ROMAN URBAN TOPOGRAPHY in Britain and the western Empire

Dilke: Ground survey and measurement in Roman towns
Fig 14 Portable sundial in the Oxford Museum of the
History of Science, probably 3rd century AD. This
type was designed for use at all latitudes, and has
those of many provinces inscribed on the reverse
(Britain is at 57º)
recent attempts to prove its use elsewhere have not, as
shown by Duncan-Jones (1980) and Millett (1982), been
entirely conclusive. Frere (1977, 92; cf Walthew 1981 b)
has rightly pointed out that the outer insulae at Amiens
measure 4 by 4 actus in pedes Drusiani; though if at Vetera
I, near Birten, the insula south of the principia is 375 by
330 pedes Drusiani, should we not leave it at that, rather
than convert it to 3 by 2 actus (Frere 1977)? Similarly,
it does not follow that for the ‘proto-forum’ in London the
overall exterior length envisaged by the Romans was 3
actus in pedes Drusiani (ibid, 101-2). The measurement
given is 106.7m, which comes to 355 rather than 360 pedes
Drusiani, and even that only if we include projections.
Round numbers in Roman buildings seem on the whole to
refer to inside measurements wall to wall, but the
application of actus to buildings is a doubtful proposition
in any case, hardly proved by a house at Cambodunum
(Kempten) adduced by Walthew (1978, 342). 1
How then were Roman towns planned and surveyed?
Detailed instructions are provided by Vitruvius, some of
which would chiefly concern the architect, some the
surveyor. For a new town, he writes (i.4), first a good site
should be selected, aspect, health, and defence being
considered (see Salway, this volume, pp 67-8). For
defence, town walls should not be square or with projections,
but should have a circuit wide enough for two men
to pass (i.5.2-3). Before building them, the architect
should see that ‘roads duly laid out, convenient rivers, or
supplies by sea through harbours ensure adequate building
material for the ramparts’ (i.5.1). We may note that
nearby water was not indispensable, as aqueducts could be
built (viii.5-6).
Both the architect and the surveyor should master
ichnographia (i.2.2), the drawing, presumably on papyrus,
parchment, or wax tablets, by rule and compasses, of a
ground plan. Town streets should be planned first, then
the forum (i.7). In a seaside town the forum should be by
10
the harbour; elsewhere it should be central. Next the
theatre site should be chosen (v.3). Temple sites should be
fixed with due consideration for the deity involved
(i.7.1-2). For baths as warm a site as possible is advocated
(v. 10.1).
As preparations for his ichnopaphia Vitruvius gives
certain ideal proportions. The forum should not be
square, like the Greek agora (this of course is a generalization),
but rectangular (v.3), his prescription being a
proportion of 3:2. He prefers to adhere to the traditional
layout of many Roman towns by siting gladiatorial shows
in the forum, rather than by constructing an
amphitheatre, which he barely mentions. The basilica, he
says, should adjoin the forum on as warm a side as
possible, its breadth being from a third to a half its-length.
He had himself designed one, perhaps in c 30 BC, at
Fanum Fortunae (Fano), with a nave measuring 120 by 60
Roman feet between the columns and a porticus 20ft wide
between the columns and the wall (v. 1.6). Vitruvius also
gives proportions for the caldarium of baths, 3:2 excluding
the apse, and for the triclinium, 2:l.
Since the basilica measurements, evidently actual as
well as theoretical, involve round numbers, one might
imagine that this would be true of rooms in houses too. In
fact this is not so, since Vitruvius gives a series of ratios,
depending on the length of the atrium, for the widths of the
alae and tablinum (vi. 3.3-S). Thus for atrium lengths of 35,
45,55,70, and 90 Roman feet, the widths of the alae will
be 11.67, 12.86, 13.75, 15.56, and 18 Roman feet
respectively. These figures are only theoretical, but they
show that, even though an architect may have liked to start
Fig 15 Units of area measurement in Roman land surveying.
Above: 1 iugerum = 2 actus quadrati = 2
x 1 lineal actus; 1 lineal actus = 120 Roman feet.
Below: 1 centuria = 200 iugera = 400 actus
quadrati = 20 X 20 lineal actus