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

First Order 1: 100,000 Second Order Class I 1: 50,000 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 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 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.