Two Face Measurement in Surveying

Definition

Two face measurement, also known as face left and face right measurement, is a fundamental surveying technique employed to verify the accuracy of distance and angular measurements by taking observations from opposite sides of a measuring instrument or theodolite. This method serves as a quality control mechanism that identifies instrumental errors and human mistakes during the surveying process.

Purpose and Applications

The primary purpose of two face measurement is to eliminate systematic errors inherent in surveying instruments. By taking measurements from both the left and right sides of an instrument, surveyors can cross-verify their data and achieve higher levels of precision. This technique is particularly valuable in:

Horizontal and vertical angle measurements

Distance determinations using taping methods

Leveling operations

Traverse surveys

Boundary establishment

Methodology

The two face measurement process involves several key steps. First, the surveyor positions the instrument and takes an initial measurement from one side, referred to as "Face Left" or "Face 1." The instrument is then rotated 180 degrees around its vertical axis, and the measurement is repeated on the opposite side, known as "Face Right" or "Face 2."

When measuring horizontal angles, the process typically involves:

1. Sighting the backsight point on Face Left 2. Turning to the foresight point and recording the angle 3. Rotating the instrument 180 degrees 4. Sighting the same points again on Face Right 5. Computing the mean of both measurements

Advantages

Two face measurement offers numerous benefits to surveying operations:

Error Detection: Significant differences between Face 1 and Face 2 measurements indicate instrumental problems or measurement errors

Improved Accuracy: The average of two face measurements provides a more reliable result than a single observation

Elimination of Collimation Errors: By using both faces, the effects of instrument misalignment are minimized

Detection of Operator Mistakes: Discrepancies reveal when the surveyor has made procedural errors

Professional Standards: Many surveying standards and regulations require two face measurements for critical work

Tolerance and Acceptance Criteria

When comparing Face 1 and Face 2 measurements, surveyors establish acceptable tolerance ranges. These tolerances depend on:

The precision requirements of the project

The quality of the surveying instrument

Environmental conditions

The distance being measured

If measurements from both faces differ beyond established tolerances, the work must be repeated or investigated to identify the cause of discrepancy.

Modern Applications

While electronic total stations and GPS technology have become prevalent in modern surveying, two face measurement principles remain relevant. Digital instruments often incorporate automated two face measurement capabilities, providing immediate feedback when readings from both faces fall outside acceptable parameters.

Conclusion

Two face measurement represents a cornerstone technique in professional surveying practice, ensuring data quality and reliability. Despite technological advances, understanding and implementing two face measurement principles remains essential for surveyors committed to accuracy and professional excellence in their field operations.