598

I have received from Mr Baker on the triangulation executed by him.

I have the honour to be,

Sir,
Your most obedient servant,

THOS. IEZZE,
Chief Surveyor.

His Honor the Superintendent,
Southland.

Survey Office,
Invercargill, July 16, 1864.

Sir,—I have the honour to send you the following report of the Trigonometrical Survey of Southland, on which I have been engaged during the summer of 1864.

Having to finish the survey of the New River Harbour, it was late in the season before I could take the field, and it was not until the middle of December that I commenced the trigonometrical survey of the Aparima Hundreds.

After carefully inspecting the ground, I selected the Otantau Flat for the base line, and after the line had been properly cleared and ranged, I measured it three times with one of Troughton and Simms’ steel standard chains which had been compared with the standard laid down by you at the Survey Office in Invercargill. I carefully registered the readings of the thermometer at every fifty chains. The respective chainages and mean readings of thermometer were as follows.

1st—1794.98 mean reading thermometer........................ 98.50
2nd—17945.51 mean reading thermometer........................ 86.10
3rd—17946.12 mean reading thermometer........................ 56.36

The differences of chainages would therefore be between the—

1st and 2nd— .53 of a link.
2nd and 3rd— .61 do do
3rd and 1st—1.14 do do

This is considerably under the maximum error allowed in measurements with a standard chain, and when the difference of temperature is taken into consideration the results of the three measurements are most satisfactory, and the mean of the three may be considered as absolutely correct for a minor triangulation.

The base line was most carefully levelled by Mr Butler, (sub-assistant surveyor.) The total rise from A. to B. is 63.09 feet. If this had been a gradual rise, the horizontal value of the measurement of the base line would have been unaffected, but as the base line was intersected by several small gullies, I calculated the horizontal value of every chain, where the rise or fall had been over .50 of a foot to the chain; the deduction for anything under that would be less than .0028 of a link to the chain, which is quite unappreciable in measurements with a standard chain.

After I had selected and built the stations on the west side of Aparima, I proceeded to the Bluff to carry on the meridian as laid down by the chief surveyor of Otago in 1857. I was unavoidably delayed by bad weather for three weeks, but succeeded in getting a very complete set of bearings to a signal fire on Observation Hill, the highest peak of the Wairio Downs. I also got an equally good set of bearings from Observation Hill to the Bluff, using a heliotrope as my object of observation instead of a signal fire. The superiority of a heliotrope as a distant signal for observation, I brought under your notice in a former report.

The length of the sides of the triangles in the Aparima Hundreds average about 2½ miles, and very few of the angles are under 50° degrees excepting where I had to close on small sides of triangles, in the Jacob’s River and Waimatuku triangulations. The meridian of my survey closes within one minute (1.00’) with the Jacob’s River triangulation, which was executed by Mr. Hately under the Otago staff.

The difference in length of the side LK of the triangle LKD in the Jacob’s River triangulation, is 15 links in 18890 is no doubt to be attributed to the fact that the Otago survey staff had no standard laid down in Southland, to compare the different chains used in the detached trigonometrical surveys.

The length of the sides of the triangles on the Oreti Plain to the east side of the Aparima, average rather over three miles, and few of the angles are under 60°. I have had great difficulty in carrying the triangulation across the Oreti River, having to cut several lines through the scrub, on the river, 20ft in width, varying from ten to twenty chains in length, the plain on either side being too low to be able to see over the scrub.

The difference of the meridian in the closure with the Winton triangulation is 3’ 30’’, this meridian having been carried on from the New River triangulation.

The meridian of the block surveys of the Winton District, has been carried on