Procedure of Chaining
It must be remembered in surveying, that under most circumstances, all distances are presumed to be horizontal distances and not surface distances.
• This dictates that every field measurement taken be either measured horizontally or, if not, reduced to a horizontal distance mathematically.
• In many instances, it is easiest to simply measure the horizontal distance by keeping both ends of the chain at the same elevation. This is not difficult if there is less than five feet or so of elevation change between points. A hand level or “pea gun” is very helpful for maintaining the horizontal position of the chain when “level chaining.” A pointed weight on the end of a string called a “plumb bob” is used to carry the location of the point on the ground up to the elevated chain by simply suspending the plumb bob from the chain such that the point of the plumb bob hangs directly above the point on the ground.
• When the difference in elevation along the measurement becomes too great for level chaining, other methods are called for. One option, “break chaining”, involves simply breaking the measurement into two or more measurements that can be chained level.
Distance Measuring (Electronic Distance Meters)
In the early 1950’s the first Electronic Distance Measuring (EDM) equipment were developed. These primarily consisted of electro-optical (light waves) and electromagnetic (microwave) instruments. They were bulky, heavy and expensive. The typical EDM today uses the electro-optical principle. They are small, reasonably light weight, highly accurate, but still expensive.
Principle of Chaining
• To measure any distance, you simply compare it to a known or calibrated distance; for example by using a scale or tape to measure the length of an object. In EDM’s the same comparison principle is used. The calibrated distance, in this case, is the wavelength of the modulation on a carrier wave.
• Modern EDM’s use the precision of a Quartz Crystal Oscillator and the measurement of phase-shift to determine the distance.
• The EDM is set up at one end of the distance to be measured and a reflector at the other end.
• The EDM generates an infrared continuous-wave carrier beam, which is modulated by an electronic shutter (Quartz crystal oscillator).
• This beam is then transmitted through the aiming optics to the reflector.
• The reflector returns the beam to the receiving optics, where the incoming light is converted to an electrical signal, allowing a phase comparison between transmitted and received signals.
• The amount by which the transmitted and received wavelengths are out of phase, can be measured electronically and registered on a meter to within a millimeter or two.
Angle Measuring
Measuring distances alone in surveying does not establish the location of an object. We need to locate the object in 3 dimensions. To accomplish that we need:
1. Horizontal length (distance)
2. Difference in height (elevation)
3. Angular direction.
An angle is defined as the difference in direction between two convergent lines. A horizontal angle is formed by the directions to two objects in a horizontal plane. A vertical angle is formed by two intersecting lines in a vertical plane, one of these lines horizontal. A zenith angle is the complementary angle to the vertical angle and is formed by two intersecting lines in a vertical plane, one of these lines directed toward the zenith.
Types of Measured Angles
• Interior angles are measured clockwise or counter-clockwise between two adjacent lines on the inside of a closed polygon figure.
• Exterior angles are measured clockwise or counter-clockwise between two adjacent lines on the outside of a closed polygon figure.
• Deflection angles, right or left, are measured from an extension of the preceding course and the ahead line. It must be noted when the deflection is right (R) or left (L).
A Theodolite is a precision surveying instrument; consisting of an alidade with a telescope and an accurately graduated circle; and equipped with the necessary levels and optical-reading circles. The glass horizontal and vertical circles, optical-reading system, and all mechanical parts are enclosed in an alidade section along with 3 leveling screws contained in a detachable base or tribrach.
A Transit is a surveying instrument having a horizontal circle divided into degrees, minutes, and seconds. It has a vertical circle or arc. Transits are used to measure horizontal and vertical angles. The graduated circles (plates) are on the outside of the instrument and angles have to be read by using a vernier.
Bearings and Azimuths
The Relative directions of lines connecting survey points may be obtained in a variety of ways. The figure below on the left shows lines intersecting at a point. The direction of any line with respect to an adjacent line is given by the horizontal angle between the 2 lines and the direction of rotation. The figure on the right shows the same system of lines but with all the angles measured from a line of reference (O-M). The direction of any line with respect to the line of reference is given by the angle between the lines and its direction of rotation.