MODERN FIELD SURVEY SYSTEMS
(EDMs And TOTAL STATION)
Surveying is the process of determining the ground features and transferring their relative position on to the paper. Surveying principles can be applied in a variety of ways to accomplish the aim of position finding. The preferred survey method for both two and three-dimensional position findings has changed through the years in response to the advances in technology. Different methods considered in modern surveying include precise leveling & traversing, Astronomical surveying, Photogrammetry, EDM, Remote sensing, GIS, GPS, DGPS, etc.
EDMs And TOTAL STATION:
There are three methods of measuring distance between points (i) Direct distance measurement (DDM) such as the one by chaining or taping (ii) Optical distance measurement(ODM) such as the one by tachometry, horizontal subtense method, or telemetric method using optical wedge attachments (iii) Electromagnetic distance measurements (EDM) such as the one by Geodimeter and Distomat.
The method of direct distance measurement is unsuitable in difficult terrain and sometimes impossible when obstructions intervene. The problem was overcome after the development of optical distance measuring methods but in the ODM method also the range is limited to 150 m and the accuracy obtained is 1 in 1000 to 1 in 10000. Electromagnetic Distance Measurement enables the accuracy up to 1 in 100000 over ranges up to 100 km.
Electronic Distance Measurement:
Electromagnetic distance measurement (EDM) equipment consists of an aiming head/ receiver unit set at one end of the line to be measured and pointed towards a reflective glass prism set up at the other end. An electromagnetic beam is emitted by the aiming head, projected towards the prism, reflected back, and analyzed to determine the distance.
Type of EDM Instruments:
Depending upon the type of carrier wave employed, EDM Instruments can be classified under the following three heads.
i) Micro Wave Instruments, ii) Visible light Instruments & iii) Infra red Instruments
Distomat is a very small compact EDM particularly useful in building construction, Civil Engineering Construction, Cadastral, and detail survey, particularly in populated areas where 99% of distance measurements are less than 500m.
ADVANTAGES OF MODERN SURVEYING INSTRUMENTS AND METHODS:
i) Traditional survey methods are laborious and time-consuming
ii) Familiar manual procedures have been integrated and automated
iii) Fully automatic electronic measurement
iv) Digital display of staff reading and distance
v) Single measurement or averaged repeat measurements.
vi) Data storage in Instruments possible
vii) Direct transfer to Personal computer of data stored in Instruments
viii) Online operation through the integrated interface to the computer.
The instrument of choice for the modern surveyor integrates an electronic digital theodolite, a electronic distance measuring instrument and a computer into single unit. The resulting hybrid instrument is called a ‘Total Station'. A total station automatically measures and displays distance and direction data (both horizontal and vertical angles) and results to its computer.
Hence the total station instrument is a combination of
a. Distance measuring instrument (EDM) .
b. Angle measuring instrument (Theodolite)
c. A simple micro processor
The total station emits a laser signal that bounces off the prism reflector at the top of the rod and then returns to the gun. The distance and slope at which the laser beam travels provides the basis on which the data collector calculates the azimuth to and elevation of the point on which the rod is placed.
COMPONENTS OF TOTAL STATION:
The total station is referred to generically as the gun. The process of sighting the rod with the gun,and then recording the locational data using the data collector is referred to as “shooting”
1.1 Optical Plumb Bob:
There is the optical plummet telescope, or optical plumb bob that looks like another knob and is located immediately to the left of the data screen. The optical plumb bob can be viewed through to see the ground directly below the center of the gun. When using this plumb bob, the gun can not be screwed into the tripod, since it will obscure the view. The gun must be seated on top of the tripod loosely, so that it can be shifted around until it is positioned directly over the marker. When looking through the knob, there are two black circles and one dot arranged in a ”bulls-eye” fashion that allow the user to line up the gun precisely on a location.
1.2 Eyepiece and Sighting Collimator:
The easiest way to line up the crosshairs on the prism reflector is to rely on the sighting collimator, or sight, which is located on the top of the gun below the carrying handle. The sight can be used to get the gun into range with great accuracy without having to look through the eyepiece. The key is to line up the triangle viewed in the sight on the prism reflector. This should place the prism reflector close enough to the crosshairs such that the focus adjustment knobs can be locked down without having to look through the eyepiece. Once the knobs are locked down, the user can look through the eyepiece and make fine-grained adjustments to line up the prism reflector on the crosshairs perfectly. However, it will take some practice using the sight and knowing how the sight works in order to do this accurately every time.
There are two ways to focus the eyepiece such that both the prism reflector and the cross hairs can be viewed clearly. First, if the prism reflector is blurry, rotate the gray ring, or objective focusing knob, that encircles the eyepiece until it is clear. Second, if the crosshairs are blurry, adjust the small black knob that is part of the eyepiece, which is known as the reticle focus knob.
Battery is attached to the gun on the right hand side when facing the eyepiece and data screen. The battery locks into place with a locking lever at the top of the battery. It is essential that the battery be re-charged daily when conducting fieldwork. There should always be at least one fully charged spare battery in case the one being used needs to be replaced.
1.4 Environmental Box:
The data collector is stored in an environmental box to prevent the elements from penetrating into the equipment. In particular, the environmental box prevents heavy misting and light rain from affecting the performance of the data collector. However, one note of caution is that condensation can build up inside the environmental box in humid and sunny conditions. In humid environments, it is recommended that the data collector should be taken out of the environmental box when not in use and allowed to “breathe.” Furthermore, it may be necessary to clean the contacts on the data collector regularly, even nightly, to prevent potential problems. Too much humidity (more than 90% relative humidity) can corrode the contacts such that data collector can not read information from the gun, and therefore can not record data. In general, the data collector should be stored in moderately cool, shady and dry conditions as much as possible when it is not in use.
1.5 HP-48CX Calculator and TDS-48GX Survey Card:
The data collector comprises a calculator with TDS-48GX survey card that is inserted into the back of the calculator. The data collector referes to the calculator when the survey program is enabled.
1.6 Batteries Of Calculator And RAM Card:
There are two parts of the data collector that use batteries: the calculator and the RAM card. It is essential that there are always spare batteries in the field. Whenever one is preparing to go into the field to conduct site mapping, make sure that extra batteries of both kinds are included in the bag in which the data collector is stored.
Other important Components of the Total Station are Data Screen, Prism Reflector, TRIPOD, Mechanical Plumb Bob, etc.