The Correction of an Altitude for Index error, Horizon, Re- fraction, Parallax, and Semi-diameter Extra-meridional correction . . Obtaining latitude by observation in the Prime Vertical Collimation. Level and Azimuth TRAVERSING WITH THE CHAIN AND CROSS-STAFF facing page 94 CHAIN-SURVEY OF AN ESTATE (Record No. I.) AID ΤΟ SURVEY-PRACTICE. PART I. GENERAL SURVEYS. CHAPTER I. INSTRUMENTS AND CALCULATIONS. Section I. GENERAL EXPLANATION. ALL Geodetical operations may be said to be generally comprised under the three heads of Surveying, Levelling, and Setting out, or locating. Surveying generally consists in taking the measurements and observations on which a plan, chart, or map of the natural and artificial features of a tract of land or water may be based; although this does not strictly apply to some special branches, such as geological surveys; the term itself is also sometimes used to include levelling. General surveys may be classified in three ways: either according to the nature and scope of the ultimate object, or in accordance with the instruments used in the work, or the scale of the intended plan. Survey work of the highest and most perfect type is known as Great Trigonometrical Surveying, which 98 B may extend over many degrees of latitude and longitude, and even include a large region as for instance, that of India, of Germany, France, or the British Isles. The utmost precision being necessary in such work, the general management of it is entrusted to scientific men, with whom rest the initiation or adoption of improved methods, instruments, and appliances; while the carrying out of the detailed routine work, its checking and superintendence, is usually delegated to inferior men that happen to be naturally well fitted to adhere to a rigid system of work not requiring the exercise of much judgment or discretionary power. These systems and appliances are fully described in books devoted to that special subject, which is beyond the scope of this work; they will hence be merely referred to here in so far as they serve as guides, or afford indications useful in ordinary practice. The least perfect type of Survey-work is termed Military Surveying. It is conducted on the principles of ordinary practice, but in a rough manner, mostly with pocket-instruments, aided greatly by good guess-work; the permissible amount of error in it is large, while extreme rapidity is of paramount importance. For such surveying no additional training or special instruction is necessary, although a knowledge of strategical requirements is a valuable adjunct. All ordinary general survey-practice lies between these two extreme types, and may be thus classified on the first-mentioned principle. 1. District Surveys, of extended tracts of country. These being of large scope, and requiring great precision, are carried out in style and method after the model of operation originally adopted in Great Trigonometrical Surveys, although less exact and cumbrous instruments and appliances are used, and a very much. less amount of refinement and exactitude, as well as time, is expended on them. 2. Parish and Estate Surveys, being more limited. in scope, require less exact instruments and appliances, but follow the general principles adopted in the filling in of Great Trigonometrical Survey work. 3. Town Surveys, which ordinarily cover about the same extent of land and range of limits as those of the last class, differ from them in requiring greater exactitude, as they have to be plotted to a much larger scale, and in involving a greater amount of labour. 4. Engineering Surveys. These having for their special object the future location of some work of public or private utility, such as a railway, canal, waterworks, drainage works, or factories, are generally confined either to a long narrow strip of country or a small tract of country or plot of land; the amount of exactitude necessary being liable to vary all over the ground in accordance with its bearing on future requirements and the nature of the intended works. 5. Route-Surveys have in view the limited object of producing a map or plan of a route taken by an explorer or traveller either by land or sea; in the latter case it is usually termed navigation, and consists in laying down the ship's course: the position of natural and artificial features of the country, or of detached points of land, in the immediate vicinity of the route, are also recorded in these surveys. 6. Mining and Subterraneous Surveys are limited. by the extent of actual or possible future mining operations, tunnels, passages, or works of construction; they generally involve a record of position or distribution of |