Unit 6. Control Surveys Flashcards
are used to establish coordinate systems for mapping.
Control surveys
are used to establish baselines from which field surveys can be performed.
Control surveys
___ for a mapping survey is provided by two or more points on the ground, permanently or semi-permanently monumented, and precisely fixed in position horizontally by distance and direction or coordinates.
Horizontal control
It is the basis for locating map features.
Horizontal control
can be established by the traditional ground surveying methods of traversing, triangulation, or trilateration, or by using GNSS surveys.
Horizontal control
For large areas, a sparse network of horizontal (and vertical) control can be densified using ___.
photogrammetry
For small areas, horizontal control for mapping surveys is generally established by ___.
traversing
Horizontal control methods (5)
- GPS
- Photogrammetry
- Traversing
- Triangulation
- Trilateration
A series of horizontal distances and directions are observed to connect points whose positions are to be determined.
Traversing
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.
Triangulation
___ consist of networks of geometric figures with almost all of the angles measured directly in the field and with relatively few of the sides measured.
Triangulation surveys
The lengths of the sides of a series of triangles are measured and the angles are computed from the lengths.
Trilateration
This is the most accurate method of establishing horizontal control.
Trilateration
the vertices of the triangles in a triangulation system
Triangulation stations
one of the lines in a triangulation system whose length is precisely measured and its direction determined; usually located at the beginning of the system
Base line
one of the lines, usually at regular intervals, whose length is precisely measured and serves as a check for length calculations
Check base
Consists of a network of connected triangles which adjoin or overlap each other.
Triangulation Systems
Triangulation Systems. Four of the commonly used types are:
Chain of Single Triangles
Chain of Quadrilaterals
Chain of polygons
Chain of polygons with an extra diagonal
Choice of figure (2)
Equilateral triangles
Squares
is provided by benchmarks in or near the tract to be surveyed and becomes the foundation for correctly portraying relief on a topographic map.
Vertical control
is usually established by running lines of differential levels starting from and closing on established benchmarks.
Vertical control
Vertical control methods (5)
Differential leveling
Trigonometric leveling
Barometric leveling
GPS
Photogrammetric methods
the sum of the angles about each station is equal to 360 degrees
STATION ADJUSTMENT
the sum of the interior angles of each triangle equal to 180 degrees
FIGURE ADJUSTMENT
The term “___” is applied to a function used to determine the strongest route through a chain of triangulation figures composed of quadrilaterals and central-point polygons.
strength of figure
The ___ determines the manner in which angular errors affect the computed lengths and positions.
shape of the figures
