The Surveyor-General has ruled that in future the “blue ...



INSTRUCTIONS FOR THE CONVERSION OF AREAS TO METRIC

The Surveyor-General has ruled that in future the “Blue Book” need no longer be used for the conversion of areas from imperial units to metric, particularly when preparing consolidated diagrams by compilation. This is good news as it has long been recognized that the results from the blue book are incorrect in that the conversions are merely truncated and not rounded off. This gives rise to differences of 1 square metre, particularly when the original units include roods and perches.

This ruling should simplify the conversion process particularly for those practitioners who do not have a Blue Book.

There are, however, two points about which the practitioner needs to be aware.

1) Regulation 1 prescribes the factors to be used for the conversion of distances and of areas from English measure and Cape measure. The distance and area factors do not correlate, and it is not clear why this was done when the regulations were drawn up. If they correlated,

1 Acre = 0,304 797 2654 X 0,304 797 2654 X 43 560 = 0,404 678 3808 Hectares

Instead, 1 Acre = 0,3048 X 0,3048 X 43560 = 0,404 686 Hectares

2) It is intended to investigate the possibility of amending the regulation in order to correct the deviation, but in the meantime the area factors must be used for the conversion of areas. I.e.:

1 Morgen = 0,856 532 Hectares

1 Acre = 0,404 686 Hectares

Using the prescribed area factor means that, for approximately each 13 Acres (7 Morgen in the case of Cape measure), the resulting conversion is 1 square metre larger than would be obtained using the corresponding distance factor.

3) In order that a consistent result is obtained, the following procedure should be used in dong the conversion whether it be by computer programme, calculator, Facit, or long multiplication:

• Convert the imperial units to Acres (or Morgen)

• Multiply by the relevant factor from the regulations

• Round off the result to the nearest square metres

This is the method which will be used in the S.G.O. to check the results.

The following examples are given to illustrate the conversion of areas as they commonly appear on pre-decimal diagrams:

• 5560 square feet = 5560/43560 = 0,127 64 Acres X 0,404 686 = 0,051 654 Hectares = 517 square metres.

• 5,4339 Acres X 0,404 686 = 2,199 023 Hectares = 2,1990 Hectares

• 1 Acre 36 489 square feet = 1 + 34 689/43 560 = 1,837 6722 Acres X 0,404 686

= 0,743 680 Hectares = 7 437 square metres

• 2 Acres 3 Roods 36,57 Perches (sometimes called Poles) = 2 +3/4 +36,57/160

= 2,978 5625 Acres X 0,404 686 = 1,205 383 Hectares = 1,2054 Hectares

• 1 Acre 2 Roods 25 Perches 110 square feet = 1 + 2/4 + 25/160 + 110/43 560

= 1,658 775 Acres X 0,404686 = 0,671 283 Hectares = 6 713 square metres

• 5,2385 Morgen X 0,856 532 = 4,486 943 Hectares = 4,4869 Hectares

• 2 Morgen 367 Cape Roods 135 square Cape Feet = 2 +367/600 +135/86 400

= 2,613 229 Morgen X 0,856 532 = 2,238 314 Hectares = 2,2383 Hectares

• 52 643 square Cape Feet = 52 643/86 400 = 0,609 294 Morgen X 0,856 532

= 0,521 880 Hectares = 5219 square metres

Note that there is no rounding off of intermediate results until the final figure for square metres or Hectares is determined.

The practitioners are reminded that the rule still applies that when doing a consolidation, each area must be converted to metric and rounded off before they are added together. This is logical as the original areas of the components are often in different units.

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