Reference of reference points Geometrical figure-triangulation Marking and distinguishing trig. points.-Subdivision Angles to connect the base line trig. points with other Comparison of French and English trigonometrical Verification of the measurement of angles Correction to be applied to angles in a plane Correction and verification of arcs and their correspond- ing angles-spherical excess Observing remote trig. points by reflection Finding angles at an inaccessible trig. point : By the measurement of an equal angle Measuring base lines by measurement of quantity— suggestion for practical solution. Tracing straight lines with a theodolite. Same, from an intermediate point in open country Same, by auxiliary parallel lines in heavy wooded Same: expedients for tracing straight lines in dense Finding inaccessible distances with the aid of a theodolite Tracing straight lines with a prismatic compass Computation of the sides of plane triangles . Maximum errors allowable in open and in hilly country. 80 26 box sextant-or theodolite. Finding approximately the parts in error By tangents intersecting other tangents Determining additional points on the curve By combined angular and linear measurements By equal (segmental) angles When the tangents intersect in an inaccessible point Extended survey operations Official instructions for the engineer in chief Contour description of locality and position of some Marking and describing trig. points for refinding same. Original documents and duplicates-conveyance to pur- Intakes for Railway, &c. purposes. Determining sidewidths for level and sidelong ground. 135 48 HYDROGRAPHICAL SURVEYS.-Submerged Districts. of the bottom-points referred to water surface points in a straight line-readings for the sound- Tidal or other gauges when the water level fluctuates Obtaining sections of the bottom by mechanism stake gauges cannot be readily erected-reduc- Hydrographical field-book-tidal currents Current meters-measuring for the course, and observ- 140 49 General objects of levelling-vertical section-cases Errors due to the earth's sphericity Levelling with a theodolite as an angular instrument— line of collimation-errors to be removed Errors to be apprehended in levelling . Tell-tale to show the angle the 'level-staff' makes with Comparison of the amounts of particular errors measurements how made-mode of proceeding 333 Levelling with the theodolite as an angular instrument- for a detail section-operations in the field Thermo-corrections on barometric readings. CONTOUR SURVEYS.-Tracing Contour Lines. Explanation and definition of contours Illustration of contour map-instruments required for Simultaneous and chain measurements for determined |