Land Surveying II MCQ PDF

STUDY FOR LAND SURVEYING II MCQ Control- Need to know where we are measuring from to calculate the position of features. Intersection Need 2 known intervisible points, set up at both known points and observe to the unknown 3rd point. Measure the two known horizontal angles....

STUDY FOR LAND SURVEYING II MCQ Control- Need to know where we are measuring from to calculate the position of features. Intersection Need 2 known intervisible points, set up at both known points and observe to the unknown 3rd point. Measure the two known horizontal angles. Advantages Only require 2 known points. Computation is simple Can calculate unknown points that you can set up on. Disadvantages Need to visit the known points (might not be accessible). Points need to be intervisible. Better long distance to give good observations. Need good weather to observe ‘good angles. Takes time Resection To calculate unknown points, you need to know 3-point co-ordinates. Popular because you could use targets where prism is inaccessible. Must be able to observe hz angles to all known points from unknown point. Useful to be able to measure distance from the unknown point to 1 known point but not essential & keep P within triangle of surrounding control points. Advantages No need to visit the known points. Saves time during the observation period. No need for known points to be intervisible. Only one set up means weather should not be such an important factor. Not dependent on GNSS Disadvantages  Computation is more difficult Geometry considerations (danger circle) Requires more known points than intersection. Using GNSS for control Expensive equipment Location needs to be suitable (no trees, buildings) Does not have to be suitable from other control points to establish a new point. Fixed control for traverse needs a second control point for orientation (must be visible). CP and RO should be far away as possible to reduce error. Accuracy of control The accuracy of any control point can only be as good as the known point from which it was derived. Specification To ensure the survey is being carried out to a minimum acceptable level of accuracy. Client states the accuracy required and the chief surveyor should work out acceptable specifications for each part of the survey. Scale of output, equipment available, purpose of the final product, method of survey all affects the specification. 1:1000 represents 0.2 metres on the ground. Ie 0.2mm:200mm=20cm=0.2 metres. All accuracy should aim to be twice as good as this, ie. 0.10m at 1:1000. But control points should usually be at least twice as good i.e. 0.05m in this case. TRAVERSING Often the initial site or established control is not sufficient to survey your work site. A traverse is a series of consecutive lines whose end points have been marked in the field. Lengths and directions have been determined from observation. By measuring angles and distances from a known point, we determine the relative position of the next point. By adding a known orientation to the first point, one can find the absolute position of the second point. Can be used in tunnels and mining projects where lack of GNSS limits control options. Advantages Limitations of topography and previous ‘fixed’ points do not apply. Much less reconnaissance required and ease of access to stations. Less observations required per station (rounds of angles and number of pointings). Control stations can be selected to be easily accessible. The linear nature of the method, it can be made to follow the route or other feature. Longer legs lead to better accuracy as well as speeding up the observation of the control network. Short traverse legs=establishment of more stations to detail from=> more errors in levelling and centering, angles, station markers etc. Disadvantages Method is liable to undetected errors. No checks are possible at intermediate points along the length of the traverse Must close onto some known point to be able to check the observations are acceptable- either a new known point or close back onto the start. Known/Free traverse Starts on a known point but ends on an unknown point. No possibility of checking the accuracy. Should not be used unless no alternative (mine or tunnel) Most companies specifications will not accept this kind of traverse. Closed Traverse This starts and ends on known point. Traverse is orientated at the start and end onto other known points. Start and end points are held ‘fixed’ so points in between are adjusted to fit. Would show any systematic errors in angle or distance Railroads Closed Loop Traverse Starts and ends on the same point. Often used to control an engineering project by enclosing the site with a series of points. If local values are used, then they cannot connect to other work.  If orientation is wrong at the start, then it is not possible to detect errors even if all other measurements are correct. Three main sources of error are angles, distance and centering. Also legs being too short Errors Generally, decreasing the lengths to target points by a factor of 10 causes the angular error to increase by the same amount. Rule of thumb adjustments- bowditch Rigorous adjustments- least square analysis/estimation LEAST SQUARES A rigorous estimation technique that yields the ‘most probable value’ by providing the ‘least’ possible corrections to a set of (good) measurements Background Least squares are to derive the most probable values and provide quality indicators. 1818-1832 first known use by Carl Gauss for the geodetic survey of kingdom of Hannover. Properties BLUE: Best Linear Unbiased Estimate Errors Gross errors: Mistakes made recording or calculating. Blunders: Usually, humans like mistakes made in booking numbers. Systematic errors: In surveying the group of errors caused by field processes (instrument, environmental, observer (mispointing on target), model errors) Random errors: The noise that we seek to mitigate and quantify by repeated rounds of angles. Redundancy: having more data than strictly necessary to solve a problem. Errors and residuals Error=Measured Value-True Value Hard to calculate true error so instead use residuals. Residuals- Difference between individual measured quantities and the MPV. Residual= Most probable Value- Observations Precision- repeatability- random error only. Small deviations from a sample mean or with MPV with normal distribution. The most probable value (MPV) for a quantity obtained from repeated observations of equal weight is the value that renders the sum of the squared residuals a minimum. Accuracy- Overall estimate including systematic. Accuracy and precision same if all errors removed.