Designing Roman RoadsAuthor(s): Hugh E. H. DaviesSource: Britannia, Vol. 29 (1998), pp. 1-16Published by: Society for the Promotion of Roman StudiesStable URL: http://www.jstor.org/stable/526811 .Accessed: 25/02/2011 06:02

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Designing Roman Roads

By HUGH E.H. DAVIES

I. INTRODUCTION

The central thesis of this paper is that the Romans used land surveys and maps as integral parts of the road design process.' This is not the conventional view, and is in direct contradiction to a fundamental assumption made by the acknowledged master of the

subject, I.D. Margary. After praising the achievements of the Roman road builders he writes: '. . .it should be remembered that no maps or compasses were available to them. . .'.2 As far as the magnetic compass is concerned, it certainly seems unlikely that it was familiar to the Romans,3 but the sun-dial offered a reliable, if less convenient, alternative method of finding the north-south direction.4 However, the supposed absence of maps is more critical, and has led to the putting forward of many exotic theories to explain how the roads were designed and laid out. Despite their ingenuity, these theories are highly impractical, yet still fail to explain how the Roman engineers solved the problems they faced. This paper shows that available knowledge and equipment were quite adequate to allow the Romans to design their roads with the same level of careful planning as they demonstrated in other fields of technical endeavour.

II. THE ROMAN ROAD NETWORK

Many writers have acknowledged the skill shown by the Romans in planning and constructing their roads.5 They praise the ability of the designers to link distant places with direct routes, and remark on the accuracy with which long straight lengths of road were laid out. The apparent regularity of the system has given rise to several suggestions that the Romans imposed a strategic boundary plan on Britain, by covering large parts of the country with regularly-shaped geometric patterns of roads.6 It is not the purpose of this paper to debate the question of whether any regular pattern exists, but to address the problem of how the network

1 The work described in this paper is based on current research for a PhD at the University of Reading, under the supervision of Professor M.G. Fulford and Dr R. Lawrence. Hugh Davies is a retired Transport Scientist.

2 Margary 1973, I7. 3 The references in The Odyssey to the navigation skills of the mythical Phaecians have suggested to some that

Homer was familiar with the magnetic compass, but there is evidence that such a device was not even available to Norse navigators in the thirteenth century (E.G.R. Taylor 1971, 81).

4 See Dilke ( I987a, 214) for a description of the portable sundial as used by the agrimensores. The direction of due south could be found accurately by marking the position of the shadow of an upright gnomon during the morning, marking it again when it reached the same length in the afternoon, and bisecting the angle between the two positions (Waugh 1973, 18).

5 See for example: Margary I971, 17; Codrington 1905, 7; Chevalier 1976, 84. 6 Examples include: a large-scale grid of roads, for which Fosse Way acts as a principal axis, with other roads

running parallel or perpendicular to it (Jones and Mattingly 1994, 94-5); a square grid with sides of about 12 miles (19 km), based on daily travel distances (Bagshawe 1994, 13); rectangular road alignments at Ripe in Sussex and Cliffe in Kent (Dilke 1971, 191-5); and in Gaul a network of equilateral triangles, with sides equal to one Roman mile (Ulrix 1963).

2

2 HUGH E.H. DAVIES

was designed on the ground. Even if we resist the temptation to seek geometric patterns, the straightforward evidence suggests that the Romans were capable of high quality planning. For example, Rackham notes that: 'Whoever set out the Fosse Way evidently knew in which direction Lincoln lay from Exeter to within a fraction of a degree'.7 Similar accuracy is often claimed for the alignment of Stane Street, between London and Chichester. The road has long been a popular subject for studys and, undoubtedly, part of the appeal comes from the remarkable accuracy with which the section from London Bridge to Ewell appears to be aligned on the East Gate of Chichester (see FIG. I). Though the designers took the alignment away from the direct line after passing Ewell, this decision appears to have been made for perfectly understandable reasons of topography, rather than uncertainty about the required overall direction.9

The following section shows how the assumption that Roman road designers did not have maps available has led to progressively more complex theories to explain their achievements.

