basics of surveying

1. SURVEYING WITH GPS
Key Words: Static, Fast-static, Kinematic, Pseudo-
Kinematic, Real-time kinematic, Receiver
Initialization, On The Fly (OTF), Baselines,
Redundant baselines, Base Receiver,
Rover
GPS has become a standard surveying technique in most surveying
practices
This could be attributed to both the ease of use as well as reduction
in hardware costs
Advantage of GPS surveys
• Three Dimensional
• Site Intervisibility Not Needed
• Weather Independent
• Day or Night Operation
• Common Reference System
• Rapid Data Processing with Quality Control
• High Precision
• Less Labor Intensive/Cost Effective
• Very Few Skilled Personnel Needed
GPS eliminates the need for establishing control before a survey

2. GPS can establish control as and when needed and establish points
at strategic locations to start and close conventional traverses
All or any of the following values could be available directly in the
field or after post-processing the data
• Latitude, longitude, geodetic height and X, Y, Z
Cartesian coordinates
• State Plane or Project coordinates
• Forward and back geodetic azimuth of the baseline
• Geodetic distance or Monument to Monument slope
distance of baselines
• Vertical angle from point to point
GPS determines the geodetic azimuth between two points directly
thereby eliminating the need for converting an astronomic azimuth
to geodetic azimuth by applying Laplace correction
State plane coordinates can be directly computed from the latitudes
and longitudes obtained from GPS
The slope distances can be reduced to the ellipsoid very accurately
as ellipsoidal height is known
Note, however that, even though the baseline components such as
distances and azimuths are accurate, the accuracy of coordinates of
new points are dependent on the quality of known points included
in the survey

3. GPS OBSERVATION TECHNIQUES
In surveying applications, accepted method is relative positioning
although Differential GPS with code receivers could be used in
projects that require a very low order of accuracy, e.g. locating
cellular phone transmission towers
In relative positioning, two or more receivers make simultaneous
phase measurements on the carrier frequencies from four or more
satellites
Only the data collected from the same satellites by the receivers
occupying the ends of a baseline can be used in the computation of
this baseline
As stated earlier, accurate baseline components can be computed
from simultaneous carrier frequency measurements from four or
more satellites by two receivers
If accurate coordinates of a point are needed, one of the receivers
must occupy a point whose coordinates are known accurately
For this reason, at least one receiver must occupy a known point in
the WGS84 datum as GPS satellite positions are defined in
WGS84 datum coordinate system
In practice, however, points in the National Spatial Reference
System (NSRS) which are in NAD83 datum are used

4. For projects requiring geodetic accuracy, National Geodetic
Survey (NGS) must be contacted or NSRS database must be
accessed to determine points available in the project area before
the project begins
If there are points previously established which are in the National
Reference System and meet the accuracy requirements, these
points can be used instead of NGS points
Coordinates of points determined in the WGS84 (NAD83) system
could be transformed to any other coordinates such as State Plane
coordinates or project coordinates either in the field, if real time
coordinates are needed, or after post processing
State Plane coordinates can be directly computed from
NAD83 coordinates by using software available from NGS
or other parties
If accurate project coordinates are needed, an accurate
transformation needs to be done by including a sufficient
number of points whose coordinates are known in both
systems
In less precise work such as in construction projects,
project coordinates can be determined by a field calibration
of the site independently using an approximate
transformation

5. There are several different field techniques for GPS surveying
using carrier frequency
These techniques could be used singly or in combination in some
surveying projects
The mode used depends on
• Accuracy needed
• Type of equipment and post processing software
available
• Sky visibility in the project area
• Other considerations such as mode of transportation,
urgency of the project, etc.
The primary objective of each of the field techniques is to resolve
the integer ambiguity resulting from carrier wave measurements
Unlike conventional surveys, planning is an important part of any
GPS survey, regardless of the technique used
Following are the techniques that are commonly used now
• Static

6. • Fast Static (Rapid Static)
• Kinematic
• Pseudo-kinematic(Pseudo-static)
• Real Time Kinematic
STATIC MODE OF GPS SURVEYING
This method, sometimes called static surveying, is used surveying
projects that require high accuracy
In this method, each receiver at each point logs data continuously
for a pre-planned length of time
The duration of data collection depends on
• required precision
• number of visible satellites
• satellite geometry(DOP)
• whether the receivers are single frequency or dual
frequency
• distance between receivers
The duration of data collection, however, should be long enough
for the post processing software to resolve the integer ambiguity
Most new generation receivers and processing software are capable
of resolving the integer ambiguity with small amount of data

