General Surveying II (Control Survey)
1. Chain of single and independent triangles 2. Chain of quadrilaterals formed with overlapping triangles 3. Chain of polygons or central-point figures 4. Chains of polygons each with an extra diagonal
Four types of triangulation systems
1. Horizontal Control 2. Vertical Control
Two types of Control Surveying
Triangulation
a method for extending horizontal control for topographic and similar surveys which require observations of triangular figures.
Least Square Method
a method of adjustment used in triangulation systems and other types of traverse surveys.
Trilateration
a method used for horizontal control surveys which is based exclusively on measured horizontal distances.
Control by traversing
a series of horizontal distances and directions are observed to connect points whose positions are to be determined.
Side Condition Equations
after the triangles are adjusted so as to close perfectly, the figure may still be imperfect because the lines AB, BD, and BC may not intersect in a common point.
Astronomic checks
are made to check accurately the position and direction of stations and lines.
Horizontal Control Surveys
are surveys made to establish geodetic latitudes and longitudes, and plane rectangular coordinates of a fundamental network of reference stations.
Vertical Control Surveys
are surveys usually run by barometric, trigonometric, or differential leveling to establish elevations for a network of monuments called bench marks.
Triangulation system
consists of series of triangles in which one or more sides of each triangle are also sides of adjacent triangles.
Measurement of base lines
define the shape and strength of a triangulation system.
Control survey
determination of the precise position of a number of stations
Angle Condition Equations
in each quadrilateral, seven angle conditions may be formed.
Figure Adjustment
in this adjustment the objective is to make the sum of the three angles in each triangle within a triangulation system equal to 180 degrees.
Adjustment of a Quadrilateral
is adjusted to remove all discrepancies and to give the most probable value of angles.
Gas pipe tower
is another type of observation platform constructed entirely of gas pipes and also used in triangulation work
Chain of Polygons
is composed of a group of triangles having within it a station which is at a vertex common to all the triangles.
Bilby tower
is made of steel and similar in construction to those used i windmills
Check base
one of the lines in a triangulation system whose length is also precisely measured and may be located at regular intervals or at the end of triangulation system.
Base line
one of the lines in a triangulation system whose length is precisely measured and its true directions determined by astronomical observations.
Vertical Control
the main objective is the establishment of a network of reference bench marks.
Erection of Signals and Towers
the stations of triangulation figure must be marked by a signal visible from stations from which it is to be sighted.
Measurement of angles
the strong feature of triangulation system depends largely on how accurate the angles are measured.
Reconnaissance
this step refers to the selection of a suitable pattern for the system, and the triangulation stations which would best fit the topography of the area and meet the requirements of the survey.
Wooden tower
to be used in triangulation work should be built at a low cost, light and slender, easy to construct, and should be sufficiently rigid to withstand ordinary storms
Chain of Triangles
is the simplest form of triangulation system since it involves a minimum of angular measurements and calculations for the amount of area covered.
Barometric leveling
is well suited for reconnaissance surveys where approximate elevations are required.
Control by Trilateration
the lengths of the sides of a series of triangles are measured and the angles are computed from the lengths.
Station Adjustment
an approximate adjustment used in triangulation systems of lower orders of precision.
Observing towers
for the measurement of angular values in a triangulation system, the stations selected must be intervisible.
Horizontal Control
generally establish geodetic latitudes and longitudes of stations.
Chain of Quadrilaterals
is the most commonly used triangulation system. It is the best adopted to long and narrow surveys where a high degree of precision is required.
1. Reconnaissance 2. Measurement of Base lines 3. Erection of Signals 4. Measurement of angles 5. Astronomic checks 6. Office computation
Steps in Triangulation Work
Reduction to sea level
linear measurements for any triangulation network which extends over, a large area, particularly over regions of widely varying relief, must be reduced to a common vertical reference datum.
Office Computations
the basic objective of triangulation computations is the determination of the best possible planimetric positions for the triangulated stations.
Control by Triangulation
when a large area is to be surveyed and where the methods of traversing would not be expected to maintain a uniform high accuracy over the entire area.
