A triangulation point is a geodetic control point whose horizontal position has been precisely determined and physically marked on the ground. These points serve as fundamental references for the establishment of national coordinate systems, map production, and engineering applications.
Triangulation Network and Classification
Triangulation points are organized within horizontal control networks and are classified according to their order of accuracy and function:
- First-Order Triangulation Network: This is a primary control network of low density and high accuracy that spans an entire country. It forms the backbone of the national coordinate system. The average side length is approximately 20–25 km, and the points are used for establishing national geodetic datums.
- Second-Order Triangulation Network: Constructed around the first-order network, it supports third- and fourth-order networks and serves 1:50,000 scale mapping needs. Its average side length is about 13 km.
- Third- and Fourth-Order Networks: These contain denser control points required for detailed mapping and engineering projects. The average side length is approximately 8 km for third-order and 4 km for fourth-order networks. Each point typically governs an area of about 50 km² and 20 km², respectively.
Installation and Marking
Triangulation points are physically marked on the ground using concrete pedestals, metallic plaques, or stakes. Site selection considers factors such as soil stability, durability, visibility, and security. Elevated and accessible locations, away from urban or obstructed environments, are preferred.
Observation and Computation Methods
The position of triangulation points is computed using triangulation, trilateration, or a combination of both (triangulateration). While triangulation is based on angle measurements, trilateration uses linear distances. In high-order networks, astronomical observations may also be conducted to determine azimuths, latitudes, and longitudes.
Applications
Triangulation points serve as essential reference data for:
- Topographic and thematic map production,
- Geometric correction of satellite imagery,
- Engineering projects (e.g., bridges, roads, dams),
- Monitoring of ground deformation and crustal movements,
- Long-term geodynamic research.