III. GETTING THE ROAD STRAIGHT

Margary expresses the general view of how the Romans set about their road planning. 'The real purpose of the straight alignments was merely for convenience in setting out the course of the road, for sighting marks could be quickly aligned from one high point to another, with intermediate marks adjusted between, probably by the use of moveable beacons shifted alternately to right and left until all were brought into line; it is noteworthy that Roman roads nearly always make important turns upon high ground at points from which the sighting could be conveniently done'."' Codrington and Chevalier postulate essentially the same approach." This begs the question of how to select the particular pieces of high ground which lie between the intended origin and destination of the road. However, even if high points conveniently present themselves, the business of linking them directly is by no means straightforward. C. Taylor goes into more detail about how the Romans might have exploited high points, in setting out a road when the two ends of an alignment are visible from each other, even if intermediate points are hidden by ridges.12 Markers would be placed on these ridges, and manoeuvred until they were in line, whereupon other markers would be aligned in the intervening valleys. He claims that marker poles would be adequate for short lengths, but for straight alignments running over ten or fifteen miles he suggests the use of '. . . fires in baskets, probably at night'. He notes that even this would not explain how road lengths could be laid out in which the ends are not inter-visible. FIG. 2 shows his proposal for the design of just such a straight length, on the line of Watling Street between Towcester and Stony Stratford. Here, Taylor suggests that teams of surveyors set out simultaneously from the two ends, designing straight alignments approximately in the right direction. When the two teams met up, they would assess how far from a straight line the two sections were, and would then progressively adjust each half until they lined up.

7 Rackham 1994, 119. 8 Three books have been devoted entirely to the road, Belloc 1913, Grant 1922, and Winbolt 1936. Authors of

other books have remarked on it, for example C. Taylor 1979 and Johnston 1979, and it has been the subject of numerous journal articles - Margary cites 22 (Margary 1973, 67).

9 It can be argued that the diversion is needed in order to pass through Alfoldean and Hardham. However, as with most examples of intermediate destinations on Roman roads, it is difficult to decide which came first, the road or the place. In this case, the balance of probability must favour the former: finding a route through the South Downs is quite sufficient a reason for the road to be placed where it is.

10 Margary 1973, 18. 11 Codrington 1905, 33; Chevalier 1976, 84. 12 C. Taylor 1979, 59-60.

DESIGNING ROMAN ROADS 3

- actual course Old London oft Stane Street Bridge

- survey lines

o posting station

Merton

, + terminal point

._ R.Wandle Ewell I

Dork k --

Brockham Warren (700 )

Dorkir?i R. Mole

Hill"(9cXY)

/ Alfoidean

R.Arun

.+R Arun

S.Downs. R. Aun

..(7001)

0 5 20 Chichester miles

E.Gate

FIG. I. Plan of Stane Street, showing the section from London (Old London Bridge) to Ewell, aligned closely on the East Gate at Chichester (after Johnston 1979).

4 HUGH E.H. DAVIES

Towcester Stony Stratford

00 .m.. .

SGHTINGNVGOO~

•SIGHTING t•

*% 0

0 0

s

FIG. 2. Diagram showing a method of laying out a long straight length of Roman road when the ends are not intervisible (after Taylor 1979, 61).

All these methods are examples of successive approximation, in which a rough solution is refined by trial-and-error, until sufficient accuracy is achieved. Perhaps the most sophisticated example of this approach has been developed by Hargreaves,13 who has proposed that the key surveying instrument is the groma, a device for establishing a survey line which is a continuation of, or is at right angles to, an existing line. Its use by Roman surveyors is reasonably well established and Hargreaves has demonstrated, using practical trials with a replica device, that it is indeed capable of giving accurate results. He has shown that acceptable accuracy could be achieved even at night, using a torch to illuminate the groma itself. Hargreaves has refined the successive approximation approach and applied it to Stane Street, providing perhaps the first detailed account of how the accurate alignment could have been achieved. He envisages up to four separate teams, operating simultaneously at night, between London and Chichester. Each would be equipped with a groma and a movable fire beacon, while fires would be lit at London and Chichester. By successively sighting the fires at adjacent locations and then changing their own position so as to get more nearly into line, a straight line course between the termini is achieved.

Despite the ingenuity of these methods, there are a number of reasons why they are fundamentally implausible. First, the operation of isolated teams of surveyors, out in unfamiliar and possibly hostile territory, maybe at night, attempting to line up movable fire beacons, seems highly impractical. Manoeuvring the equipment and communicating with other teams in the field would both pose severe difficulties. Secondly, the procedure may never reach a conclusion, because any given change of position, which may seem to straighten the line in the immediate vicinity of a particular surveyor, may in fact take it further from the overall straight route. Thirdly, the methods seem to ignore the practical difficulties of dealing with over-steep gradients, providing suitable river crossings and the numerous other engineering considerations which may need to influence the line of the road. Fourthly, the methods do not explain how a road as long as the Fosse Way could have been planned: it is not

13 Hargreaves 1990.

DESIGNING ROMAN ROADS 5

credible that routes of hundreds of kilometres could be derived from successive adjustment of local sections.

