Chains. Chains. Chains. Chains. Chains. North, bet. secs. 19 and 24. -121 4.50 4.398 Desc. 155 ft. over NW. slope, through scattering timber and dense undergrowth. -17° 2.20 2.098 -0.95 19. A simplification of the reduction of measurements on the slope is obtained by the use of two diagrams constructed on crosssection paper, as follows: The first with the vertical lines representing intervals of 20 links measurement on the slope to 2, 5 or 8 chains to suit the length of tape used; the horizontal lines representing the correction in links to be made from the measurement on the slope to obtain the true horizontal distance; slanting lines are drawn to represent various degrees of slope scaled to the proper 55465-19-3 points for the correction for the full length of the tape. The second diagram is constructed with the vertical lines representing similarly the measurement on the slope in the chain unit; the horizontal lines in this diagram representing the difference in elevation in feet, at intervals of 5 feet; slanting lines are drawn to represent various degrees of slope scaled to the proper points for the differences of elevation for the full length of the tape. (See figs. 1 and 2.) 20. The following is an example of record for the use of the long steel tape and clinometer, and reductions by the use of the reduction diagrams: 21. By a skillful use of the long steel tape on the slope, with correct determinations of the vertical angle, and proper reductions from the slope to the true horizontal distance, the surveyor obtains one of the most rapid and reliable methods of measurement. It is essential to make all reductions for distance as the work progresses, but the additional information regarding the amount of the ascents and descents is readily obtainable from the record at the convenience of the surveyor. STADIA MEASUREMENTS. 22. Under proper safeguards the stadia method of measurement affords a useful and reliable means of overcoming the difficulties of obtaining correct distances across water and over precipitous slopes that can not be reached with the tape. It is required that the wire interval or ratio be determined in the field by frequent tests under working conditions in comparison with steel tape measurement, solving the formula given in the Standard Field Tables (p. 221) for the value of the wire ratio with the horizontal distance known. The record of the stadia tests should be given in the field notes. It is essential to accurate stadia work that rods of approved construction be used, together with two targets and a properly adjusted rod level to secure true vertical readings; the readings at all times must be restricted to suitable atmospheric conditions and to distances permitting exact bisections of the targets. Possible criticism of the use of the stadia method is found in the failure to observe proper details and not in the reliability of the method if skillfully followed. 23. It is desirable to state briefly at the beginning of the field notes, with every set of returns, the general plan of making stadia measurements. The following paragraphs are illustrative of the character of such record: "All stadia measurements are made with fixed stadia wires with a ratio of 1: 132±, as exhibited by the tests shown in the field notes; the focal constant of the instrument is 1. 2 links; the rod used is a standard Philadelphia level rod graduated to feet and equipped with two targets and a rod level; all readings are made with a vertical rod." "All stadia measurements are made with fixed stadia wires with a ratio of 1: 100±, as exhibited by the tests shown in the field notes; the focal constant of the instrument is 1. 2 links; the rod used is a standard Troy level rod graduated to feet and equipped with two targets and a rod level; all readings are made with a vertical rod." |