy in order to support activities
such as mapping and GIS, property boundary
surveys, construction projects, etc.
3
In addition, established control nets with a
network of monumented control points can provide
a unified coordinate base for survey and other
activities within the area
4
Geodetic Network surveys are distinguished by use
of redundant, interconnected, permanently
monumented control points that comprise the
framework for the National Spatial Reference
System (NSRS) or are incorporated into NSRS (p
1-1, FGDC
5
Control points that are submitted to be included
in the NSRS must be surveyed to far more rigorous
accuracy and quality assurance standards than
control for general engineering, construction, or
topographic mapping
6
Standards of Accuracy and Classification of
Control Surveys
7
A Survey Standard may be defined as the minimum
accuracies deemed necessary to meet specific
objectives (McKay, Positioning Accuracy
Standards, ACSM-MSPS Workshop held in 1999)
8
Survey standards provide quality assurance as
well as consistency in a survey, and also help
re-establish missing survey monuments Control
surveys and networks are usually classified based
on the standard of accuracy of established
control points
9
Conventional control surveys have been classified
based on the relative positional accuracy between
directly connected control points as a ratio of
the horizontal separation between them
10
Directly connected points are those that have the
distance between them measured or are vertices of
a triangle that have been observed
11
Conventional classification of geodetic control
surveys are given in Chapter 4 of SU 3150
Class Notes and are repeated below
12
Order of Accuracy Maximum Closure
First Order 1
100,000 Second Order Class I
1 50,000 Class II
1 20,000 Third
Order Class I
1 10,000 Class II
1 5,000
13
It is clear that, if a higher accuracy
classification is needed when the relative
positional error is constant, then the separation
between points needs to be larger
14
Example If two, directly connected, first order
survey points A and B are 13,786 meters apart,
then the positional accuracy of one point
relative to the other is expected to be at
least 13,786x 1/100,000 0.138 meters
15
Conversely, if positional accuracy of point B
relative to A is 0.128 meters, then the relative
accuracy between them is 0.128/13,786
1/(13,786/0.128)
1/107,703
16
It is clear that, if the measurement technique
employed offers a constant precision in relative
position, higher accuracy classification can only
be achieved by increasing the separation between
points
17
If the length between two unrelated points is
computed, the accuracy of the computed length
needs to be determined by laws of random error
propagation
18
Example Assume there is point C where the
distance AC 11,420 meters and also has a
relative accuracy of 1 100,000. Now, the
accuracy of C relative to A is
11,420/100,000 0.114 meters
19
Assume also that computed distance between B and
C is 4,725 meters. Now, the accuracy of C
relative to B is given by Sqrt (0.138)2
(0.114)2 0.179 meters Note that it is NOT
equal to 4720/100,000 0.047 meters.
20
With the introduction of GPS techniques, the
accuracy standards were modified to accommodate
the higher accuracies possible with GPS, and are
given below (Geometric Geodetic Accuracy
Standards and Specifications for Using GPS
Relative Positioning Techniques, FGCS 1988)
21
Classification Minimum Accuracy
Standard AA Order 0.3 cm.
1 100,000,000 A Order 0.5
cm. 1 10,000,000 B Order
0.8 cm. 1 1,000,000 First Order
1.0 cm 1 100,000 Second Order
Class I 2.0 cm 1 50,000
Class II 3.0 cm 1 20,000
Third Order 5.0 cm 1 10, 000
At 95 Confidence Level
22
Example If control points A and B in a First
Order network and the distance between them is
6345.294 meters, then the accuracy of one point
relative to the other is Sqrt (0.01)2 (
6345.294/100,000)2
0.064 meters
23
Vertical Control has been generally classified as
follows as given in Chapter 4 of SU 3150 Class
Notes
24
Classification Relative Accuracy
Between
Directly Connected Points
First Order Class I 0.5 ?K mm
First Order Class II 0.6 ?K mm
Second Order Class I 1.0 ?K mm
Second Order Class II 1.3 ?K mm Third
Order 2.0 ?K mm K
is the distance between points in kilometers
25
Federal Geodetic Control Subcommittee of the
Federal Geographic Data Committee has now
published new accuracy standards for geodetic
networks in part 2 of their publication titled
Geospatial Positioning Standards (FGDC-007-1998)
26
New standards are supposed to supercede all
previous standards and only considers absolute
positional accuracy of a point at 95 confidence
level
27
Accuracy standards are given for horizontal
position, ellipsoid height and orthometric
height Table 2.1 Standards for Geodetic
Networks of the Geodetic Control Subcommittee of
the Federal Geographic Data Committee
28
Local Accuracy and Network Accuracy Following
definitions have been extracted from a workshop
conducted by NGS in 1999
29
The local accuracy of a control point is a
number, expressed in centimeters, that represents
the uncertainty, at 95 confidence level, in the
coordinates of this control point relative to the
other directly connected, adjacent control points
30
The network accuracy of a control point is a
number, expressed in centimeters, that represents
the uncertainty in the coordinates, at 95
confidence level, of this control point with
respect to the geodetic datum
31
For NSRS network accuracy classification, the
datum is considered to be best expressed by the
geodetic values at the CORS supported by
NGS Note that both local and network accuracies
are relative but neither is dependent on the
distance between points
32
Planning Field Reconnaissance A control
survey may consists of setting a few points to be
used for a survey project of limited extent, e.g.
a construction project, or an extensive network
of control points
33
Planning is most important when a control survey
is done in order to establish a large number of
points and/or when the survey covers a large
geographic extent
34
After the project has been studied as to the
geographic area covered, number and general
locations of points to be established, required
order of accuracy, and any other requirements
such as time constraints, a plan should be drawn
up to achieve required results
35
  • In large geodetic networks, optimal design of the
    network plays a major role in achieving
  • Desired accuracy
  • Reliability
  • Cost savings

36
Optimal design includes best locations for
network points, required precision of different
types of observations, and redundant
measurements, etc. Elements of network design
applicable for GPS networks will be discussed
later
37
Field reconnaissance is a mandatory component of
the planning process to ascertain the field
conditions such as terrain topography,
accessibility to certain locations, trespassing
issues, etc.
38
Control point locations could be marked, and
monumented if necessary, at this stage After
the field recon, a schedule including a timeline
can be prepared for the field campaign
39
In addition to above, there are other planning
issues specific to GPS that will be discussed
later
40
Fieldwork Field campaign should adhere to the
pre-prepared schedule as much as possible
41
Any variations should be evaluated to determine
the effect as to the timely completion of the
project
42
Computations/Adjustments Most observations
should be pre-processed, in the field if
possible, in order to determine if they meet
required accuracies
43
They also should be corrected for any systematic
errors such as meteorological corrections for EDM
distances
44
Finally, the network should be adjusted by Least
Squares techniques not only to determine the
coordinates of points but also to do a
statistical analysis of the results
45
Quality Analysis of Results Quality
analysis is an important part before reporting
the coordinates to the user
46
These include validity of the network adjustment
and expected variability of coordinates, etc.
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