Two Face Measurement in Surveying

Definition

Two face measurement, also known as face left and face right measurement, is a fundamental surveying technique used to enhance the accuracy of angle measurements and eliminate systematic errors introduced by instrumental imperfections. This method involves measuring the same angle or direction twice—once with the instrument in one position and once with it rotated 180 degrees (or flipped to the opposite face).

Purpose and Significance

The primary purpose of two face measurement is to:

Eliminate systematic errors caused by instrument collimation errors, which occur when the instrument's optical or mechanical components are not perfectly aligned

Reduce the impact of environmental factors such as temperature variations and atmospheric refraction that may affect individual measurements

Improve measurement reliability by providing a mathematical check on the quality of observations

Increase overall survey accuracy for both horizontal and vertical angle measurements

Methodology

The two face measurement process follows a systematic approach:

1. First Face (FL): Position the instrument normally and measure the desired angle or direction. Record all readings carefully.

2. Rotation: Rotate the telescope or instrument 180 degrees horizontally (for horizontal angles) or vertically (for vertical angles) to place it in the opposite position.

3. Second Face (FR): Repeat the measurement in the reversed position, ensuring all procedures are identical to the first measurement.

4. Analysis: Calculate the mean of both measurements, which provides a more accurate result than either single measurement alone.

Applications

Two face measurement is extensively used in:

Theodolite and total station surveying for horizontal and vertical angle measurements

Precise traverse surveys where high accuracy is required

Astronomical observations and celestial measurements

Building and construction surveys requiring stringent accuracy standards

Geodetic surveying and control point establishment

Mathematical Treatment

When two face measurements are taken, the final result is typically calculated as:

Mean Angle = (Face Left + Face Right) / 2

The difference between the two measurements serves as an indicator of measurement quality. Large discrepancies suggest instrumental problems or environmental interference requiring investigation.

Advantages

Provides built-in quality assurance and measurement verification

Mathematically eliminates collimation errors

Increases confidence in survey results

Creates redundancy that improves overall accuracy

Allows identification of instrumental malfunctions

Limitations

Doubles measurement time and field work

Requires trained personnel for proper execution

May be impractical for routine surveying tasks

Environmental conditions may change between measurements

Modern Practice

With contemporary surveying instruments such as total stations and digital theodolites, two face measurement has been largely automated. Many modern instruments can be programmed to automatically perform face left and face right measurements and calculate the mean, significantly reducing field time while maintaining accuracy advantages.

Conclusion

Two face measurement remains a cornerstone technique in precision surveying, providing reliable methods to detect and eliminate systematic errors. Though partially automated in modern practice, understanding this technique is essential for surveyors working on projects requiring exceptional accuracy and quality assurance.