Measuring Included Angles in Theodolite Traverse Surveying

In surveying, a traverse is a series of connected survey lines whose lengths and directions are measured. The stations are points where observations are made. When conducting a theodolite traverse, angles between consecutive lines are measured. These are often referred to as included angles.

Understanding Included Angles in a Traverse

An included angle at a traverse station is the angle formed by two consecutive traverse lines meeting at that station. To consistently measure these angles and ensure accuracy, a standard procedure is followed using a theodolite.

Standard Procedure for Measuring Included Angles

The generally accepted method for measuring included angles in a theodolite traverse involves setting up the theodolite at a traverse station. The instrument is then oriented by sighting the previous station (referred to as the back station) and setting the horizontal circle reading to zero or a known value. Subsequently, the instrument is turned horizontally to sight the next station (referred to as the forward station). The angle turned is then read from the horizontal circle.

To obtain the unique included angle without ambiguity and to maintain consistency throughout the traverse, the direction of turning is standardized.

• The measurement typically starts by sighting the back station.
• The horizontal angle is measured by turning the telescope in a specific direction towards the forward station.

Let's examine the options provided:

• Option 1: clockwise from the forward station - This is incorrect. Measurement usually starts from the back station, not the forward station.
• Option 2: anti-clockwise from back station - While measuring anti-clockwise is possible, the conventional and generally used method for included angles is clockwise, especially for calculating interior angles directly in closed traverses.
• Option 3: anti-clockwise from forward station - This is incorrect. Measurement starts from the back station and is typically clockwise.
• Option 4: clockwise from back station - This is the standard procedure. The instrument is set up at the station, sighted back to the previous station, and then turned clockwise to the next station. The reading gives the included angle (often the interior angle in a closed traverse if measured consistently).

Measuring clockwise from the back station to the forward station is the conventional method because it provides a consistent way to measure the angle between the previous line and the next line in sequence, usually yielding the interior angle in a closed traverse directly.

Therefore, the included angles of a theodolite traverse are generally measured clockwise from the back station.

Revision Table: Theodolite Traverse Angles

Additional Information: Surveying Angle Measurement

While clockwise measurement from the back station is standard for included angles in a traverse, other angle measurement techniques exist depending on the application:

• Bearing/Azimuth: Angles measured from a reference meridian (North or South) clockwise to the line.
• Deflection Angle: The angle between the extension of the previous line and the next line, measured either left (anti-clockwise) or right (clockwise). These are commonly used in road or railway surveys.

The choice of measurement method depends on the type of survey, the required accuracy, and the convention followed. For included angles in typical closed or open traverses, the clockwise measurement from the back station is the most common and practical method for field work and subsequent calculations.