Principle of Electronic Distance Measurement and Use of EDM:

Electronic distance measurement(EDM) is a method of determining the length between two points using electromagnetic waves. EDM is commonly carried out with digital instruments called theodolites.

EDM instruments are highly reliable and convenient pieces of surveying equipment and can be used to measure distance of upto 100 kilometer. Each piece of EDM equipment available at Engineer supply provides dependably accurate distance measurements displayed on an easy to read digital screen.

Theodolite , Types of Theodolite and its components:

Theodolite:

It is an instrument used for measuring horizontal and vertical angles accurately. It is widely used in surveying for laying off horizontal angles, prolonging survey lines, finding difference in elevation and setting out engineering works which require higher precision.

It is used in ranging the highway, railway curve, alighnment of highway and railway, canal, tunnel, etc.

Theodolites are primarily classified as:

a. Transit theodolite

b. Non-transit theodolite

• A transit theodolite is one in which line of sight can be reversed by resolving the telescope through 180⁰ in the vertical plane. Vernier theodolite is also known and transit.

• A non-transit theodolite is one in which telescope cannot be transited.

Components of theodolite:

A transit theodolite consits of the following parts:

• Telescope
• Vertical circle
• Index frame/ T-frame/ Vernier frame
• Leavelling head ( consist of two parallel triangular plates i.e. upper tribach and lower tribach.)
• Two spindles/axes/centres
• Lower plate/ scale plate
• Upper plate/ Vernier plate
• Plate levels
• Tripod

Main parts of a theodolite:

1. Levelling head (7): Levelling head is used to attach the instrument to tripod and attach the plumb bob along the vertical axis of the instrument.
2. Lower plate/circle plate (18):  An annular horizontal plate with the graduations provided all around, from 0 to 360°, in a clockwise direction. The graduations are in degree divided in to 3 parts so that each division equals to 20 min.
   • Horizontal angles are measured with this plate.
   • The size of the theodolite is defined by the diameter of horizontal circle.
3. Upper plate (17): Horizontal plate of smaller diameter provided with two verniers. On diametrically opposite parts of its circumference. These verniers are designated as A and B. They are used to read fractions of the horizontal circle plate graduations. The verniers are graduated in 20 min and each minute is divided in 3 to 5 parts making least count  20⁰ or 10⁰ .
4. Clamps and tangent screws(15, 19): There are two clamps and associated tangent screws with the plate. These screws facilitate the motion of the instruments in horizontal plane.
5. Lower clamp screw locks or releases the lower plate. When this screw is unlocked both upper and lower plates move together. The associated lower tangent screw allows small motion of the plate in locked position.  
6. The upper clamp screw locks or releases the upper verniers plate. When this clamp is released the lower plate does not move but the upper verniers plate moves with the instrument. This causes the change in the reading. The upper tangent screw allows the fine adjustment.
7. Plate level (5):
   • Spirit level with the bubble and graduation on glass cover.
   • A single level or two levels fixed in perpendicular direction may be provided.
   • The spirit level can be adjusted with the foot screw (21) of the levelling head (7).
8. Telescope (10): The essential parts of the telescopes are eye-piece, diaphragm with cross hairs, object lens and arrangements to focus the telescope.
9. Vertical circle (1):  Circular plate supported on horizontal axis of the instrument between the A-frames. Vertical circle has graduation 0-90 in four quadrants. Vertical circle moves with the telescope when it is rotated in the vertical plane.
10. Vertical circle clamp and tangent screw (11):  Clamping the vertical circle restrict the movement of telescope in vertical plane.
11. Altitude level (2):  A highly sensitive bubble is used for levelling particularly when taking the vertical angle observations.

Adjustment of theodolites:

The adjustments of a theodolite are of two kinds:-

1.  Permanent Adjustments.

2.  Temporary Adjustments.

1. Permanent adjustments: The permanent adjustments are made to establish the relationship between the fundamental lines of the theodolite and , once made , they last for a long time. They are essential for the accuracy of observations.

The permanent adjustments in case of a transit theodolites are :-

• Adjustment of Horizontal Plate Levels. The axis of the plate levels must be perpendicular to the vertical axis.
•  Collimation Adjustment. The line of collimation should  coincide with the axis of the telescope and the axis of the objective slide and should be at right angles to the horizontal axis.
•  Horizontal axis adjustment. The horizontal axis must be perpendicular to the vertical axis.
• Adjustment of Telescope Level or the Altitude Level Plate Levels. The axis of    the telescope levels or the altitude level must be parallel to the line of collimation.
•  Vertical Circle Index Adjustment. The vertical circle verniers must read zero when the line of collimation is horizontal.

2. Temporary Adjustment: The temporary adjustments are made at each set up of the instrument before we start taking observations with the instrument. There are three temporary adjustments of a theodolite:-                 

• Centering.
• Levelling.
• Focusing.

a. Setting of theodolite over the station (Centering):

• Centering of the instrument
• Appropriate leveling with the help of tripod legs.

b. Levelling up (Levelling):

• Exact/ accurate levelling is done with the help of foot screws.

 c. Elimination of parallax:

• Focusing the eye-piece
• Focusing the objective

Uses of Theodolite-Measurement of Horizontal and Vertical Angles and Bearings

Horizontal angles: A horizontal angle is the difference in direction of two intersecting lines in a horizontal plane.

Vertical angle: A vertical angle is the difference in direction of two intersecting lines in a vertical plane. It is usually measured with respect to horizontal line and inclined line of sight at observed point. The inclined may be upwards or downwards with respect to horizontal axis.

There are three methods for angular measurements:

• Normal method
• Repetition method
• Direction method ( Reiteration method)

Components of Total station:

Total station is the latest developed surveying equipment. It is a combination of electronic tachometer and EDM instrument. It can observe distance, angles, bearings, elevation coordinate etc and displayed on its screen automatically as soon as a single key is touched.

Features of Total station:

• One Transmitting and Receiving unit ( Instrument station)
• Reflection unit ( Staff points/ target point)
• Excellent hardware feature for setup of the instrument
• Provide information with sound during field observation.
• The angular observations can be taken both in clockwise or anticlockwise direction.

Uses of Total station:

It can be used in Traversing, Trigulation, Trilateration, Trigometrical levelling and detail engineering survey like road, irrigation, hydropower, tunnel, mines, etc to measure distance angles, bearings, elevation, coordinate etc.

Electronic Data Recording:

Total station can be operated like a conventional theodolite using similar surveying procedure by the data observed by total station can be recorded automatically in the electronic field book fixed in total station. The device like Data loggers and Data recorders are used for collection and storing the observed data. Large amounts of data can be stored in this small device. The observed data are transmitted from the total station to a data recorder and these are stored together with point numbers generated by the recorder and feature codes which are entered manually on site.

So, after field observation, data collected is transferred from a data recorder to a computer through memory card for further processing,

Some common notations used in observations are,

HL = Face left horizontal angles

FR = Face right horizontal angles

V = Vertical angles

HD = Horizontal distance

VD = Vertical distance

SD = Slope distance

N = North cooridinate

E = East coordinate

Z = Z- coordinate

ANG = Angle meaasurement key

ESC =  Escape key

ENT = Enter key

F₁ – F₄ =  Function key

m =  Meter unit

F =  Inch/ Feet unit