Selection of triangulation stations; applications

Selection of triangulation / trilateration stations

1. Intervisibility: every station should be visible from the adjacent stations; hills, high buildings or towers can be used, and the line of sight should clear the ground, trees and buildings (by at least a few metres, to avoid refraction near the ground).
2. Well-conditioned triangles: the angles should lie between 30 and 120 degrees (preferably 45 to 90); avoid long, thin triangles.
3. Stability and accessibility: stations should be on firm ground that will not settle, easy to reach with the instrument and signals, and have enough space to erect the signal.
4. Height: stations should be high enough to see over obstructions, but not so high that tower cost or access becomes a problem; keep the number of stations to a minimum.
5. Base line: the base line site should be nearly level and free from obstacles, near the centre of the area, so that the base can be extended into the net.
6. For trilateration: the line of sight must be clear for the EDM beam, free from strong reflecting surfaces, power lines or heat sources close to the beam, and the temperature and pressure should be measurable at both ends.
7. Permanence: stations should be placed so that they can be used for future survey work and are safe from disturbance and construction.

Field applications of triangulation

• Providing control points for topographic mapping and large-scale surveys.
• Fixing the accurate positions of towns, boundaries, bridges and dams.
• Setting out long bridges, tunnels, and the centre lines of dams or canals.
• Geodetic work: size and shape of the earth, latitude, longitude and azimuth.
• Hydrographic surveys: fixing soundings and shore points from shore stations.
• Photogrammetric control for aerial mapping.