7. However, a higher accuracy for the baseline components can be
achieved by collecting data for a longer period of time
Collection of data using two or more receivers for a period of time
is called a session
The slope line between any two antennas is called a baseline
vector or simply baseline
If more than two receivers are used multiple baseline vectors can
be determined simultaneously
Most GPS survey projects consist of multiple baselines or
networks, and the baselines can be measured individually using
only two receivers or several at a time using multiple receivers
For economic reasons it is preferable to use more than two
receivers for multiple baselines
When the baseline between a known point and a new point is
measured the new point can be used as a known point for other
baselines
Unlike in conventional surveys, the accuracy obtainable from
networks is independent of the network geometry

8. Accuracy can be increased by increasing the number of redundant
measurements
Redundant measurements are those that are over and above the
ones required to determine the coordinates of unknown points
A redundant measurement should also be independent i.e. a
measurement that is not related to or could not be generated from
other measurements
In a single session using more than two receivers, there are both
independent (non-trivial) and dependent (trivial) baselines
A baselines measured in separate sessions are always independent
Redundant measurements can also be used to check for blunders
In a network of GPS baselines, blunders can be detected by
checking the closures of loops formed by connecting, independent
baselines
If the network is such that the loops are elongated in a east-west
direction a higher accuracy in the positions can be obtained(GPS
measurements are stronger in north-south direction) and this
should be considered at the planning stage of the network

9. Networks should also have several control points, located at
strategic locations, in order to strengthen the network
These control points should be preferably above or at least equal to
the order of accuracy expected of new points
The number and locations of control points depend on the size and
shape of network (See Geometric Geodetic Accuracy Standards
and Specifications for Using GPS Relative Positioning
Techniques, Federal Geodetic Control Sub-committee, 1988)
FAST STATIC MODE OF GPS SURVEYS
Fast Static or Rapid Static was a method developed for dual
frequency receivers
A new algorithm was developed to reduce the amount of data
needed to resolve integer ambiguity
Lately, because of modifications in processing algorithms and
because a larger number of satellites are available, the amount of
data needed can be reduced even with single frequency receivers
Sometimes, the manufacturers call this also fast static
Field requirements and procedure for fast static are same as those
for static except for the short session lengths

10. However, fast static is only suitable for low order control surveys,
e.g. ground control for photogrammetric mapping
KINEMATIC MODE OF GPS SURVEYING
This is the mode of positioning from a moving platform. i.e. when
the antenna is in motion
This is the mode used in navigation where usually only a single
receiver is used
But, unlike in navigation, the kinematic method used in surveying
is a relative positioning method where one antenna+receiver is
stationary and one antenna+receiver is moving
When the moving receiver is in constant motion as in navigation it
is called ‘continuous’ kinematic
In most surveying applications, a method called ‘stop-and-go’
kinematic is used
The stationary receiver, called the base receiver, is placed at a
known point while a second receiver called "rover' will visit all
unknown points
Rover will occupy each unknown point for a very short time (less
than two minutes); Hence the term "Stop-and-Go" surveying

11. It is possible to combine both ‘continuous’ and ‘stop and go‘
methods in the same survey
It also is possible to operate more than one ‘rover’ with the same
base station
The accuracy obtained is not as good as that obtained from static
surveying but is better than that obtained in most surveys
The single most advantage of ‘stop and go’ surveying is its speed
This method also has certain limitations
• An initialization process to determine the integer biases
of at least 4 satellites is needed at the beginning
• The lock on the same four or more satellites must be
maintained during the entire survey
For this reason, kinematic GPS surveying is suitable for an area
where there are no large over-hanging trees, over-passes or such
structures in rover’s route
If for any reason a cycle slip occurs, the rover must return to any
previous point which had been determined without cycle slip

12. The Initial integer bias term can be determined in one of 3 ways
• Using a known baseline less than 20 km in length and
having an accuracy of less than 5 cm.
• Antenna swap
• Perform a static mode survey first for one of the base lines
When using a known baseline, it is necessary to use one end of the
baseline as the base station
The rover will occupy the other end to collect 3 or 4 epochs of data
(less than 2 minutes)
Antenna swap is done by first occupying the known point with the
base receiver and another point 15-30 feet away with the rover
After collecting data for 3-4 epochs two receivers + antennas are
swapped while maintaining lock
Collect data for another 3-4 epochs, return the base receiver +
antenna to the base and continue the survey with the rover as usual
In the third method, a baseline is measured by static method with
the base receiver at the known base
This now becomes a known baseline and the rest is similar to the
first method
For highest accuracy more than 6 satellites, well distributed over
the sky is preferable

