Page:The New International Encyclopædia 1st ed. v. 18.djvu/828

* SURVEYING. 726 SURVIVORSHIP. istence it is possible to commence field work in widely scattered areas, lines of spirit leveling of lesser precision being run throughout the area to be mapped at distances not exceeding six miles apart. Between these, at distances of one to three miles apart, according to the scale and contour interval, lines of ilying or secondary levels are run with only sncli accuracy as will enable them to close upon the better levels with- in limits of one or two feet, an amount imper- ceptible in contours of 10 to 50 feet interval. The three surveying bureaus of the Govern- ment, viz. the Coast and Geodetic Survey, the Engineer Corps of the Army, and the Geological Survey, are all engaged in running lines of pre- cise levels throughout the country, as they are extending the related work of triangulation and topographical mapping. The skeleton of the sur- vey being completed, the next task is to perform the topographical mapping. Starting with two or more primary triangulation stations within the area of the quadrilateral to be surveyed, a secondary scheme of triangulation is extended by plane table (q.v.) by which tertiary points are located with such freqviency as to give an aver- age of from one to three good locations per square mile, equivalent to the same number per square inch of map. Elevations are determined of all triangulation points, which are chielly hill sum- mits, by vertical angulation depending on spirit leveling. Also of all the lower relief of the coun- try by lines of spirit levels run from three to six miles apart. Where the lack of surface relief or the density of forests is unfavorable to the execution of triangulation, the primary and sec- ondary control consists of a system of primary traverses checked by astronomical positions or primarj' triangulation locations. A second sys- tem of horizontal control consists of odometer traverses of all roads and trails, or, where these are not sull'ieicnt, of stadia or taped traverses across country and about the shores of lakes and streams. Tliese traverse lines are run by compass and plane table, and are platted upon the board as the work progresses, and are adjusted upon the map, between the frequent locations resulting from the primary traverse or the plane-table tri- angulation. The distances between check points on such traverse are so relatively short, being one to four miles, that the error of location is hardly perceptible upon the scales of one or two miles to the inch, and an adjustment to the better posi- tions becomes easily possible. The primary tri- angulation and precise leveling is executed a season in advance of the topographic mapping. The determination of secondarv' positions by plane-table triangulation, road and stadia trav- erse, and flying levels proceeds together a little in advance of the sketching of the map, but by members of the same party. The results of the adjustment and transfer of all traverse lines and level elevations to the secondary locations is a control or sketch sheet upon which are two to five trigonometric locations per square inch, from four to eight inches of road traverse per square inch, and an average of one to two instru- mental elevations per square inch. Placing himself at a known position, the height of which is already determined, the topographer sketches, with the aid of hand level and by eye, the plan of the contour line passing through his position on the map, and it may be possible to locate this for a distance of half a mile, the equivalent of half an inch, in either direction. If the country be wooded it cannot be seen beyond his absolute position. Proceeding now over the road, prefer- ably by vehicle, he stops at greater or lesser in- tervals, according to the amount of change of slope, carrying diti'erences of elevation for short distances between check points by aneroid. When good elevations are insutficient more are pro- cured by flying levels or by vertical angulation, as the work progresses. In this way a network of topograpliic contouring is built up everywhere along the lines of the roads, and these are fre- quently so near together that the contours meet and fill up the entire surface of the map. Other- wise the topographer walks in between roads, pacing distances if short, or runs stadia line, if necessary, and thus he fills in the interior of the road circuits and completes the map. This gen- eral method is necessarily much modified, accord- ing to the conditions. iMixiNG Surveying. Mining surveys are of two classes: (1) surveys to determine the sur- face location and boundaries of mining claims; and (2) underground surveys to determine con- nections, lay out work, establish the relations of the underground Avorkings to the surface lines, points, and so on, and measure the ore removed or still in the mine. The surface methods em- ployed are substantially land surveying methods as modified by the local and general mining laws. Tlie underground surveys comprise carrying sur- face locations underground, lining on the galleries and shafts, running levels, etc., and laying out tramways and railroads. This work is ordinarily carried on much the same as it is on the surface. City Surveying. The methods described in the section on land surveying are adequate to the needs of the city surveyor, except that, on ac- count of the value of the land, greater accuracy should be secured. To this end, the transit in- strument is used instead of the compass and the steel tape instead of the chain. BiDLiOGR.iPiiY'. The literature of surveying is extensive. A concise technical treatment of the whole subject is contained in Johnson, Theory and Practice of Surveying (New York, 1900). Other books of equal value in their particular field are: Wilson, Topographic Surrcyiiig (New York, 1900) ; llerrinian. Precise Survei/ing atid Geodesy (New York, 1S99) and Geodetic Sur- veying (New York, 1892) ; and Gribble, Prelimi- nary Surveys and Work Estimates (London, 1897). The voluminous reports of the Coast and Geodetic Survey, the Geological Survey, and the Engineer Corps, United States Army, contain an enormous mass of information relating to their surveys throughout the United States. JIany im- portant articles on surveys and surveying are also to be found in the proceedings of the engi- neering societies and in the volumes of the prin- cipal engineering journals. See Geode.sy': Map; Engineering Instruments; Engineering, Mili- t.vby'; pliotogr.piiic surveying; leveling; Coast and Geodetic Survey: Geological Sur- vey; Plane Table; Stadia; etc. SURVEYING, INIarine. See Hydrography. SURVEYS, National. See Geological Sur- vey, United States; Coast and Geodetic Sur- vey, LTnited States. SURVIVORSHIP. A term commonly em- ployed to describe a doctrine prevailing in some jurisdictions to the effect that where two or more