224
THE NEW ZEALAND GAZETTE.
No. 9

true length at 62° Fahr. The steel band when in use is to be held with a tension of 14lb., and the ends marked on flat boards spiked into the ground. These flat boards should have a hollow filled with lead, for receiving the end-marks of the band, to be made by a sharp instrument. Three boards must be used, the last being always carried forward. The base should be measured thus at least three times, and the mean of the measurements taken. Angles of inclination must be observed, so that a vertical section of the line can be made for reduction to true level. Bases of verification are to be measured in the same way. When for any reason it is inconvenient to prepare the ground, a base line may be measured 2ft. or 3ft. above the surface, the steel band being supported on adjustable stands, and, with suitable precautions, may be measured on an incline. The final length of baseline must be reduced to sea-level.

MINOR TRIANGULATION.

4. The surveyor must be provided with a 5in. theodolite, standard steel band, thermometer, prismatic compass, aneroid, and straining apparatus.

5. Should a major triangulation cover the area to be surveyed no measured base will be necessary, for the distances of minor trigonometrical stations will be obtained by breaking down the larger triangles, always on the Ray Trace system if practical.

6. Trigonometrical stations for minor work should be, as nearly as practicable, 2½ miles apart. To extend the meridian of the circuit from the geographical into the settlement survey, one of the geodesical or major trigonometrical stations is to be chosen as origin, the bearing being deduced from those given on the standard maps. If a theodolite, with three verniers, is used, at least three sets of observations are to be taken to each minor trigonometrical station in the series; the circumference of the instrument is to be divided into at least three arcs, using each one successively as an origin—thus nine readings will be observed; but if a theodolite with two verniers is used, at least four sets of observations are to be made, the horizontal arc being divided by four, and each used as an origin respectively—thus eight readings will be observed on different parts of the limb. The number of readings here set forth should never be less, but it is optional with the surveyor to take more. In each set, the instrument must be turned in one direction until the origin station is again bisected, and the return reading of the vernier is to be entered in the field-book. The angles at each trigonometrical station are to be observed in like manner, so as to complete the three angles of each triangle, and the defect, ±180°, is to be applied to correct the summation. Points are to be selected so as to have well-conditioned triangles—no angle being less than 30° nor greater than 120°, unless under very exceptional circumstances. As far as practicable, crossing triangles, or one bearing over another bearing, are to be avoided; each triangle should appear on the maps distinct from others, and it is advantageous to carry the triangles forward in a polygonal series, by which a continuous check is obtained; care must be taken to close on to adjacent triangulations wherever practicable. Vertical angles are to be observed between stations with similar care, the datum heights for computation being taken from the standard maps.

7. The logarithms used in computations should be taken out to seven places, and the angles to seconds. After the sides and angles are known, the position of all stations are to be calculated on the meridian and perpendicular of the initial station of the circuit, or of the survey district, as may be decided by the Chief Surveyor of the district, and a table of the same prepared. Only in cases where it is impossible to adopt the circuit origin is the district origin to be used. From this table the skeleton maps must be constructed by standard scale. The difference of height between two trigonometrical stations is to be obtained from the vertical angles taken at both stations.

8. In executing the survey of an isolated section or of a block, if a base has to be measured, minor triangulation is to be carried from it to the land to be surveyed; but, if the work is to be based on major triangulation already executed, triangles are to be carried thence in the most direct course to such survey, and no more work is to be executed than is necessary for checking the linear measurements.

9. With average care the degree of error in minor triangulation need not exceed 2 links to the mile, and this is the extreme error allowable; the error in the summation of angles of a triangle must not exceed 30". All work having error in excess of this will require revisal.

10. Minor trigonometrical stations are to be constructed in the following manner: Gas-pipes, 2in. internal diameter, cut to 2½ft. lengths, are to be inserted into cast-iron plates with sockets, secured by an iron pin. The alphabetical letter or number of the station is to be cut on the upper end of the pipe with a cold-chisel. The pipe thus constructed is to be sunk in the hole prepared for it to a depth of 2ft. 3in., with the metal plate downwards. The hole must be then refilled, and the loose soil firmly beaten down. Round this a circular ditch, 20ft. diameter, 1ft. deep, and 18in. wide, should be dug. On high rocky peaks where a ditch cannot be dug, a circle of stones should be made. When in use the trigonometrical tube should have a pole carrying a black-and-white flag, or a ball, bunch of brushwood, or other suitable signal inserted into it, and the pole must be properly stayed; or a light wooden pyramid may be erected over it, with calico or boards tightly tacked or battened to the sides all round for about 3ft. from the top; signals may be varied in accordance with special instructions. It is not desirable to build trigonometrical mounds, but in low positions these may be necessary; the surveyor will in such cases exercise his own judgment. If mounds be built, the exterior rim should be of stone or sods, with earth in the centre. In positions where the nature of the soil may require modifications special directions will be given.

11. The trigonometrical work only is to be mapped on one sheet, which should show trigonometrical stations (two concentric pink circles) with their alphabetical letters or numbers and local names, which should be the original Native ones where obtainable, the base line in red, other lines in black, bearings observed from each station (in blue), calculated mean distances (black), the observed angles (in the middle of each triangle) summed up (black). A few of the streams should be shown, so as to localise the trigonometrical stations readily. There should also be a note giving the results of the different measurements of the base line, which should also be given in detail in a special report. Scale, 40 chains to an inch.

TOPOGRAPHICAL SURVEY.

12. Combined with the trigonometrical operations, a topographical survey is to be made showing