عنوان مقاله [English]
Each numerical or geometric quantity, or sets of numerical quantities, which are used as reference or basis for other quantities, are called base levels or datums.
In geomorphology, two different baseline levels are considered, one of which is flat, which forms the basis of the accurate calculation in horizontal surveying in which the curvature of the earth's surface is considered. The other is the elevation base level according to which the height of points are measured and calculated. In other words, in geodetic surveying the coordinates of different points are obtained from a series of initial points with known coordinates by base line parameters. Due to the development of different methods in surveying, several datums or baselines have been defined and produced.
Today, several datum have been defined and produced for development of geodetic surveying and satellite geodetic methods, and there is no need to have more than one datum in this method. But this does not mean a complete replacement of all datums by a single one, since in practice there are initial surveying parameters and quantities that each depend on the different datums. On the other hand, the overall change of datums will require a century to transform all the information in the maps of a country into a new base level. For example, the issue of determining the sovereignty over the border lines of the US state has been on since centuries ago, and even by drawing up a map of disputed borders, military and civilian users still prefer the old systems of surveying measurement over new ones due to special reasons. Consequently, making general changes in determining the base level not only does not eliminate the problem, but may add another variable to initial problems.