Total Station
Dheeraj Kumar
Introduction
A total station is an optical instrument used in modern surveying. With this device, as with a
transit and tape, one may determine angles and distances from the instrument to points to be
surveyed. With the aid of trigonometry, the angles and distances may be used to calculate the
actual positions (x, y, and z or northing, easting and elevation) of surveyed points in absolute
terms.
It is a light weight, compact and fully integrated electronic instrument combining the capability
of an EDM and an angular measuring instrument such as wild theodolite. It can perform the
following functions.
–
Distance measurement
–
Angular measurement
–
Data processing
–
Digital display of point details
–
Storing data is an electronic field book
A standard transit is basically a telescope with cross-hairs for sighting a target; the telescope is
attached to scales for measuring the angle of rotation of the telescope (normally relative to north
as 0 degrees) and the angle of inclination of the telescope (relative to the horizontal as 0
degrees). After rotating the telescope to aim at a target, one may read the angle of rotation and
the angle of inclination from a scale. The electronic transit provides a digital read-out of those
angles instead of a scale; it is both more accurate and less prone to errors arising from
interpolating between marks on the scale or from mis-recording. The readout is also continuous;
so angles can be checked at any time.
The other part of a total station, the electronic distance measuring device or EDM, measures the
distance from the instrument to its target. The EDM sends out an infrared beam which is
reflected back to the unit, and the unit uses timing measurements to calculate the distance
traveled by the beam. With few exceptions, the EDM requires that the target be highly reflective,
and a reflecting prism is normally used as the target.
The reflecting prism is a cylindrical device, at one end is a glass covering plate and at the other is
a truncated cone with a threaded extension. It is normally screwed into a target/bracket on the top
of a pole; the pointed tip of the pole is placed on the points to be surveyed.
The total station also includes a simple calculator to figure the locations of points sighted. The
calculator can perform the trigonometric functions needed, staring with the angles and distance,
to calculate the location of any point sighted.
Many total stations also include data recorders. The raw data (angles and distances) and/or the
coordinates of points sighted are recorded, along with some additional information (usually
codes to aid in relating the coordinates to the points surveyed). The data thus recorded can be
directly downloaded to a computer at a later time. The use of a data recorder further reduces the
potential for error and eliminates the need for a person to record the data in the field.
The determination of angles and distance are essentially separate actions. One aims the telescope
with great care first; this is the part of the process with real potential for human error. When the
telescope has been aimed, the angles are determined. Only then does one initiate the reading of
the distance to the target by the EDM. That takes only a few seconds; the calculations are
performed immediately.
The total station is mounted on a tripod and leveled before use. Meanwhile, the prism is mounted
on a pole of known height; the mounting bracket includes aids for aiming the instrument. The
prism is mounted so that its reflection point is aligned with the center of the pole on which it has
been mounted. Although the tip of the pole is placed on the point to be surveyed, the instrument
must be aimed at the prism. So it will calculate the position of the prism, not the point to be
surveyed. Since the prism is directly above the tip, the height of the pole may be subtracted to
determine the location of the point. That may be done automatically. (The pole must be held
upright, and a bubble level is attached to give the worker holding the pole a check. It is not as
easy as one might expect to hold the pole upright, particularly if there is any wind; as a result,
multiple readings may be required.
When the instrument is set up and turned on, it sets itself to be pointing to zero degrees (north)
when power is first supplied. The user must then re-set the instrument to zero degrees when it is
actually pointing north
There are two adjustment knobs for rotating within the horizontal plane. One rotates the
telescope to make a sighting, with the readout of angles displaying changes. The other, however,
permits the user to rotate the entire instrument and to keep the current angle unchanged during
the process. That effectively re-orients the zero or north setting. That can be very helpful when
setting up or re-setting the instrument, but, of course, it can be devastating if one makes that
adjustment by mistake and thereby changes the north setting. This particular instrument was
designed in such a way that it was too easy to re-set the instrument when one only wanted to
make a sighting.)
The pole is designed to be placed on the survey point in a vertical position; it cannot be placed
on a point on the face of a wall. In fact, a prism pole can rarely be placed against the face of a
wall because of the bulk of the prism, the pole, and the target to which the prism is attached. For
siting a point on the wall, one can use reflecting sheet or even prism can be kept without its pole
and target.
The instrument measures to within 1-5 seconds for vertical and horizontal angles. The electronic
distance measuring device (EDM) measures to within 2 mm. and 3 parts per million; so the error
will be no more than the sum of 5 mm. and 3 parts per million of the measured distance from
instrument to prism. Instruments are available which measure to tighter tolerances, but for shortrange work
baseline
backsight
Total station
foresight
Fig. 1 : Measurement using a total station
Salient features
•
Dynamic angle – Scanning System ensures highest measuring accuracy.
•
With standard telescope.
