Geodetic survey is different from plane surveying because of ________.
🌍 The Fundamental Difference: Earth's Shape
The primary distinction between the two main branches of surveying lies in how they treat the shape of the Earth.
Geodetic Surveying: This method considers the true shape of the Earth, which is a spheroid. It accounts for the Earth's curvature. All lines measured on the surface are treated as arcs, and calculations involve spherical trigonometry. It is essential for high-accuracy work over large areas.
Plane Surveying: This method ignores the Earth's curvature and assumes the survey area is a flat, horizontal plane. All measurements are treated as if they are on a flat surface, and calculations use plane geometry and trigonometry. This is suitable for smaller areas where the effect of curvature is negligible.
🔍 Analysis of the Options
Understanding why the correct answer is the best choice involves looking at the other options.
- (b) The curvature of the earth is considered: This is the correct answer. It is the defining technical and methodological difference. The decision to account for curvature dictates the instruments, calculations, and precision of the survey.
- (a) Very large area is covered: This is a consequence, not the fundamental difference. We perform geodetic surveys *because* the area is large, which makes the Earth's curvature significant. The large area is the reason for choosing the method, but the method itself is defined by its treatment of curvature.
- (c) Undulations of the topography: Both plane and geodetic surveys must deal with the natural ups and downs (undulations) of the terrain. This is typically handled through processes like leveling, and it doesn't define the difference between the two main types.
- (d) The large difference of elevations: Similar to topography, both methods are used in areas with large elevation differences. Accounting for height is a standard part of surveying, regardless of whether it's plane or geodetic.
Cause vs. Condition
Think of it this way: The curvature of the Earth is the fundamental physical property (the cause) that makes the two survey types different. The large area is the condition under which this property becomes too important to ignore.
📊 Quick Comparison Table
| Feature | Plane Surveying | Geodetic Surveying |
|---|---|---|
| Earth's Curvature | Ignored (Treated as flat) | Considered (Treated as a spheroid) |
| Area Limit | Generally < 260 km² | Generally > 260 km² |
| Lines | Level lines are straight; Plumb lines are parallel | Level lines are curved; Plumb lines converge at Earth's center |
| Accuracy | Less accurate for large areas | Highly accurate |