Map projections and grids: to what are grid lines (eastings and northings) parallel in a standard grid system used on maps? Choose the most appropriate reference.

Introduction / Context:
Topographic and engineering maps often overlay a rectangular grid for coordinate referencing (eastings and northings). Understanding how the grid relates to the underlying projection and meridians is essential for converting between grid bearings and true bearings, and for applying convergence corrections.

Given Data / Assumptions:

• The grid is derived from a map projection (e.g., Transverse Mercator).
• Grid north is parallel to the central meridian of the projection’s grid.
• Magnetic north differs due to declination and temporal variation.

Concept / Approach:

In common grid systems (like UTM), the grid north direction is defined to be parallel to the central true meridian of the grid zone. Grid lines (northings) run parallel to that meridian; eastings are perpendicular to it. Away from the central meridian, geographic meridians converge, causing a small angle (convergence) between true north and grid north. Magnetic meridian directions differ again by declination, which is unrelated to the geometric grid.

Step-by-Step Solution:

Verification / Alternative check:

Map marginal notes typically provide the grid convergence to convert between true and grid bearings, confirming that grid north is tied to the central true meridian, not to magnetic north.

Why Other Options Are Wrong:

Magnetic meridian depends on location/time; equator is east–west; “none” is incorrect because the relationship is well-defined.

Common Pitfalls:

Confusing magnetic with grid or true north; ignoring grid convergence when computing bearings across wide areas.

Final Answer:

line representing the central true meridian of the grid