•
Two – axis compensator automatically determines and corrects standing – axis tilt.
•
The data can be recorded and stored in a cassette or Card or Internal memory of the
instrument, and can be downloaded to or up loaded from a computer by supplied software
from manufactures (Leica Survey Office’ for Leica Model, T-com for Topcon and
Sokkialink for Sokkia Total station). The data is recorded in a format compatible to
Software programme and computer evaluation of the data, so that the out put can be
obtained in desired usable form.
•
Online control by computer
•
Total Station can be operated with a multi function Keyboard, which is associated with a
display screen which is some cases, gives a graphic display also.
•
Built-in Co-ordinate geometry functions for ancillary calculation.
•
Key-board-control
o All the functions are controlled by operating key board.
•
Digital panel
o The panel displays the values of distance, angle, height and the co-ordinates of the
observed point, where the reflector (target) is kept.
•
Remote height object
o The heights of some inaccessible objects such as towers can be read directly. The
micro-processor provided in the instrument applies the correction for earth’s
curvature and mean refraction, automatically.
•
Traversing program
o The co-ordinates of the reflector and the angle or bearing on the reflector can be
stored and can be recalled for next set up of instrument.
•
Setting out for distance, direction and height
o Whenever a particular direction and horizontal distance is to be entered for the
purpose of locating the point on the ground using a target, then the instrument
displays the angle through which the theodolite has to be turned and the distance
by which the reflector should move.
Use
With a total station one may determine angles and distances from the instrument to points to be
surveyed. With the aid of trigonometry, the angles and distances may be used to calculate the
coordinates of actual positions (X, Y, and Z or northing, easting and elevation) of surveyed
points, or the position of the instrument from known points, in absolute terms. The data may be
downloaded from the theodolite to a computer and application software will generate a map of
the surveyed area. Some total stations also have a GPS interface which combines these two
technologies to make use of the advantages of both (GPS - line of sight not required between
measured points; Traditional Total Station - high precision measurement especially in the
vertical axis compared with GPS) and reduce the consequences of each technology's
disadvantages (GPS - poor accuracy in the vertical axis and lower accuracy without long
occupation periods; Total Station - requires line of sight observations and must be setup over a
known point or within line of sight of 2 or more known points).
Total Stations are popular for the following reasons:
• One can work in an area where GPS units won’t work due to trees or other obstacles.
The total station doesn’t require any satellite coverage.
• The most accurate measurements are possible. The survey-grade GPS unit will give
around 20mm precision, but the total station, if used properly will give 5-10mm
precision.
Field Checklist for Total Station work
• tripod with cover
• Total Station
• Lens cover
• Back sight rod/tripod and prism and lens cover
• Foresight rod/tripod and prism and lens cover
• Manufacturer’s manual
• List/maps of points
• Field notebook
• Survey tape
• Binoculars
• Hammer and nail indent maker
• Clippers (for removing vegetation for line of sight) if necessary
Working Procedure
Physical Set-Up
Total station
• Stand yellow wooden/metallic tripod over the chosen point (this varies depending on
scenario), and loosen bottom screws on all the legs. Unclip the strap.
• Without spreading the legs, pull the tripod up to chin level and tighten the screws.
• Spread the legs of the tripod so that they are evenly positioned over the point. Unscrew
the yellow cap on the top. Arrange the tripod so that the openings between the legs are
facing the area where you will take most of the foresight shots.
• Remove the total station from the case, keeping one hand on the top handle and one
hand on the bottom at all times. Place the total station on the silver flat surface on the
tripod and attach it using the screw underneath the tripod. Be sure to set the total
station so that Face 1 of the total station is facing you, and the tribrach is situated so
that the bubble is also facing you (see pictures at the end of the manual)
Backsight
• There are two options for the backsight. You can use either a pole with prism OR
tripod with adaptor and prism.
• Attach prism to the top of the tripod/pole. Make accurate centering & leveling so as to
position the pole/tripod exactly on the ground point.
• Note down the height of prism centre from ground point (height of foresight)
• Make sure that the rod and prism are facing approximately the direction of the total
station. The operator will give you directions to make sure that it is perfectly pointed
later on.
Foresight
• There are two options for the fore sight. You can use either a pole with prism OR
tripod with adaptor and prism. Attach prism to the top of the tripod.
• Stand the rod/tripod and prism, and place the tip of the rod/tripod on the unknown point
of interest that you want to shoot (foresight shot). Make accurate centering & leveling
so as to position the pole/tripod exactly on the ground point Note down the height of
prism centre from ground point (height of foresight)
• Make sure that the rod and prism are facing approximately the direction of the total
station. The operator will give you directions to make sure that it is perfectly pointed
later on.
Instrument Orientation
There are two orientations to be carried out
a. Station Orientation
b. Back sight Orientation
In station orientation the absolute coordinates of the station (which is occupied by the total
station) is fed to the total station.
