Total station

INTRODUCTION
A total station is an optical instrument used a lot in modern surveying and, in a minor way, as well as by police, crime scene investigators, private accident deconstructionists and insurance companies to take measurements of scenes. It is a combination of an electronic Theodolite (transit), an electronic distance meter (EDM) and software running on an external computer known as a data collector.
The original name of Total-station was electronic tachometer, but Hewlett-Packard introduced the name Total-station over 30 years ago and the name immediately caught on with the profession.
With a total station one may determine angles and distances from the instrument to points to be surveyed. With the aid of trigonometry and triangulation, 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.








Type of Total-station
In the early days, three classes of total stations were available-manual, semiautomatic and automatic.
Manual Total-station:-
It was necessary to read the horizontal and vertical angles manually in this of instrument. The only value that could be read electronically was the slope distances.

Semiautomatic Total-station:-
The user had to manually read the horizontal circle for these instruments, but the vertical circle readings were shown digitally. Slope distances were measured electronically and the instruments could, in most cases, be used to reduce the values to horizontal and vertical components.
Automatic Total-station:-
This type is the most common Total-station used now days. They sense both the horizontal and vertical angles electronically and measure the slope distances, compute the horizontal and vertical components of those distances, and determine the co-ordinates of observed points. The co-ordinates information obtained can either be stored in the total-stations memory or by using an external data collector.

PARTS OF A TOTAL STATION
I. Optical plummet
II. Display
III. Battery compartment
IV. Leveling tube
V. Leveling screws
VI. Vertical clamp
VII. Telescope
VIII. Horizontal clamp




FIELD EQUIPMENTS
Modern electronic survey equipment requires surveyor to be more maintenance conscious than they were in the past. They have to take care about power sources, downloading data and integrity of data, including whether or not the instruments and accessories are accurately adjusted and in good form. When setting up a crew to work with total-station and a data collector, it is helpful to supply the party chief with a checklist to help the crew maintain its assigned equipment and handle the collected data upon returning to the office. It is also important that each crew should be supplied with all necessary equipment and supplies. These should be stored in an organized and easily accessible manner.
Preparing an equipment list carefully will assure the survey crew a sufficient equipment inventory to meet the general needs of boundary, layout, and topographic surveys. This procedure will confine what are needs to maximize productivity when using a total station with a data collector.
The maximum equipment inventory required is as follows:
1. Total-station set
a. Total-station instrument in a hard case
b. Battery charger
c. Extra battery
d. Memory module/card, serial cable
e. Rain cover
f. User manuals
g. Tripod
h. Tape measure
2. Prism set
a. Prism
b. Prism holder
c. Centering rod
3. Back sight set
a. Prism
b. Prism holder
c. Prism carrier
d. Tribrach
4. Data processing
a. Laptop computer with serial port or USB port
b. Serial cable or USB-serial adaptor
c. Terminal application
d. Application program: MS Excel, Adobe illustrator, etc.
e. Data backup device and media (Zip, memory card, etc.)
5. Survey tools
a. Stakes, nails, paint, marker
b. Hammer
c. Thermometer
d. A pair of radio (with hand-free headsets)
e. Clipboard, field note, pen
f. Compass
g. GPS


Temporary adjustment of a total-station
Setting-up the instrument:-
Extend the legs of the tripod as far as is required and tighten the screws firmly.
Set-up the tripod so that the tripod plate is as horizontal as possible and the legs of the tripod are firm in the ground.
Now, and only now, place the instrument on the tripod and secure it with the central fixing screw.
Leveling-up the instrument:-

After setting up the instrument, level it up approximately with the bull’s-eye bubble.
Turn two of the foot-screws together in opposite directions. The index finger of your right hand indicates the direction in which the bubble should move.
Now use the third food-screw to centre the bubble.
To check, rotate the instrument 180o. Afterwards, the bubble should remain within the setting circle. If it does not, then readjustment is required (refer to the user manual.)
For a level, the compensator automatically takes care of the final leveling up.
The compensator consists basically of a thread-suspended mirror that directs the horizontal light beam to the centre of the crosshair even if there is residual title in the telescope.
If now you lightly lap a leg of the tripod, then you will see how the line of sight swings about the staff reading and always steadying at the same point. This is the way to test whether or not compensator can swing freely.
SETTING UP A BACK SIGHT:-
A back sight is a reference point for the horizontal angle. At the beginning of a new survey, a back sight can be set at an arbitrary point and marked. The best way to set up a back sight is to use a prism carrier and a tribrach on a tripod. The procedure for leveling up and centering of the prism is the same as that for the total-station. If there is no plummet in the tribrach and the prism carrier, use the plummet of the total-station and then exchange the total station above the tribrach with a prism carrier. A prism should be put right on the reference point when sighting is possible from the total station.



