Survey-Triangles of the Ordnance Survey of Great Britain and Ireland-Primary Triangles-Secondary Triangles-Tertiary Tri- angles-General Form of Triangulation-Reconnaissance and Selection of Stations-Determining necessary Heights of Stations- Clearing out Line between two Stations not visible from each other-Selection of Base Lines-Permanent Marking of Stations and Base Lines-Instrument Stations and Scaffolds-Signals- Sun Signals or Heliographs and Heliostats-Heliostats and Helio- graphs-Gauss' Heliotrope-Wharton's Improvised Heliostat- Night Signals-Phase of a Signal-Measurement of Base: Measure- ment with Steel Tape-Straining Apparatus-Absolute Length of Tape-The Coefficient of Expansion-The Modulus of Elasticity - Correction for Sag-Correction for Pull-To Eliminate Corrections for Sag and Pull-Correction for Temperature-Corrections for Fractional Part of Tape Length-Measurement of Bases with Steel and Brass Wires-Accuracy of Steel Tape Base Measure- ments-Measurement with Rods-Broken Base-Reduction of Base to Horizontal-Reduction of Base to Mean Sea Level- Extending a Base-Extension of the Triangulation from the Measured Base-Instruments-Observation of the Angles: Method of Repetition-Method of observing Horizontal Angles adopted on Ordnance Survey of United Kingdom-General Method of observ- ing Angles Time for observing Angles-Accuracy of Measure- ments of Angles—Reduction of Angles to the Centre of Station- Correction of the Errors of the Angles-Calculation of Spherical Excess-Adjustment of the Angles-Example of Correction and Adjustment of Angles of a Triangle-Correction by Reciprocals of Number of Observations-Correction by Reciprocals of Squares of Number of Observations-Adjustment of Angles inversely as "Weights" of Observations-Calculation of the Sides of the Triangles-Calculation by Spherical Trigonometry-Calculation by Delambre's Method-Calculation by Legendre's Method- Summary of Operations in Computation of Triangles by Legendre's Method Calculation of the Astronomical Co-ordinates of the Stations-Polar Spherical Co-ordinates — Rectangular Spherical Co-ordinates-Surveying of Interior Detail-Levelling-Cost of Trigonometrical Surveys—Instructions for Secondary Triangulation -Locating Stations-Reading Angles-Closing Triangles-Base Lines-Observations for Azimuth-Stone Line Bench Marks- Cutting Timber-Descriptions of Stations-Tertiary Triangula- tion Triangles — Stations Instrument Stations - Observing Angles-Marking Stations-Map Projections-Projection to be adopted — Rectangular Projection — Trapezoidal Projection — Simple Conic Projection-De L'Isle's Conic Projection-Bonne's Projection-The Polyconic Projection-Projection of Maps of the Survey of India-Projection of Maps of the Ordnance Survey of PAGES TABLE I. FOR HEIGHTS BY MERCURIAL BAROMETER 168, 482, 484, 485 II. FOR HEIGHTS BY ANEROID BAROMETER LIST OF TABLES. 170, 486, 493 III. FOR HEIGHTS BY BOILING POINT THERMOMETER 171, 494, 497 IV. FOR REDUCTION OF INCLINED SIGHTS IN TACHEO- TABLE FOR REDUCING MEASUREMENTS ON SLOPE TO HORI- TABLE FOR REDUCING MEASUREMENTS ON SLOPE TO HORI- TABLE FOR CONVERSION OF DEPARTURE INTO DIFFERENCE OF TABLE OF LENGTHS OF A DEGREE OF LONGITUDE AT DIFFERENT 273, 498 9 10 348 349 SURVEYING AS PRACTISED BY CIVIL ENGINEERS 100 0 Instruments: Chain. For the purpose of making actual linear measurements on the ground, the chain is most used. It consists of strong links of steel or iron wire of from No. 7 to No. 12 W.G., connected by rings, with a brass handle at each end (Fig. 1). In English-speaking countries there are two different lengths of chain in common use, the Gunter's or 66 ft. chain and the 100 ft. chain. The 66 ft. chain is most used, and possesses the advantage in computing areas that 10 square chains is equal to 1 acre. Both chains consist of 100 links, SURVEYING WITH THE CHAIN ONLY. CHAPTER I. 20 and every tenth link is distinguished by a brass tablet, as shown in Fig. 1. The first 10 links from the end is marked by a brass tablet with one point; the tablet at 20 links has two points, that at 30 links three points, at 40 links four points, and 50 links or the centre of the chain is marked by a circular tablet. Each 10 links is marked from the other end similarly, so that the chain can be read both ways. Each link of the 66 ft. chain is therefore part of 66 ft. or 7.92 in., and each link of the 100 ft. chain is 1 ft. Distances are thus measured with the 66 ft. chain in chains and decimal parts of a chain, or links as they are called, as 7.85 chains or A 7 chains 85 links; with the 100 ft. chain the measurements are of course in feet only. In the United Kingdom nearly all railway work and ordinary surveying is executed with the 66 ft. chain, the use of the 100 ft. chain being principally confined to works of water supply, sewerage, and municipal works. The 100 ft. chain is, however, universally used in British India and the United States, and generally in the Colonies. In countries where the metric system is used, the usual length of chain is 20 metres. This is almost exactly 66 ft., 20 metres being equal to 65.6 ft. Reading the Chain. In taking measurements with the chain, we must look for the nearest brass tablet short of the point that is being measured to, and count the number of links from it forward to the point in question. If the nearest tablet indicates 20 links, and the point being measured to is 6 links beyond it, the distance is thus 26 links. Custom soon enables one to read the chain at a glance. Some confusion may arise at first from the fact that the tablet with four points indicates 60 links as well as 40 links, according as we reckon from one end of the chain or the other; in the same way the tablet with two points may either indicate 20 links or 80 links. A little practice, however, soon enables one to tell which is the correct reading. A look at the end of the chain or at the 50-link tablet will always decide the point. Laying out Chain on Ground. -When not in use, the chain is rolled up in a bundle (see Fig. 2) and fastened with a leather strap. To lay out the chain on the ground, take both handles in one hand, and throw forward the chain, keeping hold of the handles. When fastening up the chain after use, take it up at the centre link and bunch it up double, two links at a time. |