In backsight orientation either bearing of the base line or coordinate of the back station is fed to
the total station. This is done only after the perfect sighting to the backsight point. This is
required in order to orient the total station with respect to the known azimuth.
Measurement
The inclined distance and horizontal & vertical angles to target (Fore sights) are measured
automatically at a push of button.
Angle
The angle measurement system of the total station in similar to that of an electronic theodolite,
the dynamic angle-measuring system used in the total station makes use of a large number of
graduation for each reading of the angle on Horizontal or Vertical circle of the instrument. This
helps to eliminate as for as possible the effects of graduation errors.
Most modern Total Station instruments measure angles by means of electro-optical scanning of
extremely precise digital bar-codes etched on rotating glass cylinders or discs within the
instrument. The best quality total stations are capable of measuring angles down to 0.5 arcsecond. Inexpensive "construction grade" total stations can generally measure angles to 5 or 10
arc-seconds.
Distance
Measurement of distance is accomplished with a modulated microwave or infrared carrier signal,
generated by a small solid-state emitter within the instrument's optical path, and bounced off of
the object to be measured. Electromagnetic modulated beam or wave is generated in the main
instrument held at one end of the line to be measured. This modulated beam is directed toward a
reflector held at the other end of the line from where it is reflected back toward the main
instrument, in a parallel path, where the distance traveled by the electronic signal is determined
by measuring the “phase-difference” between the transmitted and reflected signals.
The modulation pattern in the returning signal is read and interpreted by the onboard computer in
the total station, and the speed-of-light lag between the outbound and return signal is translated
into distance. Most total stations use a purpose-built glass prism as the reflector for the EDM
signal, and can measure distances out to a few kilometers, but some instruments are
"reflectorless", and can measure distances to any object that is reasonably light in color, out to a
few hundred meters. The typical Total Station EDM can measure distances accurate to about 0.1
millimeter, but most land surveying applications only take distance measurements to 1.0 mm.
Some modern machines are 'robotic' allowing the operator to control the machine from a distance
via remote control. This eliminates the need for an assistant staff member to hold the reflector
prism over the point to be measured. The operator holds the reflector him/herself and controls the
machine from the observed point.
The other parameters like horizontal distance, co-ordinates, levels etc. are also calculated
automatically and displayed digitally. The display may be alphanumeric and graphic as
well. A number of trigonometrical functions and setting out parameters can also be
performed.
Initial setting parameters like bearing of a line, co-ordinates and height of the station
occupied, the height of instrument and that of the reflector height needs to be input in the
instrument memory before starting the actual measurement.
It can also be used in trekking mode for continuous measurement on a moving reflector
for setting out points of interest.
Observations
•
A number of functions, settings and adjustments are available in the control panel
mounted on the system to set up the Total Station for specific tasks.
•
Co-ordinates, height, and horizontal direction (angle) of instrument stations can be stored
in the theodolite and are then available as station co-ordinates for the next traverse
stations.
Applications
Total stations are the primary survey instrument used in many mining applications.
Underground Mining
As the development drifts in an underground mine are driven, a total station will be used to
record the absolute location of the tunnel walls (tope), ceilings (backs), and floors. This data can
then be loaded into a CAD program, such as SURPAC, AutoCAD, and compared to the designed
layout of the tunnel.
At regular intervals, the survey party will install stations. These are small steel plugs that are
drilled into the walls or the back. The plugs are installed in pairs. For wall stations, two plugs are
installed in opposite walls, forming a line perpendicular to the drift. For back stations, two plugs
are installed in the back, forming a line parallel to the drift.
When the survey crew wants to set up the total station in a drift, they use a set of plugs to locate
the total station.
Surface applications
1. Establishing National Grid bases at new areas:
2. Establishing boundaries in case of mining lease areas
3. Documentation of land holdings
4. Establishing control stations around OCPs
5. Training the present generation to face future challenges
6. Other applications
a. Surveying the positions of benches
b. Surveying the positions of the working faces, (to monitor the production)
c. Monitoring the Positions of Machines in the Mine,
d. Measurement of O.B. Dumps.
e. Measurement of Stock piles,
f. Detail Surveying on the Surface,
g. Monitoring the pit slopes for Stability,
h. Delineating the position of water in the sump.
i. Updating the mine’s Information System (IS)
j. Surveying the positions of boreholes, drilling sites etc.
k. Strengthening the Survey Control Network.
Manpower Requirements
• One operator and one person for each prism. At least one prism is necessary.
• There are systems that can be operated by one person.
