Survey are performed by salaried officials or by those who are "authorised" on either Crown lands or sold lands which will be brought under the operation of the Transfer of Lands Statute. Plans of surveys made by "unauthorised " persons are not officially recognised. All "authorised" surveyors must comply with the regulations issued as departmental instructions or bearing the force of an "Order in Council." Particulars of the examinations for selecting "authorised" or certificated surveyors are given at the end of this chapter.

Each colony is divided into survey districts, each of which is under the control of a salaried inspecting officer. These again are subdivided into divisions, and each is assigned to an "authorised" surveyor, to whom orders for departmental work are sent. The "authorised" surveyors are not salaried, but are remunerated by fees fixed by regulation, and they are removable at the pleasure of the Surveyor-General. When a vacancy occurs selection is made from those holding a certificate of competency obtained by examination. The inspecting officer of each district makes periodical inspection of "authorised" surveyors' work in the field, their instruments, field books, &c., and certifies to all accounts. Inspecting surveyors are under immediate control of the Surveyor-General.

The higher classes of surveying, including minor triangulation, are performed exclusively by staff officers; the ordinary sectional or block surveying and laying out of towns and roads is performed by "authorised" surveyors. The system of examination and supervision is so complete that a very high standard of efficiency has now been attained.

SUBJECTS OF EXAMINATION OF "AUTHORISED " SURVEYORS TO LANDS DEPARTMENTS, COLONY OF VICTORIA, AUSTRALIA.

a. Construction, adjustment, and use of theodolite, level, and other modern instruments.

b. Principles and practice of subdivisional, topographical, and road surveying.

c. Practical trigonometry.

d. Computation.

e. Plotting by co-ordinates and otherwise.

f. Drawing.

Additional special subjects are:

a. Practical geodesy, including determination of latitude, longitude, and meridian, and computation of relative latitude and longitude from triangulation.

b. Spherical trigonometry.

c. Setting out curves and computations in connection therewith. d. Levelling and mensuration of earthwork.

These examinations generally extend over five to seven days of eight hours each. About 30 per cent. of the candidates pass. The examinations in the other colonies are practically the same. Certificates are granted in some instances without examination, the fact of their being so obtained being stated on them. In the colony of Victoria certificates are granted without examination only under the following conditions:

1. Having passed some examination equivalent in the opinion of the Board to that prescribed in Victoria, in Great Britain, United States of America, India, or some British colony, and having been in practice for not less than one year.

2. Having been engaged in the colony under some authorised surveyor for not less than six months, and having a favourable report on his qualifications from the Surveyor-General or the inspecting surveyor in charge of the district in which the applicant has been employed.

CHAPTER XII.

TRIGONOMETRICAL OR GEODETIC * SURVEYS.

Ordinary Small and Large Surveys.-A small survey may be defined as one in which the length of the main chain lines does not as a rule much exceed a mile. An ordinary large survey may be defined as one of greater extent or area, but in which the length of the main lines is still limited to about a mile. In these cases the lengths of the lines are actually measured and the stations are easily visible from each other, so that no special signals or signalling apparatus are required. This class of survey is dealt with in Chapters I. and II. et sequitur.

Trigonometrical Survey.-A trigonometrical survey is one undertaken for the mapping of a large extent of country, and in which the length of the sides of the main triangles is great, the maximum being about 100 miles, while the average will be about 40 to 60 miles.

Base Line and Primary Triangles.-The whole of a trigonometrical survey is based upon the accurate measurement of a "base line."

The base line is laid out in the most favourable situation, accurately measured, and from its extremities angles are observed to surrounding stations, which stations form with the base line triangles. These triangles are gradually and systematically extended in size as hereafter described (page 456), until they reach approximately the size of the largest triangles to be measured in the survey. The sides of these triangles are computed from the actually measured length of the base and the observed angles, and they then serve as bases upon which to form other triangles of like extent whose angles only are measured, the sides being com

* Strictly speaking the term geodetic applies only to measurements undertaken to determine the figure of the earth. It is, however, commonly applied to trigonometrical surveys.

puted from the angles and the previously calculated sides. In this manner the whole country to be surveyed is covered with a net work of "primary" triangles averaging 40 to 60 miles in the side, the whole being computed from the one accurately measured base and the observed angles.

All the angles of each triangle are measured so that the errors of measurement of the angles may be ascertained and corrected.

In laying out and deciding upon the stations of these "primary triangles" any available maps or sketches should be utilised, and these stations should be determined on by a careful reconnaissance of the country and examination of each proposed station.

Base of Verification.-During the progress of the "primary triangulation" a "base of verification" should be measured from time to time. Its length as calculated from the original base and the angles of the intervening primary triangles is compared with its length by actual measurement, and this gives a check upon the measurement of the angles of the primary triangulation. A similar "base of verification" is also to be measured at the conclusion of the primary triangulation.

Secondary Triangles. Within the "primary triangles" are formed "secondary triangles." As many of these as possible should be fixed upon and their angles observed with the large theodolite during the progress of the primary triangulation. The remainder may afterwards be observed with a smaller instrument. The sides of these "secondary triangles " will average 10 to 12 miles in length, and are computed from the observed angles.

Tertiary Triangles.—Within these secondary triangles "tertiary triangles" are formed, averaging in open country 3 to 4 miles in the side, and in enclosed country perhaps 1 or 2 miles in the side. The sides of these triangles also are computed from the observed angles.

Interior Filling-in.-When the above skeleton triangulation of the country has been completed, the interior filling-in, survey of roads, streams, villages, towns, &c. &c., is performed by traversing with the theodolite and chain, compass, plane table, and ordinary chain surveying, as described in Chapters I. and II. et sequitur, this work being made to "check in" between the stations of the "tertiary triangles" within certain stipulated limits of accuracy.

Triangulation Stations. All prominent points, such as churches, mills, &c. &c., may of course be taken advantage of as stations for the tertiary triangulation, and also for the primary and secondary triangulation when suitable. In the absence of such existing marks the stations must be fixed by suitable marks as hereafter described. In the case of churches, &c., being selected as stations the instrument cannot as a rule be set up over the centre of the station, in which case it is set up as near it as possible, and the observed angles taken with the instrument at that station are "reduced to the centre of the station" as hereafter described.

Whole Survey Reduced to Sea Level. The measurement of the base is reduced to mean sea level or other fixed datum and to a standard temperature. All the computed lengths of the sides of the triangles are therefore also as reduced to mean sea level and to the standard temperature. The consequence is that the result of the survey is a representation of the country as projected on to the surface of a sphere whose radius is that of the earth at mean sea level, the whole being as at the standard temperature adopted. Apart from considerations of temperature, a line 4 miles long, at a height of 1 mile above mean sea level, actually measured on the ground, would thus differ from its length as represented on the map by about 6 ft. On a scale of 500 or 25 in. to the mile it would be scarcely possible to scale a distance of 4 miles to 6 ft. Upon reducing the measured length to mean sea level it should of course agree exactly with the length as represented on the map. The above is merely cited to show that for all practical purposes actual measured distances will agree with their scaled lengths on the map.

BASE LINES OF THE ORDNANCE SURVEY OF GREAT BRITAIN

AND IRELAND.

Hounslow Heath Base.---The first base measured was the Hounslow Heath Base, about 5.20 miles long. The length of this base as measured with glass rods was determined in 1784 to be 27404.0137 ft. reduced to mean sea level and to a temperature of 62 F., the standard temperature of British imperial linear measures. The height of the base above mean sea level was taken as 54 ft. This base was remeasured in 1791 with a steel