1 / 46

GEODETIC CONTROL SURVEYS

- Definition, Standards of Accuracy,

Classification, Specifications, etc.

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.

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

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

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

Standards of Accuracy and Classification of

Control Surveys

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)

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

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

Directly connected points are those that have the

distance between them measured or are vertices of

a triangle that have been observed

Conventional classification of geodetic control

surveys are given in Chapter 4 of SU 3150

Class Notes and are repeated below

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

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

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

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

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

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

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

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.

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)

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

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

Vertical Control has been generally classified as

follows as given in Chapter 4 of SU 3150 Class

Notes

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

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)

New standards are supposed to supercede all

previous standards and only considers absolute

positional accuracy of a point at 95 confidence

level

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

Local Accuracy and Network Accuracy Following

definitions have been extracted from a workshop

conducted by NGS in 1999

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

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

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

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

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

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

- In large geodetic networks, optimal design of the

network plays a major role in achieving - Desired accuracy
- Reliability
- Cost savings

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

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.

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

In addition to above, there are other planning

issues specific to GPS that will be discussed

later

Fieldwork Field campaign should adhere to the

pre-prepared schedule as much as possible

Any variations should be evaluated to determine

the effect as to the timely completion of the

project

Computations/Adjustments Most observations

should be pre-processed, in the field if

possible, in order to determine if they meet

required accuracies

They also should be corrected for any systematic

errors such as meteorological corrections for EDM

distances

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

Quality Analysis of Results Quality

analysis is an important part before reporting

the coordinates to the user

These include validity of the network adjustment

and expected variability of coordinates, etc.