From 1853 the Dominion of New Zealand was subdivided into six Provinces - Auckland, New Plymouth (renamed Taranaki in 1859), Wellington, Nelson, Canterbury and Otago - and each carried out their own surveys under their own provincial government's lands and surveying department. At that time there was no standardisation of measuring equipment, so there was inconsistency between survey circuits.
Chief Surveyor of Taranaki Province, T. Humphries (think Humphries Castle rock outcrop above North Egmont on Mounga Taranaki), was responsible for the initial survey of Taranaki from 1876 using the method of triangulation which he perfected. Elsewhere across the country by the early 20th Century problems were becoming apparent due to a lack of a primary national triangulation (PNT) as any survey that crossed circuit boundaries made it difficult to provide accurate maps on a national scale.
In 1909 a primary triangulation began with the establishment of four baselines in the North Island. A baseline is a fundamental requirement of the triangulation network.
The first was in the Wairarapa established in 1909
This geocache recognises the Eltham Okaiawa baseline, established in 1910
The Waitemata and Matamata baselines were established in 1911.
The Eltham - Okaiawa Baseline showing intermediary benchmarks
From the four baselines the rest of the North Island was surveyed in large triangles.
The Eltham base
This base was first suggested by H. M. Skeet in 1897 when he was carrying out the triangulation of the Taranaki district. Using existing stations as the terminals - ELTHAM 1253 at the north end and B 1243, since renamed KARIMOI at the south Okaiawa end - the base follows a straight line for 10 miles sloping generally downwards.
In actuality the country is exceptionally rough for base measurement, with many streams and gullies on the line. In measuring across gullies, additional height had to be provided for support-stands, some up to 16 feet above ground level. In addition to the natural roughness of the line, artificial obstacles such as barbed wire fences and large boxthorn hedges were frequent with 120 fences of all descriptions having to be crossed in the course of the measurement. During the preparatory work, stiles were built over 40 hedges with ditches and banks, and 400 yards of plank footways were required to cross streams, ponds, and swamps.
Measurement of the base took 46 days between April and June 1910.
The actual length of the Eltham base is 79 604.347 links ∓0.0128 (that's 2.5mm error over 16,013 metres)
The triangulation of the country from these baselines involved triangles of up to 20 miles in length. Such was the care and accuracy of the measures that the closure distance of the 18 triangles off the Eltham line averaged 0.802 of an inch with a mean error of an angle being 0.569 of a second (of a minute of a degree). This equates to an error of 1/90 000. Impressive!
The intermediary benchmarks
The benchmarks on the local roads have mostly disappeared or become buried over the intervening years;
ELTHAM 1253 established before 1900, by an unknown surveyor, but probably H. M. Skeet is on private farm land on a prominence known locally as Birk's Hill 1.5 miles west of Eltham. Marked by a trig beacon.
B KARIMOI 1243 established 1879 by H. W. Climie is also on private farm land near the intersection of Ahipaipa Road and Scott Road. Also marked by a trig beacon.
BLK VIII ELTHAM BASE AN8B established 1910 by T. G. Sole as part of the Eltham baseline survey, is on the north side of Skeet Road, 130 yards west of the intersection with Umutahi Road.
This is the location of the Ammo can geocache.
Articles researched in writing and placing this cache
Lawn, C. A., 'The Pioneer Land Surveyors of New Zealand, Part II 1854 – 1876, Chapter 14 Taranaki Provincial Surveys', updated 2005 Derek Brown, M.N.Z.I.S. Retired. (Retrieved August 2017)
Lawn, C. A., 'The Pioneer Land Surveyors of New Zealand, Part IV Biographical Notes', updated 2005 Derek Brown, M.N.Z.I.S. Retired. (Retrieved August 2017)
Lee, L.P., 'First Order Triangulation of New Zealand 1909-49 1973-74'. Department Lands & Survey, 1978. (Retrieved August 2017)
Land Information New Zealand, 'LINZ Geodetic database'. (Retrieved August 2017)
Lovell-Smith, Melanie . 'Modern mapping and surveying', Te Ara - the Encyclopedia of New Zealand, (Retrieved August 2017)
Mackenzie, James. Conference of the Director of Commonwealth Lands and Surveys, the Surveyor General and the Government Astronomer of New Zealand, and the Surveyors General of the States of the Commonwealth of Australia, Melbourne, 20 – 26 May, 1912, Report upon Trigonometrical Survey of the Dominion of New Zealand, Appendix H, (Retrieved August 2017)