Lab 2: Compass Traversing and Detailing

A foundational understanding of several surveying terms is essential for this lab.

Traversing

This is a branch of surveying where the directions of survey lines are established using a compass, and their lengths are measured, often through chaining or taping. A traverse is the path created by connecting the survey stations.

Open Traverse: A traverse where the starting point is known, but the endpoint is not.

Closed Traverse: A traverse where both the starting and ending points are known, forming a closed loop. This configuration is beneficial because it allows for an accuracy check of the fieldwork.

Key Concepts

Bearing: The bearing of a line is its direction relative to a given meridian.

Fore Bearing (FB): The bearing of a survey line taken in the direction of the traverse. For a line AB, it is the bearing measured from station A towards station B.

Back Bearing (BB): The bearing of a line measured in the opposite direction of the traverse. For a line AB, it is the bearing measured from station B towards station A.

Local Attraction: This refers to the influence of nearby magnetic materials (like transmission lines or transformers) on the compass needle, causing it to deflect from the earth’s magnetic field.

Closing Error: In a closed traverse, errors in angle and length measurements can cause the plotted endpoint to not coincide with the starting point. This discrepancy is known as the closing error.

The following measurements were recorded in the field.

Table 1: Distance Measurements

Table 2: Bearing Measurements

3.1. Interior Angle Calculation

The interior angles of the closed traverse were calculated as follows:

3.2. Error Adjustment

3.3. Corrected Interior Angles

The correction was added to each calculated interior angle:

3.4. Corrected Bearing Calculations

The bearings were adjusted starting with line AB, which had the smallest deviation from the ideal 180° difference between its fore and back bearings. An initial correction of +15′ was applied to the bearings of line AB.

Table 3: Final Corrected Bearings

Closing Error: The initial plot of the traverse did not close, indicating the presence of measurement errors, which is a common occurrence in surveying. The source attributes this to errors in both angle and length measurements.

Local Attraction: The report mentions the concept of local attraction, which are magnetic disturbances from sources like power lines that can affect compass readings. While it is stated that no magnetic effect was observed, the differences between fore and back bearings (ideally 180°) were not uniform, suggesting some uncorrected local attraction might have been present. For instance, the difference for line DE was 181°30′, while for CD it was 178°.

Data Inconsistencies: There are minor inconsistencies in the document’s tables. For example, in the “BEARINGS MEASUREMENT” table, the Back Bearing for line AB is listed as 205°, while the calculation for Angle A uses 240°30′ as the Back Bearing of FA. Additionally, some corrected bearing values in the final table do not precisely match the preceding calculations, likely due to rounding or minor calculation errors in the original document.

The compass traverse survey was successfully completed, and the data was processed to adjust for observational errors. The details of the surrounding area were mapped using the established traverse as a reference framework. The exercise provided valuable practical experience in compass surveying, data correction, and detail mapping. The final corrected data and the resulting map are considered to be within an acceptable range of accuracy for the methods employed.