13. Kinematic post processing software is needed to obtain the point
coordinates
Kinematic GPS is similar to radial surveys with Total stations
Kinematic GPS can use multiple bases and/or multiple rovers in
the same survey, if necessary
REAL TIME KINEMATIC GPS SURVEYS
Real time kinematic (RTK) refers to a stop-and-go method where
the coordinates of points are available in real time
In this method, a radio communication link is maintained between
the base receiver and the rover, and the base receiver supplies the
pseudo-range and carrier phase measurements to the rover which
in turn computes its position and display the coordinates
The rover keeps updating coordinates as it moves as long as the
lock on satellites is maintained
Kinematic GPS surveying is generally suitable for any type of
surveying or mapping, but for stakeout surveys, RTK is essential

14. Some RTK receivers have the capability of resolving the integer
ambiguity On The Fly (OTF), and this technique can only be used
with dual frequency receivers
This means that there is no need to maintain the lock on satellites
while the rover is in motion
New observables are generated by taking linear combinations of
observations made on these codes and carriers (wide laning)
The integer ambiguity can be resolved very quickly by this
technique while the receiver is still in motion
Wide laning techniques are used in some high-end receivers even
if OTF is not being used

15. PSEUDO-KINEMATIC (OR PSEUDO-STATIC)
This is a combination of both static and kinematic methods
It has the speed of kinematic method but there is no need to
maintain lock on 4 satellites
However newer receivers and algorithms can resolve the integer
ambiguity much faster and the need for pseudo-kinematic surveys
is somewhat diminished
There is a reference (or base) receiver and a roving receiver, and
the reference receiver remains at the reference point during the
entire survey while the roving receiver visits the unknown points
There is no initialization as in ‘stop and go’ method
Each point is occupied for 5-10 minutes for baselines 10 km. or
less
Each point must be revisited multiple times (at least once more)
and these visits must be separated by at least 1 hour and preferably
not more than 4 hours
Multiple observations at the same site at different times capture
different epochs along the satellite's orbit, and allow the satellite
configuration to change and to resolve the integer ambiguity

16. This technique is suitable for areas where there are obstructions to
signal and crew movement or if the receivers are not equipped with
kinematic software
Pseudo-kinematic is the least precise of all methods but is more
productive than static
Stop-and-Go kinematic method is suitable for details surveys as
topographic mapping or boundary survey work whereas pseudo-
kinematic is suitable for lower order control such as
photogrammetric control etc.
The GPS survey technique used in a given project depends on
• Accuracy requirements
• Urgency of the project
• Local terrain conditions
• Available equipment, etc.
A combination of these methods can be used in some projects

17. COMMON ERRORS IN GPS OBSERVATIONS
• Setting up over wrong point
• Not using well adjusted tribrachs
• Not observing long enough during a session
• Poor planning (selection of points that may cause cycle
slips or multipath or poor PDOP)
• Interruption due to power failure (not checking
batteries prior to departure)
• Reading and recording wrong antenna height
Some recommend that instead of using a tribrach with an optical
plummet a rotatable tribrach adapter with a plate vial be used
Errors such as the following may be introduced at the processing
stage as well
• Incorrect datum or coordinates for known points
• Incorrect linear units for the project
• Entering incorrect point names and/or antenna heights

18. POSSIBLE ERRORS IN KINEMATIC SURVEYING
• Antenna height may change between points, especially if a
prism pole with a sliding mechanism is used
• Not properly plumbing the antenna over the point at time of
measurement
If no OTF is available, it is sometimes necessary to raise the
antenna over some obstructions in order to maintain lock
For this reason a method to raise the antenna will be useful
QUESTIONS
1. What are the advantages of GPS surveying over conventional surveying methods ?
2. What is the single factor that determines whether or not a GPS survey is possible in
an area and/or a project ?
3. What is a ‘baseline’ in GPS surveying ?
4. What information/quantities pertaining to points and baselines are computed by post-
processing software ?
5. What is an ‘epoch’ in GPS terminology ?
6. Why is the ‘static’ GPS survey method so named ?
7. What is the reason for ‘minimum session length’ in static surveying ?
8. What factors determine the length of a session in static surveying ?
9. What factors determine the GPS surveying method suitable for a given area/project ?
10. What is the purpose of rover ‘initialization’ in kinematic surveying ?
11. What are the three ways of rover initialization in ‘stop-and-go’ kinematic surveying ?
12. What is the fastest initialization method ?
13. How much data is collected at each point in stop-and-go GPS surveying ?
14. What is the purpose of re-occupation of points in pseudo-kinematic method ?
15. What are the time limitations on re-occupation ?
16. Which GPS surveying method would you use for establishing control with geodetic
accuracy ?
17. Which GPS surveying method would you use if you need to complete a job urgently ?

19. 18. Which GPS survey method is suitable for a project that does not need very high
accuracy but the project is in a downtown area where there are tall buildings and
over-passes ?
19. What type of receivers are needed to do true fast static surveys ?
20. Everything else being equal, real time kinematic GPS or conventional method with a
total station would you prefer for a stakeout survey ? Indicate your reasons.