surveying iii (topographic and geodetic surveys) - Modern Prepper

differential leveling. The theodolite, other measuring equipment, and the trigonometric functions of atriangle are all used for trigonometric leveling. In point of accuracy leveling, trigonometric leveling isinferior to precise spirit leveling, particularly in flat areas. In mountainous country, the trigonometricmethod is of great value, and the results are comparatively more accurate.There are many accuracy classifications and requirements for leveling. This lesson begins by discussingthe orders of accuracy and requirements and continues by describing the equipment, methods,procedures, records, and computations for precise differential leveling. The final portion of this chaptercovers the process of determining elevations by the indirect method of trigonometric leveling.PART A - ACCURACY AND REQUIREMENTS4-1. Spirit Level Lines. Spirit level lines usually determine elevations referred to the datum. Theselines are run according with rigid specifications that prescribe the conditions under which the line maybe run and the tolerances that are permissible for the recognized orders of accuracy. When planning newlevel lines, the surveyor must take into consideration the ties to all previously determined elevations ofacceptable orders of accuracy, the best distribution of new elevations for the contemplated survey, thenature of the terrain involved, and the most practical routes to run the new line. All pertinentinformation must be assembled for study, and suitable guide maps must be prepared to show the routingof the proposed lines. Generally, the level man is not responsible for such planning, but he shouldunderstand the established requirements as he will be in a position to apply sound judgment if localconditions force slight deviations from the initial plan.4-2. Benchmarks. The value of a line of levels depends upon the accuracy, distribution, andpermanency of established benchmarks or those tied in during leveling. Benchmarks are permanentobjects, natural or artificial, bearing a marked point whose elevation above or below the adopted datumis known. Temporary benchmarks (TBMs) are intended to serve for short time periods.4-3. Leveling. Leveling is classified as the degree of accuracy according to the methods andinstruments used and the closure specified. The orders of accuracy are first, second, and third order.First order is the most precise. This lesson will only address the required accuracies of first- and secondorderleveling.a. First-Order Leveling. Since the uses of first-order leveling are so important, the criteria are verystrict. First-order leveling is used to establish an area's main level net and to provide basic verticalcontrol for the extension of level nets of the same or lower accuracy. This order of leveling providesdata for mapping projects, special site construction projects, cadastral and local surveys, and geodeticresearch involving earth sciences.(1) First-order level lines must start and end on proven, existing benchmarks of the same order.New levels must be run between the startingEN0593 4-2