• Once the data is collected, it must be uploaded onto a computer to process
Components of a Total Station
EDM
Electronic theodolite
On-Board Micro-processor
Data Collector (built in or separate unit)
Data Storage (internal or memory card)
Prisms
Tri-pod
Survey Rod (adjustable height)
Micro-processor
• Averages
multiple
angle
measurements
• Averages
multiple
distance
measurements
• Computes horizontal and vertical
distances
• Corrections for temp, pressure and
humidity
• Computes inverses, polars, resections
• Computes X, Y and Z coordinates
Fig. 2: Components of Total station
A
“RESECTION”
P
C
B
Fig. 3: Method of Angular measurement at a point
Field to Finish Operations
Memory card
USB and Compact
Flash
Automatic target
recognition
Control/operation
(robotic)
Specifications
1. Range
a. Reflectorless:
b. Single Prism
2. Accuracy
a. Angles
b. Distance
3.
4.
5.
6.
L.C
L.C
Data Storage
Display
Transfer remotely
(radio/cell phone)
Measurement
and basic
comps
Fig. 4: Field to finish operation
Final Comps,
checks and
outputs
upto 1200 meters
upto 5500 m
1” - 5”
3mm + 2ppm (prism)
4mm + 3ppm (reflectorless)
0.5” (Angular)
upto 0.2mm (Distance)
20000 points / flash card with 256MB memory
11 digits
Continuing Evolution of Measurement Technologies
Leica Smartstation
Merging TS and GPS
Broadcast of
Real-Time
Corrections
Topcon Imaging TS
Merging TS and Lidar
Terrestrial Photogrammetry
Google Earth
High Resolution Satellite Imagery
Fig. 5: State of the art measurement techniques
Major Manufactures
PENTAX Total Stations : http://www.pentax.co.jp
Nikon Total Stations : http://www.tdsway.com/products/nikon_tstations
Leica total station : http://www.leica-geosystems.com/corporate/en/products/total_stations/
Trimble total stations : http://www.trimble.com/totalstations.shtml
Sokkia total stations : http://www.sokkia.net/eu-index.html
Topcon total stations : http://www.topconpositioning.com
Smartstation
•
Recent developments include a GPS unit with the total station
•
Fully integrated data storage and data processing, Bluetooth data transfer or GPRS
•
It is a total station with integrated GPS receivers. All commands, displays, functions,
operations and computations relating to GPS reside in the total station. With Smartstation
there is no need to setup control points, long traverses or resections. Just Smart Station is
set up at chosen point and GPS determine the position. The survey work becomes easier,
quicker and it needs only few setups.
•
Total stations need local control points over which they can be set up, from which they
can traverse, and to which they can measure to resect their positions.
•
On the other hand, GPS receivers can determine their positions within a few seconds to
centimeter-level accuracy using data from GPS reference stations that may be 50km or
more away.
•
GPS rover receivers are fast and efficient to use but need an open view of the sky in order
that they can receive the satellite signals. They are at their most advantageous in wide,
open areas. By contrast, total stations can measure and stakeout where GPS cannot be
used: underground, to points under trees and bushes, in city canyons, on construction
sites where there are large obstructions.
Fig. 6: Smartstation
Errors in Total Station
Telescope errors
Observation errors
Levelling errors
Manufacturing errors
Fig. 7: Errors in Total station
The Telescope
•
Parallax error
•
Collimation error
Fig. 8: Error due to Telescope
Parallax error
The affect can be large. The solution is simple:
•
Focus the telescope on an object at optical infinity (say over 200m)
•
Then focus the eyepiece so that the cross-hair is crisp
•
Move your head and watch if the cross-hair moves
Collimation Error
•
An error of adjustment
•
Occurs when the centre of the cross-hair is not aligned with the optical axis of the
instrument
Fig. 9: Collimation Errors
•
Can be detected by observing the difference between face left and face right
–
For Horizontal angles, the difference is 180°
–
For Vertical angles, they should sum to 360 °
•
Can be removed by adjusting the cross-hair
•
Can be eliminated by taking the average of the face left and face right readings!
Plate Bubble Error
•
There are two level bubbles The pill or bulls-eye bubble
–
The horizontal plate bubble
–
(and the vertical index bubble)
Pill bubble
•
Is only approximate, to be used as a guide only
•
Can be corrected by adjustment once the instrument is correctly levelled
Fig. 10: Tribrach
Plate Bubble
•
Is the serious one
•
Is subject to errors of adjustment
•
The affect can be greatly reduced by averaging the displacement of the bubble during setup
Plumbing Errors
•
Difficult to detect, we need to test the tribrach/instrument combination in the workshop
•
The optical plummet is still a telescope, we need to focus the cross-hair and image
correctly
•
Errors are reduced by interchanging the jigger with targets when traversing
Observation Errors
•
These are treated as random and can be modelled by the Normal Distribution
•
They can be reduced by repeated measurement
•
This improves accuracy and precision
•
Pointing to the wrong target is NOT an error!
•
Reading the display incorrectly is NOT an error!
n
xmean =
σx
mean
∑x
i =1
n
n
=
σx
n