SURVEYING TOPIC 2

Measurement of Distance

The accurate determination of the distance between two points on any surface is one of the basic operations of plane surveying. Such linear measurements are understood to mean the horizontal distance. In surveying, the commonly employed methods of linear measurement include pacing, taping, tachymetry, graphical, mathematical, mechanical, photogrammetric, and electronic distance measurement.

Pacing

consists of counting the number of steps or paces in a required distance

Pace

is defined as the length of a step in walking

Stride

is equivalent to two paces or double step

Pace Factor

To pace a distance it is necessary to first determine the length of one's pace.

Distance by

The use of graduated tape is probably the most common method of measuring or laying out horizontal distances. It is a form of a direct measurement which is widely which is widely used in the construction of buildings, dams, bridges, canals, and many other engineering as well as non-engineering activities.

Tachymetry (or tacheometry)

is another procedure of obtaining horizontal distances. It is based on the optical geometry of the instruments employed and is an indirect method of measurement

A transit or theodolite

is used to determine subtended intervals and angle on a graduated rod or scale from which distances are computed by trigonometry.

Stadia Method.

This method provides a rapid means of determining horizontal distances. It was introduced in 1771 by James Watt of Scotland and was at that time referred to as a micrometer for measuring distances. The equipment for ______ measurements consists of a telescope with two horizontal hairs called ______ hairs and graduated rod called a _______ rod.

Subtense Bar Method.

The subtense bar is a convenient and practical device used for quick and accurate measurement of horizontal distance. The bar, which is precisely 2 meters long, consists of a rounded steel tube through which runs a thin invar rod. At each end of the frame the target marks are housed. It is mounted horizontally on a tripod and place perpendicular to the line of sight by means of a sighting device on top of the bar.

DISTANCE by GRAPHICAL and MATHEMATICAL METHODS

By graphical or mathematical methods, unknown distances may be determined through their relationship with known distances geometrically. These methods are widely employed in plane table surveys, and in triangulation work. Determining distances by scaling from maps or aerial photographs could also provide sufficiently accurate results.

DISTANCE by MECHANICAL METHODS

There are few mechanical devices which could also be employed for the measurement of distances. These devices, however, are only applicable for low precision surveys or where quick measurements are desired.

The odometer

is a simple device that can be attached to a wheel for the purposes of roughly measuring surface distances. The wheel is rolled over the distance to be measured and the number of revolutions of the wheel is directly registered by the device. Since the circumference of the wheel is known, the relationship between revolution and the distance could then be established. It is similar in function to the distance recorder of a vehicle speedometer.

A measuring wheel

is very similar in operation to an odometer except that it is more portable and self-contained measuring device. It basically consists of a small wheel which is attached to a rod and handle, and can be pushed by an operator. As the wheel of the device is rolled along a line to be measured, a built-in calibrated recorder automatically gives a distance reading in meters and decimals of a meter.

An optical rangefinder

operates on the same principle as a rangefinder on a single-lens reflex camera. This device, which is usually hand-held or mounted on a small tripod, can be used to determine distances approximately simply by focusing.

ELECTRONIC DISTANCE MEASUREMENT

The development of electronic distance measurement (EDM) has provided a significant advance in surveying instrumentation and techniques. These electronic instruments allow a rapid and accurate determination of linear distances, horizontal and vertical angles suitable even for large extent surveys requiring higher orders of precision.

DISTANCE by PHOTOGRAMMETRY

The term photogrammetry refers to the measurement of images on a photograph. The types of photographs used are those taken from an aircraft with the axis of the camera pointed vertically towards the terrain photographed.