
Old Hampshire Mapped



Scale Lines
Notes

map scale

Many maps use a scale line to indicate the scale of the map.
This is particularly useful in the present age of easy
photocopying when the map might be unknowingly enlarged or
reduced in the process; the scale line is changed equally
and remains a true indicator of the map's scale. The more
we work with maps and plans the more it seems best (just
good manners) to always provide a scale line. Simply quoting
a numerical scale is not good enough.
It is useful to express the scale of a map as a numerical
ratio. Thus the familiar scale of:
1 inch to 1 mile
can be numerically expressed as
1 to 63360
It is pretty quick, with a pocket calculator, to measure a
scale line on an old map and work out the map's scale.
An example:
scale line 10 miles = 67.3 mm long
It seems easier to measure accurately in millimetres, mm,
than fractions of an inch. The scale is got thus:
calculate 10 x 63360 x 25.4 / 67.3
= 239139.87
Remember: 63360 inches in 1 mile; 25.4mm in 1 inch.
BUT
: the old english mile was not the present day
statute mile, so this calculation fails for old maps.
For example Saxton's mile has been shown to be about 1.25
statute miles.
Remember too that your measurement of 67.3 mm is not precise,
it is probably no better than plus minus 0.2 mm, an error
of plus minus 0.3% in the example. This error must be
reflected in the way you express the scale. You must round
off some. For the example above the scale can be given no
more accurately than:
1 to 239000
BUT: the calculation assumes that the miles on the
scale line of the map are modern statute miles. On early
maps this is an unreasonable assumption: BEWARE.

a mile

A legal statute mile was incidentally established in 1593, in
an Act, 35 Eliz I cap 6, against new building works within
3 miles of the gates of the City of London:
... a myle to conteyne eight furlongs and every furlong
to conteyne fortie luggs or poles and ev'ry lugg or pole
to conteyne sixteen foot and half ...
The mile comes out at the familiar 1760 yards, each of 3 feet
of 12 inches; 1 mile is 5280 feet, 63360 inches.
Before this time a variety of customary miles were in use,
and they remained in use for a long while after. The
Elizabethan statute mile gradually came into more common use,
but was only well established by an Act of 1824, 5 Geoge IV
cap 74.
Early maps often carried more than one scale line, for
'great', 'middle' and 'small' miles or whatever, reflecting
the uncertain nature of the customary distance represented.
'mile' derives from the Latin milliarius
thousand
or miliarium
milestone
; standing for a distance of mille
passum a
thousand paces
. A pace was 5 feet (but, just
how long was the foot?)

towns

The length of the miles being uncertain, the meaning if the
scale line is uncertain. An alternative is to use the
position of places on the map to calculate the map's scale.
You can measure the distances between a selection of
towns, choosing towns whose position you can tell both now
and then, and average a lot of calculated scales worked out
from the distances. This project uses a computer program:
DISTAB.exe
to do this job.
A handy list of map scales and placetoplace distances
is available if you want to do calculations by hand.


References

Connor, R D: 1987: Weights and Measures of England, The:
Science Museum; HMSO (London):: ISBN 0 11 290435 1
