Page 2 Advertisements Column 2

Wellington Independent, Volume XXIV, Issue 2806, 6 March 1869, Page 2

Order of procedure for Surveys. 8. To ensure an harmonious conformation of the various surveys, after a previous reconnaissance, the order of procedure should be — First, Major Triangulation; Secondly, Minor Triangulation; and Thirdly, the traverses of the boundaries of blocks, their rivers, streams, road lines, and sectional subdivisions. Method to be pursued when the order of procedure cannot be adhered to. 9. But owing to circumstances over which there is no control, the above order cannot always be adhered to, and must therefore be modified in such a manner as to render the surveys easily susceptible of incorporation with the process immediately preceding. Thus a block of un-surveyed land of moderate extent may be commenced by erecting one or more principal stations at distances averaging 8 to 10 miles apart so as to embrace the whole block and to become eventually points for the Major Series. The Minor Triangulation may then be proceeded with. Or small isolated blocks and sections may be at once submitted to the ordinary process of a chain survey after erecting minor trigonometrical stations to be hereafter fixed at intervals of from 2½ to 5 miles apart, to which the traverses can be referred. New Surveys adjacent to Trigonometrical points to take initial elements from them. 10. But in new surveys it will now be generally found that the localities lie adjacent to and within moderate distances from some fixed trigonometrical points affording the initial elements for commencing the new survey, viz. : — a point of departure, the direction of the meridian, and a base of ascertained length for the extension of the triangulation. In such cases advantage is to be taken of all or any of the above data to prevent further surveys partaking of the character of detached operations. MAJOR TRIANGULATION. Description of instrument to be used for Major Triangulation. 11. For Major Triangulation the instrument used should be an eight inch Transit Theodolite somewhat similar in construction to the altitude and azimuth instrument. Base Line. 12. The base line from which the sides of the triangles in extension derive their measures ought to be selected on an even plain convenient for chaining and of not less than 2 to 2½ miles in length. As the accuracy of the succeeding work depends upon this initial measure, especial care and attention is directed to the line being quite straight and clear of impediments in the lay of the chain, to the expansion or contraction of the chain due to temperature, and to its stretch during measurements. Standard length of chain, and its adjustment to 62 ° Fah. 13. Previous to commencing the measurements a standard length of a chain should be laid down by marks between two blocks firmly let into the ground at a distance equal to the length of the standard chain when adjusted to a temperature of 62 ° Fahrenheit (the standard of British imperial linear measures), making an allowance for expansion or contraction of 0.0000065 being that to be adopted of iron on unity for every degree of Fahrenheit above or below 62°. Method to be observed for the measurement of the Base Line. 14. The measurement may be made with the ordinary steel chain, which, when once adjusted, or nearly so, to the standard length laid down as above directed, should not be re-adjusted until the whole of the measurements are completed. The chain is to be tested with this standard length before and after each measurement (noting temperature) and its stretch carefully recorded. During measurements it should have a tension equal to 20lbs, as indicated by a spring balance, and its extremes are to be marked upon boards fitted for the purpose with pieces of lead let into the upper sides, and with spikes underneath to fasten them firmly in the ground. Variations of temperature during measurements should be observed. Amount of error to be admissible in the measurement of the Base Lines. 15. The base line should be measured at least three times and if well executed the amount of error need not exceed three inches for every hundred chains of its length. Appendix A furnishes examples illustrating the measurements of the base lines on the Wharekaka and Opaki plains. Note — If the base line lies elevated above mean sea level a reduction is necessary. Log. reduction in feet = Log. length of base in feet + Log. height above sea level + Const. Log. 2.68009. Selection of Stations. 16. The stations are to be selected upon the highest and most commanding hills whenever practicable, but if by reason of bush or other impediments this is not so, then care must be taken that their positions secure an unobstructed view of the stations in advance. The network of triangles ought to be so arranged as to form a succession of polygonal figures, hexagonal being the most convenient, and as nearly equilateral as circumstances will permit. This however being difficult in practice, the rule to be observed is that no triangle for extension should contain angles less than 30 ° or more than 90 ° . Efforts should be made to obtain the bearings of