Azimuth Mark:-
An Azimuth Mark is a back sight without a prism. It should be geometric point or a vertical lane to aim at, with precision. Once a nice azimuth mark is found on the telescope, keep a detailed sketch and comments in the field book.
An azimuth mark may substitute for a back sight for certain total-stations, in case it is not necessary to define the error. Some CPU and data processing applications to require back sight, setting up of additional azimuth marks beside the back sight is useful to check whether the configuration has not gone wrong.

MEASUREMENT WITH TOTAL-STATION
When both the total-station and back sight are finally leveled and centered, the hardware setup is over and the software setup is to be started. The software setup of a total station differs from one make to another. One has to follow the user’s manual of each instrument. Most total stations memorize these settings, but it is better to check through he setup menu in order to avoid a false setting.
System: Choose appropriate existing interfaced for data output
Angle Measurements: tilt correction/tilt compensator (2 axis)
Horizontal angle increments: At right angle (clockwise)
Unit setting: Angle in degrees/min/sec, distance in meter, temperature in 0C and pressure in hPa
EDM settings: Select IR laser, fine measuring mode, use RL with caution. Set appropriate value for the prism constant.
Atmospheric Parameters: Get ppm for the diagram from the manual of the equipment or let the total station calculate from hPa and degree centigrade
Communications: Set all parameters the same for a total station and data logger/PC. They are baud rate, data bits, parity, end mark and stop bits. Refer the manual for each device.










ADVANTAGES OF TOTAL-STATION
The advantages of total-station include;
I. Quick setting of the instrument on the tripod using laser plummet
II. On-board area computation program to compute the area of the field
III. Greater accuracy in area computation because of the possibility of taking arcs in area computation
IV. Graphical view of plots and land for quick visualization
V. Coding to do automated mapping. As soon as the field jobs are finished, the map of the area with dimensions is ready after data transfer
VI. Enormous plotting and area computation at any user required scale
VII. Integration of database
VIII. Automation of old maps
IX. Full GIS creation
X. Location language support


PORRO–PRISM Double-PORRO-prism

DISADVANTAGES OF TOTAL STATION:-
I. Their use does not provide hard copies of field notes. Hence it may be difficult for the surveyor to look over and check the work while surveying.
II. For an overall check of the survey, it will be necessary to return to the office and preparing the drawings using appropriate software.
III. They should not be used for observation of the sun, unless special filters, such as the Troelof’s prism, are used. If not, the EDM part of the instrument will be damaged.
IV. The instrument is costly, and for conducting surveys using total station, skilled personnel are required.

INSTRUMENT ERRORS IN THE TOTAL STATION
Ideally, the total station should meet the following requirements:

a. Line of sight ZZ perpendicular to tilting axis KK
b. Tilting axis KK perpendicular to vertical axis VV
c. Vertical-circle reading precisely zero at the zenith
If these conditions are not met, the following terms are used to discribe the particular errors:
a. Line-of-sight errors. Or collimation error c
b. Tilting-axis error a deviation from the right angle between the tilting axis and the vertical axis
c. Vertical-axis tilt tangle user manual. Vertical-axis tilt does rate as begin an instrument has not been adequately leveled up, and measuring in both telescope faces cannot eliminate it. It influences on the measurement of the horizontal and vertical angle is automatically corrected by means of a two-axis compensator.
d. Height-index error (the angle between the zenith direction zero reading of the vertical circle ,i.e. the vertical circle reading when using a horizontal line of sight) .is not 100gon (900 but 100gon )
By measuring in both laces and then averaging the index error is eliminated it can also be determined and stored.
The effects of these three errors on measurements of horizontal angles Increase with the height difference between the largest points.
Taking measurements in both telescope faces eliminates line –of-sight errors and tilting errors .The line-of-sight error (and for highly precise total stations, also the tilting errors ,which is generally very small) can also be determined and stored .These errors are then taken into consideration automatically whenever an angle is measured, and then it is possible to take measurements practically free of error ,and telescope face The determination of these errors ,and their storage are described in the appropriate.












CONCLUSION

A total station is an optical instrument used a lot in modern surveying and, in a minor way, as well as by police, crime scene investigators, private accident deconstructionists and insurance companies to take measurements of scenes. Now a day it is the most useful instrument in the field of survey. Its speed and accuracy is very helpful to the surveyor for taking measurement.













REFERENCE

1. Advanced Surveying (Satheesh Gopi, R. Sathikumar & N. Madhu)
2. www.Gogle.com
3. www.wikipeadia.com