Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 84

Erection of Survey Marks and Monument Setting
Erection of Survey Marks
National triangulation points or traverse points are way apart from each other and
invisible in general conditions; geodetic survey marks are therefore needed to show
the specific position of the point as the target. Geodetic survey marks are usually
classified into ordinary survey marks and tower structures. The former is only used
as the target point with a height of 4.3 m and 6.3 m, while towers are chiefly used to
elevate the instrument and position of the target point when two adjacent points are
not intervisible.
Monument Setting
The survey mark is a permanent mark of the control point position. Field observa-
tion is referred to the center of the mark, and the plane coordinates and height of the
point obtained will be the position of the survey mark center. If the mark is
destroyed or displaced, the surveying results and position coordinates will be
meaningless. Hence, when the monument is set, one should firmly abide by the
principle of “quality first.” Meanwhile, the monument should be stable to ensure
permanence.
A monument can be classified as that of the first- and second-order triangulation
(traverse) points or that of the third- and fourth-order triangulation (traverse) points.
A monument is generally filled with concrete chiseled from granite, bluestone, or
other hard stones with identical specifications. Monuments consist of disks and
pillars, both with a mark sunken into the center of their top surfaces. The survey
mark can be made of metal or vitreous enamel. There are many types of monu-
ments, which are different in terms of the different orders and places of
monumentation under the principle of ensuring their stability and permanence.
Generally, a monument of first- and second-order points is composed of pillars
and upper and lower disks, as shown in Fig. 3.11 , while that of third- and fourth-
order points is composed of pillars and one disk.
Completion of the technical design and erection of survey marks and monuments
marks the position of each control point in the horizontal control network on the
Earth's surface. However, extensive distance and angle measurements, as well as
adjustment computations, still need to be made before the coordinates of the control
points can be determined.