intermediate TPs. In each case, a “higher” TP(as TP1) and a “lower” TP (as TP~~) was used,resulting in two different HIs for each. Computedby way of the higher HIs, the elevation of BM2came to 851.98 ft. Computed by way of the lowerHIs, it came to 852.00 ft. The mean (average) of851.99 ft was taken as the correct elevation.INDIRECT LEVELINGIndirect methods of leveling encompassboth trigonometric and barometric leveling.TRIGONOMETRIC LEVELING uses verticalangles and a horizontal distance to computethe difference in elevation, BAROMETRICLEVELING uses the difference in atmosphericpressures that are observed by a barometer or analtimeter to determine the elevation differences.Indirect methods of leveling will be discussed atthe EA2 level.PRECISION IN LEVELING;MISTAKES AND ERRORSIN LEVELINGLeveling, like any other surveying operation,is carried out by following a prescribed ORDEROF PRECISION—meaning that the instrumentsyou use and the methods you follow have to bethose that can give you the specified standard ofaccuracy.PRECISION IN LEVELINGFIRST-ORDER leveling is used to establishthe main level network for an area and to providebasic vertical control for the extension of levelnetworks of the same, or lower, accuracy insupport of mapping projects, cadastral (recordingproperty boundaries, subdivision lines, buildings,etc.), and local surveys. Level lines must start andend on proven, existing BMs of the same order.New levels must be run between the starting BMbeing used and at least one other existing BM andmust show there is no change in their relativeelevations.SECOND-ORDER leveling is used to sub-divide nets of first-order leveling and to providebasic control for the extension of levels of thesame, or lower, accuracy in support of mappingprojects and local surveys. Second-order levels aredivided into two classes: Class I and Class II.CLASS I is used in remote areas where the linemust be longer than 25 mi because routes areunavailable for the development of additional orhigher order networks and for spur lines. CLASSII levels are used for the development of nets inthe more accessible areas. In Class I leveling, itis required that all lines start and close onpreviously established BMs of first or secondorder. New levels have to be run between theexisting BM being used and at least one otherexisting BM to prove that they have not changedtheir relative elevations. The criteria for Class IIare the same as for Class 1, except that Class IIlines are run in one direction only.THIRD-ORDER leveling is used to subdividean area surrounded by first- and second-orderleveling and to provide elevations for theimmediate control of cadastral, topographic, andconstruction surveys for permanent structures.The following criteria should be observed in third-order leveling:1. All lines have to start from, and close on,two previously established BMs of third, orhigher, order of accuracy if the new levelingindicates they have not changed in their relativeelevations.2. In the United States, third-order linesshould not be extended more than 30 mi fromBMs of first or second order. In foreign or remoteareas, the distance may be extended according tothe evaluation of the existing control and thesituation. They may be single-run (one direction)lines but should always be loops or circuits thatclose upon BMs of an equal or a higher order.3. When a line from previously establishedthird-order marks is extended, the maximumlength of the new line is greatly reduced. Thedistance and allowable error are to be carried backthrough the existing line to the nearest tie BM ofthe second or higher order.4. Balanced sights should not be greater than300 ft. BS and FS distances maybe measured bypacing and approximately balanced between BMs,Rod readings are read to thousandths and the rodwaved for extended rod readings. The bubble ischecked to make sure it is exactly centered beforeeach sighting and reading. Turning point pins orplates or well-defined points on solid objects areused for TPs.FOURTH-ORDER leveling is used to sub-divide an area within a third-order network. Thisis the method of leveling used in connection “withthe location and construction of highways,railroads, and most other engineering works thatconcern the SEABEEs in advanced base projects.14-21
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