We should begin by asking not how the Romans built such straight lengths of road but why? Far from being the convenient way of linking high points, the whole business seems so complicated, tedious and even dangerous, that it is fair to ask: 'Why bother?'. The likely answer is that the straight sections are not part of the planning process at all, but the outcome of it. It seems inescapable that the required bearing of a proposed road was known in advance to the engineers, so that the road alignment could then be designed to follow this bearing as closely as possible, while also taking account of local geographical features. A section of road would have been designed as a straight line only when it was convenient to do so. We should see these straight sections, so characteristic of Roman roads, as having been chosen, not found. The required information on bearing, along with other geographical data, would have been obtained by surveying and map-making. The process should be seen as supportive of, but distinct from, that of road design, with separate roles for surveyors and engineers. Johnstonl4 has recognized this distinction, though he does not go into detail about how either group would have gone about their duties. What these duties are likely to have been is discussed below.

IV. THE PROBLEM OF ROAD DESIGN

There are many similarities between the problems faced by the modern road designer and his Roman counterpart. As direct a route as possible needs to be planned linking the selected end- points and any intermediate destinations, while avoiding steep or marshy ground, finding suitable crossing-points for rivers, and locating supplies of suitable material to form the structure of the road. There are of course some differences. A conquering force would have had few worries about the opposition of land-owners. Also, Roman design constraints for maximum gradient were far less stringent than they are for modern motor roads, reducing the need for cuttings or tunnels. However, in most respects, little has changed in the designer's task. The steps of the planning process used by today's highway engineers are described by O'Flaherty.15 The process starts by identifying the boundary of a region between the end- points, being about a third as wide as it is long. Within this region, various corridors are identified, inside which roads could be designed. The search gradually narrows, until a number of possible road alignments are available for closer study. Finally a preferred line is chosen and its position on the ground defined precisely. Maps of appropriate scale are used at each stage, with such items as ground levels, soil types, water-courses, and the use of land, all being studied in progressively more detail, as various options are identified and compared. It can be seen therefore that it is not possible for the modern highway engineer to select the best route for a road without examining many alternatives. Probably the Roman engineers, under pressure to build roads as quickly as possible in support of the occupation of Britain, would not have evaluated as many alternatives as is common today. But modern practice demonstrates that finding a satisfactory route is not straightforward, so that the Roman engineers would have required accurate information about the area through which they planned to build. Using O'Flaherty's figure of a planning region having a width of one third its length, would suggest that perhaps 29 km of width would have been needed for Stane Street. Good survey techniques would have been needed to gather information, and maps would be essential for displaying the results, particularly from large areas like this. Though no maps of the type being proposed have been found, there are many clues as to how they could have been produced, based on

14 Johnston I979, 69. 15 O'Flaherty 1993, 4.

6 HUGH E.H. DAVIES

knowledge of other forms of map-making carried out during the Roman period. Before reviewing this knowledge, it will be useful to look at a more recent example of the map-maker at work, afforded by the well-documented beginning of the Ordnance Survey.

V. THE ORDNANCE SURVEY AND EARLY MAP-MAKING

The formal founding of the Ordnance Survey (OS) in 179116 offers an example of map-making before the era of computers and satellites. The circumstances which led to the setting-up of the OS offer parallels with the Roman era, since the motivation was the need for improved maps to support the design of military roads, and the work was carried out by army personnel. Though many maps had been produced before the eighteenth century, battles with the Scots had demonstrated that improved roads needed to be planned, and the Army realized that existing maps were inadequate for the purpose. The commonly used scale of one inch to the mile was inadequate and, crucially, there was little detail of topography. William Roy was appointed to begin a survey in Scotland in the late 1740s, but conflicts with France prevented much progress for forty years, until it was decided that a full national survey was needed. This was commenced in 1784, still under Roy's control, and the first base-line was carefully laid out and measured across about 5 miles (8 km) of Hounslow Heath. It is from this base-line that all subsequent measurements have been made.

From the ends of this base-line, at King's Arbour and Hampton Poorhouse, near Bushey Park, a third point, St Ann's Hill, visible from each end of the base line, was sighted and the angles carefully measured. This gave a triangle, of which two angles were known, together with the length of the side between them, see FIG. 3. Using trigonometry, the length of the other two sides could now be calculated, thus giving the distance to the sighting-point from either end of the base-line. The third angle could also be calculated, and checked for accuracy, by using the Pythagoras theorem which showed that the three angles of any triangle must add up to 180 degrees. Thereafter, a series of prominent points, such as hill-tops and church steeples, was identified so that each could be sighted from two points whose positions were already known. The position of each new point could then be estimated, and then used to fix the position of further points. These formed a network of what were called the primary triangulation points. By including some points in the survey which lay on the Greenwich Meridian, and others across the Channel, the primary triangulation points could be related to an international grid. Within each triangle, subsequent surveys fixed the position of secondary points in the same way, followed by a third set. Any local features could then be located by using ground surveys to relate their positions to the nearest triangulation points.

