When Compass Surveying Isn't Recommended

Compass surveying is a fundamental technique used in surveying to determine direction using a magnetic compass. It relies on the principle that the compass needle aligns itself with the Earth's magnetic field to indicate magnetic north. This method is particularly useful for reconnaissance surveys and for establishing control points in open areas. However, certain environmental conditions and survey area characteristics can make compass surveying unreliable or inadvisable.

Analyzing Survey Suitability for Compass Methods

Let's examine the scenarios presented to understand where compass surveying faces challenges:

• Survey of long narrow strips: Compass surveying is often well-suited for tasks like mapping roads, railways, or pipelines, which are typically long and narrow. Traversing along the strip using a compass to establish bearings is a standard practice. This option represents a suitable application.
• Area division into triangles inconveniently: While triangulation is a common geometric method used to extend control in surveying, compass surveying itself primarily focuses on measuring bearings and distances. Even if an area is difficult to divide into neat triangles, compass traversing techniques can still be employed effectively by establishing a series of connected straight lines (a traverse) and measuring the interior or exterior angles and lengths. The ability to divide into triangles is more a characteristic of triangulation methods than a strict limitation for compass traversing itself, though large, complex areas might necessitate careful planning.
• Obstructions preventing long straight lines: Compass surveying, especially through techniques like chain and tape traversing or offset methods, is designed to work around obstacles. Surveyors can establish shorter lines, use offsets to bypass obstructions, and create convenient tie lines to connect survey points. This adaptability makes compass surveying viable even when long, unobstructed sightlines are not possible.
• Places with magnetic rock or iron ore: This is a critical limitation for compass surveying. Areas rich in magnetic materials like iron ore or certain types of rock can cause significant local attraction. This phenomenon disrupts the magnetic needle's alignment with the Earth's true magnetic field, leading to inaccurate bearing readings. The deviation of the needle from magnetic north due to these local magnetic influences is known as local attraction. Such inaccuracies render the survey unreliable and make compass surveying inadvisable in these locations.

Conclusion on Compass Surveying Limitations

Based on the analysis, compass surveying is most strongly inadvisable in locations where magnetic interference significantly affects the compass readings. Option 4 highlights precisely this scenario, where the presence of magnetic rock or iron ore causes local attraction, compromising the accuracy of direction measurements which are fundamental to compass surveying.