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datum point[′dad·əm ‚pȯint]
in geodesy, a mark that fixes a point on the earth’s surface whose elevation relative to the level surface has been determined by leveling. In the USSR the elevations of datum points are computed relative to the zero point on the tide gage at Kronstadt.
There are two kinds of datum points: fundamental and temporary. Fundamental datum points are reinforced-concrete pylons set in the ground every 50–80 km on all class 1 leveling lines, on the most important class 2 leveling lines, and near the most important maritime water-measuring installations. Temporary datum points are installed every 5–7 km on leveling lines of all classes and are subdivided into ground datum points, which are set in the earth, and wall datum points, which are set into the walls of permanent structures.
The fundamental and ground datum points throughout a country constitute the state leveling grid. The datum points of the state leveling grid serve as starting (reference) points for determining the elevations of intermediate points on the earth’s surface during topographic surveying and other types of surveying work. They are also used for scientific purposes, to study the difference in sea levels.