1.14.2. Magnetic compasses

The magnetic compasses used in surveying may be classified as:

1. Prismatic Compass
2. Surveyor's Compass
3. Trough Compass
4. Tubular Compass

In surveying, there are several types of magnetic compasses that are commonly used. Each type serves specific purposes and has unique features. The main types include the Prismatic Compass, which is popular for various survey work, the Surveyor's Compass, which is less commonly used, and the Trough and Tubular Compasses, which are typically used as accessories with other surveying tools.

Think of compasses as tools similar to different types of smartphones. Just as some phones are tailored for photography, while others are geared towards business productivity, each compass is designed for specific surveying tasks, whether it’s precise angle measurements or serving as an accessory to another instrument.

The compass works on the principle that a freely suspended magnetic needle takes the direction of the magnetic force at a place. It provides a reference direction with respect to which all angles can be measured.

The fundamental principle behind a magnetic compass is based on magnetism. A magnetic needle, when suspended freely, will align itself with the Earth's magnetic field, pointing towards the magnetic north. This allows surveyors to establish a reference direction for measuring angles accurately. The north pole of the magnetic needle acts as the primary reference point in compass surveying.

Imagine the magnetic needle of a compass is like a fish swimming in a river — it naturally aligns itself with the current (the Earth's magnetic field), which ensures that anytime you want to measure a direction (like taking a bearing), you can rely on that 'swim' direction of the needle.

The Prismatic compass is very popular for survey work, and the Surveyor’s compass is rarely used. However, the principle of operation of both the compasses is the same, but they are made differently.

The Prismatic compass is favored for its design and functionality in demonstrating the direction. It includes features like an eye vane for sighting, an object vane for aiming, and a graduated ring for taking precise readings. The Surveyor's compass, while less popular now, uses similar principles but differs in the absence of additional features like the prism that enhances visibility and precision of readings.

Think of the Prismatic compass like a high-tech watch that not only tells time but also gives you health data and notifications. In contrast, the Surveyor's compass is like a classic watch that tells time but doesn’t have those extra smart features. Both tell time effectively, but one adds more convenience and accuracy.

Trough and tubular compasses are used as add-ons to other instruments. They are not complete surveying instruments by themselves but are used only to indicate magnetic meridian direction.

Trough and tubular compasses serve as supplementary tools to other surveying instruments like theodolites or plane tables. Their role is to help indicate the magnetic meridian direction, allowing the primary instruments to be set accurately in the correct orientation. Thus, even though they are not standalone tools, they contribute significantly to the accuracy of measurements taken with larger devices.

Consider a trough compass as a GPS app that you use alongside your paper map. The GPS helps you find the correct route (magnetic direction), but the paper map (main surveying instrument) provides the detailed layout of streets and landmarks. Both work together for effective navigation, just like the compasses assist other surveying tools in aligning the survey accurately.

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