Electron Distance Measurement Surveying EDM | Tools and instruments

ELECTRONIC DISTANCE-MEASURING (EDM) EQUIPMENT 

When electronically determining the straight-line distance horizontal or slope between two points or stations, the equipment used

1. Sends an electronic impulse of known velocity or rate of speed, and 

2. Measures the time it takes for the impulse to travel the length of the interval between the points. 

Then, by using the equation distance = rate x time, the length of the interval is determined. Two types of electronic distance meters (EDMs) are commonly used. 

They are the 

1. The electromagnetic (microwave) instruments and 

2. The electro-optical (light wave) instruments.

 Electromagnetic (Microwave) EDM Instruments

 The first generation of electromagnetic equipment was very precise for measuring long distances; however, it was too bulky and heavy for the practicing surveyor’s needs. Over the years, with improvements in technology electromagnetic EDMs have become smaller and more portable, and are being equipped with direct readout capability. To use them, two identical and interchangeable instruments are set up at both ends of the line that needs measuring. 

The line must be unobstructed but need not be intervisible. This means observations in fog or during other unfavorable weather conditions are possible.      

Electro-Optical (Light Wave) EDM Instruments 

Electro-optical EDMs use the velocity of light waves to determine the distance between two points. The earliest of these instruments, the Geodimeter, was developed during the same decade as the electromagnetic.
Like the electromagnetic instruments, the first generation of electro-optical instruments was heavy, bulky, and not well suited to the needs of the practicing surveyor, however, through later development, modern electro-optical EDMs became smaller, lighter, and easier to use, and required less power.
Modern short-range instruments have ranges from 0.3 miles to 3 miles. Longer range instruments, using coherent laser light, have ranges from 50 feet to 36 miles.
To use an electro-optical EDM, the instrument is set up at one end of the line being measured and a reflector at the other end of the line. As with the electromagnetic EDM, the line must be free of obstacles. However, unlike using the electromagnetic device the stations at both ends of the line must also be intervisible. After setup, the EDM sends a modulated beam of light to the reflector, which returns the light pulse back to the EDM. When the instrument receives the reflected light flash, it converts the readings into linear distance between the EDM and the reflector with corrections made for atmospheric conditions.

 Direction of EDM Measured Lines. 

An EDM transmitter by itself is useful for determining only the length of a line. With some of the older EDM models, distance and direction are determined by separate setups of an EDM and a theodolite over the same station. With more recent EDM systems, the EDM transmitter is mounted on the theodolite or is built into the theodolite.

Reduction of Slope Distance 

To reduce the slope distance of a line to horizontal distance, either the vertical angle of the line measured from the instrument or the difference in elevation between the ends of the line must be known.

ELECTRONIC POSITIONING SYSTEMS 

The initial positioning systems require experience with navigational systems on board aircraft, and the Doppler systems deal with signals received from satellites. Electronic positioning systems consist of specially designed short-to-medium range EDMs that are attached to, or built into, a theodolite and can be used to determine distances and directions from a single setup of the instrument. Although many different electronic positioning systems are manufactured, each individual instrument is classed into one of three general groups as follows:

1.  Combined theodolite and EDM. 

Instruments within this group consist of an optical-reading repeating or direction theodolite with an attached EDM transmitter that can be removed for independent use of the theodolite.

2.  Computerized theodolite and EDM. 

The instruments in this group are similar to those within the combined theodolite and EDM group but have built-in electronic computers.

3.  Electronic tachometers. 

The equipment in this integrated, digitized, electronic system consists of a digitized theodolite, microprocessor, and EDM transmitter incorporated into one instrument. The instruments in this group also can be equipped with solid-state memory and magnetic tape or punched paper-tape storage units for storage of data. The equipment consists of an electronic digitized theodolite, an EDM unit, a microprocessor, a keyboard and display register, and a data storage unit. By inputting certain controlling data, such as temperature and atmospheric pressure.  Slope reduction using difference in elevation correction system needs, and by proper manipulation of the instrument controls, the operator can obtain horizontal angles, vertical angles, slope distances, horizontal distances, relative elevation, and coordinates of an unknown point. The data obtained is displayed by a liquid crystal display that can be transmitted and stored in a separate data collector. Complete operating instructions are provided with the tachometer.

LASER EQUIPMENT 

Laser light is of a single color, where light waves are in step with each other, and the light beam spreads only slightly as the distance from the light generator to the target increases.
These characteristics make the laser useful for surveying equipment used in various types of construction layout. Although a wide variety of special-purpose laser instruments exists, most have been designed for construction layout and are classified into two general groups as follows:
1.  Single-beam laser alignment instruments. These instruments project a single beam of light that is visible on targets under all lighting conditions. Included in this group are laser-equipped theodolites and transits, and lasers used for alignment of pipes, drains, and tunneling equipment.
2.  Rotating laser levels.
 These are instruments where the laser beam is rotated by rapidly spinning optics to provide a reference plane in space over open areas.

Single-Beam Laser Alignment Instruments 

A typical single-beam laser alignment instrument is mounted on a transit-like framework with horizontal and vertical motions, a spirit level that is parallel to the axis of the laser, and both vertical and horizontal circles. A telescope is attached to the laser housing to allow the operator to sight the location of the transmitted laser spot. A separate fanning lens allows the laser beam to be converted to a horizontal or vertical line instead of a spot.

Rotating Laser Level 

In this instrument, the laser unit is mounted vertically on a platform containing two orthogonally mounted sensors that act like spirit levels and deviate from center when the platform is not level. The amount of deviation is detected electronically, and the consequent electrical impulses drive servomotors that automatically level the base and make the axis of the laser vertical. 

The laser beam is emitted at an angle 90 degrees to the axis of the laser by an optical train, and the optics rotate to form a horizontal reference plane. This device also can be side-mounted so the axis of the laser is in a horizontal position, and a vertical plane can be formed by the rotating beam. An electronic sensing device, parallel to the axis of the laser, allows self-plumbing of the rotating beam. 

The instrument is self-leveling and self-plumbing within a range of 8 degrees. 

Beyond 8 degrees, it will not operate. This is a safety feature. The tolerance specified for the position of the reference plane with respect to true level or true vertical is 20 seconds of arc. Thus, in a distance of 330 feet, a deviation of 0.03 feet is possible. 

Laser  Rod 

A  laser  rod  equipped with a laser  detector  contains  a sliding  battery powered sensor  on  the  front  face  of  the  rod.
 When  within  0.45 feet  above  or  below  the rotating  laser  beam,  this  sensor  locks  onto the  beam  and  emits  a  beep  that  indicates that a  reading  should be taken.

Uses and Advantages of the Laser Plane. 

Some uses and advantages of the laser plane include the following:
1.  The laser plane replaces the horizontal line of sight of the engineer’s level, and the laser beam replaces a string line.
2.  The operation of setting a grade stake to a given elevation is the same as using an engineer’s level, except that there is no need for instructions from the operator of the instrument.
3.  It is not necessary to have an operator stationed at the instrument to indicate when to get on line or obtain a rod reading.
4.  When a laser target is properly attached to a machine used in operations, such as grading, paving, and tunneling, the operator of the machine can stay on the proper alignment and grade. 

Categories: Surveying