A treatise on land-surveying; comprising the theory developed from five elementary principles; and the practice with the chain alone, the compass, the transit, the theodolite, the plane table, &cIllustrated by four hundred engravings, and a magnetic chart . s together take the name of Alidade. If a point shouldbe too high or too low to be seen with the ahdade, a plumb-line,held between the eye and the object, will remove the difficulty. A telescope is sometimes substituted for the sights, being sup-ported above the ruler by a standard, and capable of pointingupward or downward. It admits of ad
A treatise on land-surveying; comprising the theory developed from five elementary principles; and the practice with the chain alone, the compass, the transit, the theodolite, the plane table, &cIllustrated by four hundred engravings, and a magnetic chart . s together take the name of Alidade. If a point shouldbe too high or too low to be seen with the ahdade, a plumb-line,held between the eye and the object, will remove the difficulty. A telescope is sometimes substituted for the sights, being sup-ported above the ruler by a standard, and capable of pointingupward or downward. It admits of adjustments similar in principle PART VIII.] PLAXE TABLE SURVETIXG. 305 to the 2d and 3d adjustments of the Transit, Part IV, Chapter 3,pages 242 and 246. But even without these adjustments, whether of the sights orof the telescope, a survey could be made which would be per-fectly correct as to the relative position of its parts, however farthe line of sight might be from lying in the same vertical planeas the edge of the ruler, or even from being parallel to it; just aain the Transit or Theodohte the index or vernier need not to beexactly under the vertical hair of the telescope, since the angulardeviation affects all the observed directions (452) Method of Radiation. This is the simplest, though notthe best, method of surveying with the Plane-table. It is especi-ally applicable to survey-ing a field, as in the it and the following fi-gures, the size of the Tableis much exaggerated. Setthe instrument at any conve-nient point, as 0; level it,and fix a needle (having ahead of seahng-wax) in theboard to represent the sta-tion. Direct the alidade to any comer of the field, aa A, the fiducialedge of the ruler touching the needle, and draw an indefinite line byit. Measure OA, and set off the distance, to any desired scale, fromthe needle point, along the line just drawn, to a. The fine OA isthus platted on the paper of the table as soon as determined in thefield. D
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectsurveying, bookyear18
