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Unformatted text preview: 1 GY 301: Geomorphology GY 301: Geomorphology Lecture 6: Coastal Geomorphology UNIVERSITY OF SOUTH ALABAMA Last Time The Alidade and Plane Table Surveying Alidade procedures Reading elevations Surveying tricks Alidade & Plane Table ht p:/ www.hugha.co.uk/Tanzania/TanReporter52_files/LR006.jpg An early model alidade ht p:/ www.harborfreight.com/cpi/photos/96600-96699/96669.gif Measurements with the Alidade and Stadia Rod Setup of the instrument includes: 1. Leveling the plane table 2. Drawing magnetic north reference line 3. Measuring the instrument height 4. Recording of scale (ensure all features to be mapped will fit on map) 5. Recording of ground elevation directly below the center of the plane table ht p:/ www.pssac.org/SoilMappingHistory/Plane%20Table%20and%20Alidade.gif Ray Measurement Geometry Measurements from the alidade station to a data point are termed rays The ray is drawn on the plane table to track distance and direction, and the elevation of the ray endpoint is calculated in field notes A correction must be made for the difference between the alidade cross hair (CH) intercept and the instrument height (IH) of the alidade (see below): IH=3.5ft Cross hair (CH) = 5.7ft Elevation=161ft Elevation @ Rod = 161ft + (IH-CH) = 161ft + (3.5ft-5.7ft) = 161ft 2.2ft = 158.8ft 5.7ft Ray Measurement Geometry When slope angles and distances become large the alidade telescope must be inclined to view the stadia rod A trigonometric formula must be used to take the inclination of the telescope into account (see below): Horizontal map distance Vert. angle (VA) Upper stadia Lower stadia Stadia rod Elevation change Net horz. offset = SD * (cos (VA-30)) 2 Net vert. offset = SD * (tan (VA-30)) 2...
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- Fall '10