Sec. 95. The National Level Net
It is essential that heights of points of a geodetic reference net (bench marks) be ascertained. The principal method to determine the point heights of a reference net is by levelling. The national level net in this country includes lines of levels of first, second, third and fourth orders (primary, secondary, and tertiary nets).
First-order (high-order) levelling is a precise surveying operation.
Modern techniques of first-order levelling make it possible to keep the random and constant errors as low as ±0.5 mm per km of a levelling line. It is the purpose of second-order levelling to provide an accurate level net to be then filled by lower (third and fourth) order levelling.
Second-order lines of levels are usually run along railways, highways, dirt roads, and along large rivers.
Errors of closure in
second-order circuits and lines should not exceed in absolute value
5mm*
,
where L is the length around a circuit or line in kilometres.
Third-order level nets are run inside a second-order circuit.
A third-order levelling must be accurate enough for the errors of closure in the traverses or circuits to be less than 10mm* ,
network of survey points is densified. The survey is conducted by the method of cross sections.
The survey should cover the principal route and trial routes, morphological features and waterwork locations, water gauge stations, the contour of high tides and ice, flood areas, the low water lines, washouts, shoals, exits, tributaries, flood lakes, cut-offs, existing structures, drill holes, and bench marks.
Topographical surveys use phototheodolites, tacheometers and plane tables (see Chapters 25, 26 and 27). If aerial photographs are available, analytical, stereotopographic or combined aerial photographic survey methods are used.
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