On the chord system.-A corresponding mode of traversing with the chain, which dispenses entirely with the cross-head, may be adopted by prolonging each traverse line in the same way, and at the extremity of one chain, setting out either half a chain, a whole chain, or a chain and a half, at its extremity as a chord; thus making practical use of another set of angles while merely dealing with their chords, and conducting the traverse in exactly the same way in every other respect. If this chord method is applied in further detail, using quarters or eighths of a chain, and the angle (B) corresponding to any chord set out should happen to be required, then B chord but if the chord be set out at the extremity of two or three chains instead of one, then 2R or 3R must be substituted for R in the above formula. This method must always be kept distinct from the rectangular offset system; it is not quite so rapid, as it requires three measurements instead of two, and for certainty requires also that both traverse lines should be prolonged both ways to repeat the measurements. Record or field-book of chain survey. In all cases the record of measurements should be accompanied by a sketch showing the skeleton surveywork, and the position of some of the important features of the tract surveyed, such as roads, rivers, marked buildings, churches, etc., with their names; the surveystations and principal survey-lines should be distinctively designated by the letters or numbers used for them in the record. Some recorders use alphabetical letters to designate station-points, and numbers for survey-lines in the order of their measurement; another method is to use numbers enclosed by circles for the stations, and to enter in the sketch the total distances between stations; the latter is more convenient for a stranger to plot from. Special sketches or diagrams of lines measured are required whenever intricate measurements, as of buildings, happen to occur, also for corners where more than two lines of measurement meet, and whenever the ordinary unaided form of record fails in conveying clear and sufficient explanation. Field-records are best made with pencil in graphite, chalk, or indelible dry ink, as liquid ink is troublesome and liable to blot from rain; alterations in figures should never be made, corrections should be effected by obliterating or entirely striking out or through the error or erroneous portion, and rewriting it clearly. The accompanying field-sketch, Plate II., together with Field-record No. 1, illustrate the system now in vogue in surveying estates when the chain alone is used. Chain surveys for engineering purposes are conducted much in the same way, and hence do not require any special illustration or notice. The next reduced plan, Plate III., together with Fieldrecord No. II., illustrates a small portion of a town survey, comprising one street only, surveyed for plotting on the scale ofth or 10 feet to the inch. The parallelograms that form the network of this survey are double, each street having two lines of chain measurement. The offsets in this case are rectangular, and were taken with much care, but under such circumstances, wherever accuracy is very necessary, additional oblique offsets are imperative. For surveys of the same kind |