In surveying, a horizontal plane is fundamentally defined by its relationship to gravity.
A horizontal plane is a plane perpendicular to the local direction of gravity. As stated in definitions, in plane surveying, this means it is a plane perpendicular to the local vertical line. This contrasts with a horizontal line, which is simply a line lying within a horizontal plane.
Understanding the Horizontal Plane
Think of the horizontal plane as a perfectly level surface at a specific elevation. Because gravity acts downwards, the horizontal plane is always at a right angle (90 degrees) to the pull of gravity at that location.
- Gravity's Influence: The definition explicitly links the horizontal plane to the direction of gravity.
- Vertical Line: The vertical line represents the direction of gravity. The horizontal plane cuts across this vertical line at a 90-degree angle.
- Plane vs. Line: It's important to distinguish between a horizontal plane (a flat surface) and a horizontal line (a line within that surface).
Why is it Important in Surveying?
The concept of a horizontal plane is crucial in surveying for:
- Establishing Elevations: Surveying instruments like levels are designed to create a horizontal line of sight, which lies within a horizontal plane, allowing surveyors to determine differences in elevation between points.
- Measuring Horizontal Distances: Horizontal distances are measured between points projected onto a horizontal plane, removing the effect of changes in elevation along the slope.
- Creating Maps and Plans: Maps and site plans are typically representations of the land projected onto a horizontal plane.
Key Takeaway
| Concept | Definition | Relation to Gravity |
|---|---|---|
| Horizontal Plane | A plane perpendicular to the local direction of gravity | Perpendicular |
| Vertical Line | Represents the local direction of gravity | Parallel |
Understanding the horizontal plane is fundamental to performing accurate measurements and calculations in surveying, especially in plane surveying where the curvature of the Earth is often disregarded over short distances.
