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
Class II
1: 50,000
1: 20,000
Third Order
Class I
Class II
1: 10,000
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
AA – Order
A – Order
B – Order
First Order
Second Order
Class I
Class II
Third Order
Minimum Accuracy Standard*
0.3 cm. + 1: 100,000,000
0.5 cm. + 1: 10,000,000
0.8 cm. + 1: 1,000,000
1.0 cm + 1: 100,000
2.0 cm + 1: 50,000
3.0 cm + 1: 20,000
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
First Order – Class II
Second Order – Class I
Second Order – Class II
Third Order
0.5 K mm
0.6 K mm
1.0 K mm
1.3 K mm
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 preprepared 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.