TERMS USED IN PRISMATIC COMPASS Explained & defined in details

Let’s break down the topic into two core areas: **meridians**—which guide our directional reference in compass surveying—and **local attraction**—which is the error induced by nearby magnetic influences. Each plays a pivotal role in ensuring that a prismatic compass delivers accurate bearings in the field. ## Types of Meridian In surveying, a “meridian” is a fixed reference line used to measure directions or bearings. In the context of prismatic compasses, there are generally **four types**: 1. **True Meridian** This is the line or plane that runs from the geographic north to the geographic south pole. When using astronomical methods (like observing the sun or stars), surveyors can determine true north. Bearings referenced to the true meridian provide a basis for accuracy and are essential when converting measurements to map coordinates. 2. **Magnetic Meridian** The magnetic meridian is defined by the alignment of the magnetic needle in the prismatic compass. It points toward the magnetic north, which can differ from true north due to the Earth’s magnetic variation. Because the magnetic brand is inherently used by the instrument, all initial compass readings are taken relative to this meridian. 3. **Grid Meridian** When surveys are tied to a grid (such as those found on many topographic maps or in a Universal Transverse Mercator (UTM) coordinate system), the grid meridian is used. This is a "synthetic" reference line established by the mapping projection. Grid north usually differs slightly from true north by a known amount, and adjustments are made to align the survey measurements with the grid system. 4. **Arbitrary Meridian**

For some local surveys or specific mapping projects, a reference line chosen by the surveyor—often for convenience or to suit local conditions—is used. This arbitrary meridian isn’t linked directly to true, magnetic, or grid north but is established as a fixed reference within the project. Its use can simplify field procedures while still allowing later conversion to a recognized reference if needed. Each meridian serves a distinct purpose. The true and grid meridians are essential for connecting field surveys with established mapping systems, while the magnetic and arbitrary meridians are often employed during the on-the-ground measurement process. ](https://esenotes.com/compass-surveyingtraverse-meridian-types-of-bearing/) ## Local Attraction **Local attraction** refers to the disturbance in the magnetic field of a prismatic compass caused by nearby ferromagnetic materials or objects. These could be anything from surveyor’s equipment, vehicles, or even the mineral composition of the ground. Here’s how it affects and is handled in practice: - **Nature of the Influence:** The magnetic needle is designed to align with the Earth’s magnetic field. However, if nearby objects exert their own magnetic forces, the needle can deflect away from the true magnetic meridian. This deviation means that the observed bearing is not the true magnetic bearing but one tainted by local effects. - **Detection and Correction:** To identify local attraction, surveyors typically take readings in two opposite (or back) directions along the same line. Ideally, reversing the direction of observation should yield complementary bearings (for example, if you add 180° to the forward bearing, you should get the back bearing). Any discrepancy between the two indicates local attraction. The error is usually half the difference between the forward and back readings, and once determined, the correction is applied to all measurements taken during that session. - **Practical Considerations:** Local attraction isn’t constant. It can vary depending on the location, the setup of the survey site, and even the handling of the instrument. That’s why establishing a routine for error detection is essential. Understanding and mitigating local attraction ensures that despite the presence of unintended magnetic influences, the survey data remains reliable. Proper acknowledgment of local attraction and its correction is a cornerstone of precise compass surveying. It ensures that bearings can be confidently related back to one of the standard meridians, whether it be magnetic, true, grid, or arbitrary. ](https://esenotes.com/compass-surveyingtraverse-meridian-types-of-bearing/) Both the classification of meridians and the concept of local attraction are vital for accurate survey measurements with a prismatic compass. Recognizing which meridian your initial readings relate to—and making the necessary adjustments for any local magnetic interferences—empowers surveyors to convert raw field data into reliable surveying information that aligns with mapping standards.