Surveying Control and Intersection Techniques

Control

• Control is knowing the starting point for measurements to determine the position of features.

Intersection

• Needs two known, intervisible points.
• Observe the unknown third point from the two known points.
• Measure the horizontal angles between the known points and the unknown point.
• Only requires two known points.
• Simple calculation.
• Computation of unknown points can be achieved if the points are set up on known points.
• Disadvantages: Requires visiting known points (which might not always be accessible). Points need to be intervisible. Good weather or long distance needed for good observations. Time consuming.

Resection

• Calculates unknown point using three known point coordinates.
• Useful when targets are inaccessible.
• Needs to observe horizontal angles to all known points from the unknown point.
• Useful for measuring distance from unknown point to one known point, not necessary for all known points. Should be within a triangle of surrounding control points.
• Advantages: No need to visit known points. Saves time during the observation. Does not need known points to be intervisible. Only one setup. Not dependent on GNSS.
• Disadvantages: Computation is more complex. More known points are needed compared to intersection. Geometry considerations (danger circles). More expensive equipment. Location must be suitable (no trees or buildings).

Computation

• Computation becomes more difficult when intersection or resection are used.
• Geometry considerations (e.g., danger circles).
• Requires more known points than intersection.
• Using GNSS for control.
• Expensive equipment needed.
• Location must be suitable(e.g. no trees or buildings).

Accuracy

• Control accuracy is determined by the accuracy of known points.
• Specifications need to consider the accuracy required.
• Scale of results, equipment, and the purpose of the survey influence specifications.
• Accuracy should aim for twice the precision of 0.2m at 1:1000 scale, so for example 0.1m at this scale. Control points should ideally be twice as accurate.

Traversing

• Used when the initial site or control is insufficient to survey the work site.
• Consecutive lines of points and distances need to be measured in the field.
• Measurement of angles and distances from a known point determines the new points’ relative position from the known point(s).
• Adding the known orientation to the first point establishes the second point's absolute position.

Traverse types

• Known/Free traverse: Starts on a known point but ends on an unknown point. Accuracy cannot be checked. Not suited for most projects due to lack of checks. Commonly not permitted by most companies.
• Closed traverse: Starts and ends on known points. Traverse is oriented at the start and end points to other known points .Start and end points are fixed so the points in between the start and end are adjusted accordingly. Shows systematic errors in angle or distance.
• Closed loop traverse: Starts and ends at the same point. Often used to control engineering projects. Points enclose the site for connection to other work.

Errors

• Orientation errors at the start will affect all subsequent measurements.
• Main sources of error are angles, distances, and centering.
• Short legs are also a contributing factor to errors.
• Reducing distances to known points by a factor of 10 increases angular error by the same amount.
• Rule of thumb and rigorous adjustments (least square analysis/estimation) are important techniques.

Least Squares

• A method for estimating the most probable value, adjusting measurements for most accurate results.
• Provides probable values and quality indicators.
• First used by Carl Gauss in the 1818-1832 geodetic survey of Hanover.

Other Errors

• Gross errors: calculation and recording mistakes.
• Blunders: common human errors (e.g., incorrect number input)
• Systematic errors: caused by instrument errors, and environmental factors.
• Random errors: errors that are difficult to control.
• Redundancy: Having more data than needed.

Residuals

• Difference between measured values and most probable values.
• Used instead of true error as true error is difficult to calculate.

Precision

• Repeatability of measurements.
• Random errors only. Small deviations.
• Normal Distribution.

Accuracy

• Overall measurement estimate, including systematic error.
• Same as precision if all errors are removed

This quiz covers essential concepts of control surveying, including intersection and resection methods. Learn the principles of establishing measurements from known points to unknown locations and understand the advantages and disadvantages of each technique.