Surveying technical The South African measurement system and its origin by Tomasz Zakiewicz, Chief Directorate: National Geo-spatial Information

The history of the South African measurement system is, without doubt, somewhat confusing, as is the relationship between old South African units of length, English measure, and the legal and international metre. The English and metric systems have influenced the establishment of the “commercial” Cape units, and, also, of the unit of the Geodetic Survey of South Africa.

he earliest units of land reverse, the Sears-Johnson-Jolly / measurement in South Africa metre relationship of 1928 Twere derived from the units (1 Eng. ft = 0,304 799 471 8 that were used by the Dutch pioneers Int. metres) was adopted by the in the 17th century. The “Rhynland Trigonmetrical Survey Office (now ”, then in use in Holland, Chief Directorate: National Geo-spatial comprising of twelve “Rhynland feet”, Information, CD: NGI). Farm surveys, remained the official unit of length however, were connected to the S.A. in the Cape for two centuries. During Gauss Conform (Lo) System using the this period, because there was no “South African Geodetic Foot” (S.A.G. accurate standard of reference at foot). the Cape, measuring rods and tapes The “South African Geodetic foot” was in use became unconnected to the the fundamental unit of the survey “Rhynland rood”. This situation led to of the national triangulation network, significant discrepancies in surveys. At known as the Geodetic Survey of the beginning of the 19th century, the South Africa (1879-1907). This problem was brought to the attention survey was initiated, and supervised, of the authorities and, in 1858, a Sir David Gill (1843-1914), her Majesty's by Sir David Gill (1843-1914), her commission was appointed “for the Astronomer at the Royal Observatory at the Majesty's Astronomer at the Royal purpose of ascertaining and fixing the Cape of Good Hope between 1879 and 1907. Observatory at the Cape of Good unit of land measure in the Colony Hope, between 1879 and 1907. When of the Cape of Good Hope”. The the survey commenced in 1883, the commission, under the chairmanship and the “English foot”. In 1922 it was Cape Geodetic Standard consisted of of Sir Thomas Maclear, confirmed slightly modernised, i.e., referred to the “Cape Standard Bar A”. It was one what Hertzog, then the Deputy the “international metre” using the of two identical ten-foot iron bars, Surveyor-General, had suspected in Benoit's 1896 relation of 1 Eng. ft. = brought from England in 1839, and 1844, noting that, although the “true” 0,304 799 734 8 Int. metres [5]. used by Sir Thomas Maclear for the Rhynland foot was equal to 1,030 verification of the measurement of English feet, this ratio looked different The “Weights and Measures Act Lacaille's Arc of 1752. The “Standard in the Cape, where 1 Rhynland (Cape) No. 32 of 1922 (as amended by Acts: Bar B” was returned to England foot = 1,033 English feet. No. 13 of 1933, No. 13 of 1958, in 1842 after measurement of the No. 44 of 1964, No. 55 of 1969, Zwartland baseline, between 1840 In 1859, the recommendations of the No. 76 of 1973, No. 24 of 1998, and and 1841, while Maclear's Arc, which commission were lawfully declared its newest version: No. 18 of 2006) extended from Cape Agulhas via Cape by the Act No 9, which established provided the ratio between the “Cape Point to Springbok, was completed the “Cape foot” (1 Cape rood = 12 foot” and the “international metre” only in 1848 [5]. Cape feet) – a unique and truly South (see Table 1), which was used for African entity, used in the whole converting fairly short distances (up The “Cape Bar A” was standardised country, except KwaZulu Natal, where to approximately 6 km). Moreover, by Benoit in 1886, in Paris, at Gill's the English foot dominated. Similarly, in surveys on a local system, where request. As a consequence, all a “Cape Morgen”, equal to 600 length determination and co-ordinates geodetic baselines were expressed in square Cape , was created as were adjusted to a base measured terms of the “international metre”. an equivalent unit of area. The ratio in “English feet”, the Benoit's In order to reduce his results, and of 1 Cape foot = 1,033 English feet relation was employed. Since precise define them in , Gill used was binding on all land surveys, and, leveling started in South Africa in Clarke's 1866 ratio, the only foot/ for a period of over 60 years, was the the late 1920s, for the conversion of metre ratio available at that time legal relation between the “Cape foot” elevations from metres to feet, and (1 Eng. ft = 0,304 797 265 4 Legal

