55

plain convenient for chaining end 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 con-
traction of the chain due to temperature,
and to its stretch during measurements.

Standard length of chain, and its adjustment to 62° Fahr.

13. Previous to commencing the measure-
    ments 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 impe-
    rial 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 measurements 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 re-
    corded. 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. Va-
    riations of temperature during measure-
    ments 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 neces-
sary. 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, hexa-
    gonal 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
    both diagonals when the triangles arrange
    themselves in quadrilaterals, as is some-
    times the case where the breadth of open
    country is limited.

Stations to be marked with boxes or blocks of totara, and with an iron peg, and to have piles or mounds raised round them.

17. An excellent and also an economical
    method of marking the centre of the sta-
    tions is with boxes made of the heart of
    totara or other durable wood, into which
    the pole of the signal flag is inserted.
    These should be let into the ground pre-
    vious to commencing the observations for
    bearings. In distant parts of the country
    it will be sufficient to place a sawn totara
    block 2 feet long and 6 inches square. An
    iron peg of the usual Government pattern
    is also to be placed at the station in case
    of accident from fire. To render the sta-
    tions more easily distinguishable a pile of
    stones or a mound of turf about five feet in
    diameter at the base, and five or six feet in
    height is raised round the station.

• Top left: 2' 6" x 3 1/2"
• Top right: 6"
• Bottom left:
  • Black: 8"
  • White: 6"
• Bottom right: Diagram of station setup
  • δω
  • 1' 6"
  • 5' 0"