At the start of many projects in remote areas there is a need to perform primary geodetic control on which all subsequent surveys are "hung". Depending on the size of the project this may be one, two or a whole network of points. Generally long distances are involved.
The surveys (these days) are generally based on the International Terrestial Framework (ITRF) and involve specialised very long base line (VLB) GPS fixes over extended time periods.
Fixes in the vertical plane are particularly challenging as gravitational affects and earth tides have to be taken into consideration. In the case of projects whose end purpose can be influenced by gravity, the movement of water in particular, GPS fixes cannot be applied and optical heighting techniques still have to be used. This is extremely important and many irrigation projects have incurred massive cost overruns because this fundamental consideration was ignored.
Most of the pictures in this page were taken from work the firm did in establishing the primary control and precise heighting traverses for the Lesotho Highlands Water Development project which now includes several of the biggest arch wall dams in Africa.