Vertical Control and Benchmarks
1. Vertical Control
Vertical control in surveying refers to the process of determining the elevation or height of points on the earth’s surface relative to a reference level, usually mean sea level (MSL). It helps surveyors know how high or low a point is compared to another point.
Vertical control is important in engineering works such as:
Construction of roads
Railways
Drainage systems
Buildings
Dams
It is usually established through levelling or modern methods such as GPS surveying.
Types of Vertical Control:
Primary vertical control – Very accurate control points established over large areas.
Secondary vertical control – Less accurate than primary but used to extend control.
Tertiary vertical control – Used for local surveys and construction projects.
2. Benchmarks (BM)
A benchmark is a fixed reference point whose elevation is known relative to mean sea level. Surveyors use benchmarks as starting points when measuring elevations of other points.
Benchmarks are usually marked on permanent objects such as:
Concrete pillars
Buildings
Bridges
Metal plates fixed in the ground
Types of Benchmarks:
GTS Benchmark (Great Trigonometrical Survey Benchmark)
Established by national survey authorities.
Very accurate and widely spaced.
Permanent Benchmark
Fixed by local authorities or survey departments.
Placed on permanent structures like buildings or bridges.
Temporary Benchmark (TBM)
Established temporarily during a survey project.
Used as a reference point for a short period.
Arbitrary Benchmark
A point given an assumed elevation for convenience when the actual elevation is not known.
- Teacher: Sharon Chepkemoi Langat