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Unit (length) Equivalent national survey system, to use the specified factors, which were then Foot 0,304 8 m also provided by the Act No. 76 of Foot (Cape) 0,314 858 1 m 1973 [2]. The 5th of July 1974, when Foot (Geoetic Cape) 0,314 855 575 16 m this act came into operation, is the Foot (South African Geodetic) 0,304 797 265 4 m date of the official introduction of the International Metric System (SI) in 0,025 4 m South Africa. 1 609,344 m Nautical mile (International) 1 852 m There are many versions of linear and area conversion factors, of which Rood (Cape) 3,778 297 2 m some are incorrect. The CD:NGI has Rood (Geodetic Cape) 3,778 266 9 m done thorough research into this 0,914 4 m matter and has compiled a list of Unit (area) Equivalent conversion factors, for length and 4 046,86 m2 areas, which are relevant in South Are 100 m2 Africa today. Hectare 10 000 m2 The list is based on the legislation Morgen 8 565,32 m2 legally binding in South Africa [4] Square foot 0,092 903 04 m2 (see Table 1). Thus, the conversion Square inch 0,000 645 16 m2 factors have the force of law. Square mile 2 589 988 m2 Some of these units of length might Square yard 0,836 127 36 m2 be applied in engineering metrology only, and not necessary in land Table 1: List of conversion factors, for length and areas, which are relevant in South Africa today. records. The “are” is not recognised in South Africa in connection with surveying and mapping. There are metre). Due to the implications with and the initial point of the datum, also other units, still to be found the “legal” and “international metres”, Buffelsfontein (near Port Elizabeth), on old “Natal” survey diagrams and it appeared that the results of the defined the Cape Datum, which plans (i.e., chains – used for length; geodetic triangulation were computed formed the basis of the South African perches and roods – used for areas), not in “English feet” but in terms of geodetic system, and which served but they do not have the same legal the untrue unit, now called the “South well for a century. The Cape Datum status as those listed in the table. African Geodetic foot”. This caused was replaced, on 1 January 1999, Table 1 contains equivalents of units, the reference ellipsoid to be renamed by the Hartebeesthoek94 Datum, expressed in terms of a metre (m), the “Modified Clarke 1880 ellipsoid”. using the WGS84 reference ellipsoid which is the only length unit of the The parameters of this ellipsoid and the ITRF91 (epoch 1994.0) SI and one of seven “base units”. The co-ordinates of the Hartebeesthoek others are: kilogram (kg), second (s), Radio Astronomy Telescope as the ampere (A), kelvin (K), mole (mol) datum point. and candela (cd). The “square metre” When the Gauss Conform projection (m2) is the “derived unit” of an area, was launched in South Africa in of the SI. The “millimetre” (mm) and 1921, another two fictitious units the “kilometre” (km) are respectively were introduced: the “Geodetic Cape a submultiple and a multiple of the SI foot” and the “Geodetic Cape rood”. unit of length, but are not SI units. Thus, up to 1970, all the rectangular The “hectare” (ha) and “nautical mile trigonometrical co-ordinates in South (international)” are classified as units Africa were given in: which may only be used with the SI for a limited time. The “micron”  The South African Geodetic foot (μ) may not be used at all [3].  The Geodetic Cape foot (not the Furthermore, the Act No. 76 of 1973 same as the Cape foot) specified that other factors might too  The Geodetic Cape rood (not the have a legal standing. Any equivalent, same as the Cape rood) which can be derived from the “main” It was lawfully established that, from equivalents “is also an equivalent 1 January 1970, all units of land of the units concerned”. Moreover, measure in South Africa were to be as stated in the Act No. 18 of 2006, based on the “international metre”. “all regulations made in terms of From that date, an amendment in the Measuring Units and National the Land Survey Act No. 9 of 1927 Measuring Standards Act, 1973 (Act (repealed by Act No. 8 of 1997) No. 76 of 1973), must be regarded made it compulsory, for the purpose as having been made in terms of of converting co-ordinates on the this Act”. This means that, all the

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conversion factors provided by the Act registration purposes, by the then References Director-General of Surveys, who No. 76 of 1973 are still valid. [1] DR Hendrikz: South African Units judged, in his Circular No. 2 of 1971: It should be stressed that, the terms of Length and Area, Department “it has been decided to abandon the "British foot" ("British Imperial foot") of Lands, Trigonometrical Survey, writing of areas, such as 45,236 1 ha Special Publication No. 2, 1944. or "English foot" appear in literature with a space after the 3rd decimal figure and are deemed to mean the same. [2] Measuring Units and National and to advocate that as in the past the The “South African Geodetic foot” was Measuring Standards Act, 1973 (Act four figures be grouped together viz. commonly known as “English foot”. No. 76 of 1973), Government Gazette 45,2361 ha”. Here, a space was left No. 4326, 5 July 1974. Hendrikz wrote [1]: “For all ordinary between the number and the symbol topographical and cadastral purposes [3] The International Metric System (SI), of a unit, as required [2]. These the S.A.G. foot may be mistaken for Guide to the use of the SI in South rules were highlighted in an old Land Africa, The Council of the South the British foot”, or the English foot, Survey Act No. 9 of 1927 African Bureau of Standards, M 33a, as just explained. (Regulation 24.1), but an amended 1992 Finally, it is worth noting that, in Land Survey Act No. 8 of 1997 does [4] Measurement Units and Measurement Table 1, the grouping of thousands not contain such information. Standards Act, 2006 (Act No. 18 of 2006), Government Gazette No. (in threes) and the use of the English-speaking countries (plus 29752, 28 March 2007. decimal comma, as opposed to the China, India and Japan) use the decimal point, was effected, to be decimal point. There seems to be [5] T Zakiewicz: “Units of Length Measure in accordance with the applicable a general tendency to formally & Geodetic Standards at the Cape, legislation of South Africa: “where the declare its use worldwide and 1813-1912”, History of Surveying and Land Tenure, Collected Papers, magnitude of a quantity is expressed the International Organisation for Vol. 2, The Institute of Professional in terms of a unit, a comma on the Standardization, ISO, also tends Land Surveyors & Geomaticians of the line is used as the decimal sign in the towards using the decimal point Western Cape, May 2004. numerical part of the expression and over the decimal comma. In view the digits are separated into groups of the South African law, however, Contact Tomasz Zakiewicz, of three digits on either side of the the comma shall still be employed in National Geo-Spatial Information, comma by means of spaces…” [2]. South Africa as “the only recognised Tel 021 658-4335, The exception was made, for land decimal indicator for all numbers” [3]. [email protected]

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