Sighting was sometimes carried out at night using lamps or flares, but it proved to be difficult to co-ordinate the showing of these lights, so it was preferred to observe by day." From the beginning of the survey, accurate measurements depended on the development of the Ramsden Theodolite for taking angular measurements. The device had an integral telescopic sight to improve accuracy. Heights of all points were calculated relative to several reference points whose heights were obtained by direct measurement from sea level. Finally, the easting and northing of all the triangulation points were calculated with reference to the Greenwich Meridian. 18

16 Owen and Pilbeam 1992, 3. 17 Seymour 1980, 35. There is a parallel with the earlier discussion on road planning: if the Ordnance Survey found

it difficult to co-ordinate the showing of lights at night on fixed high points, the Romans would have had even greater difficulty doing the same with movable fire baskets.

18 Seymour 1980, 38.

DESIGNING ROMAN ROADS 7

King's Arbour

St Ann's Hill

Hampton Poor House

(Bushy Park)

FIG. 3. The triangle formed by the first base-line of the Ordnance Survey of Great Britain on Hounslow Heath, together with the first sighting point on St Ann's Hill.

VI. ROMAN MAP-MAKING AND SURVEYING

Julius Caesar commissioned a world map during his dictatorship, but did not live to see it completed.19 Though there are several literary references to it, there is no record of its shape or form. Pliny the Elder describes a map of the Roman world prepared by Agrippa at Augustus' request and completed by the Emperor after Agrippa's death. The map was displayed in Rome on a portico named after Agrippa. It was built up by assembling information obtained from travellers, set against a world view derived from Greek astronomers such as Eratosthenes, whose extensive work is described by Strabo.20 Opinions differ about what the map looked like, though textual information suggests some similarity with the Peutinger Table.21 Unlike Agrippa's map, this has come down from Antiquity in a visible form, though unfortunately the section showing Britain is nearly all lost. The map is not of true scale, being elongated horizontally, presumably because it was originally produced on a long narrow papyrus roll, and later copied onto parchment. This suggests the document was used as a travellers' guide, showing the principal roads and places likely to be encountered, with sufficient detail to estimate travel times and make correct decisions at junctions. Towns are shown as symbols, with some of them being shown in perspective. The map is best described as an itinerary, to assist travellers, and could only be produced once the road system was in place. The same can

19 Dilke 1985, 40. 20 Nicolet 1994, 1oo. 21 Miller 1962.

8 HUGH E.H. DAVIES

be said of the Dura Shield,22 which has attached to it a parchment depicting roads in the area of the Roman legionary base at Dura Europos, on the Euphrates.

Another example of a travellers' guide is the Antonine Itinerary, though it contains none of the graphical elements of the Peutinger Table or the Dura Shield. It comprises simple lists of places along various routes together with the distances between them. It has been a valuable source of information on place-names in the Roman period,23 but, again, could only have been produced after the roads were built.

Perhaps the most famous map from Antiquity is attributed to Ptolemy of Alexandria.24 It seems unlikely that Ptolemy's world map, or anything like it, would have been used other than for the most general overall planning. In particular, Scotland, though recognizable, appears to be tilted through a right-angle. It has recently been shown that the error can be ascribed to Ptolemy's mistaken corrections to information provided earlier by Marinus of Tyre.25 Jones and Keillar show that when Marinus' figures are used, Scotland is orientated correctly. However, even after this correction is made, the map of Britain remains heavily distorted, so that there are serious errors in the relative positions of towns. As an example, consider the two towns Londinium and Noviomagus (Chichester) and suppose that Ptolemy's map was the best available planning tool for a road between them.26 As noted earlier, this road, now known by its Anglo-Saxon name of Stane Street, appears to have been achieved with an error in bearing of less than one degree. The Ordnance Survey Map of Roman Britain shows that the bearing of Londinium from Noviomagus is 31 degrees, whereas the tabulated figures in Ptolemy's Geography imply that the same bearing is only 9 degrees.27 Thus Ptolemy has Londinium lying almost due north of Noviomagus, rather than north-east of it. The difference of approximately 22 degrees would mean that anyone building a road starting at Noviomagus and using Ptolemy's figures would miss Londinium by about 30 km.

The lack of accuracy is perhaps not surprising given the nature of the available information, which seems to have been based mainly on journey times by land or sea, with distances and directions being estimated by the travellers who supplied the data. By comparing these and making adjustments to achieve a degree of consistency, the results were fitted in to the overall view of the Earth, its shape and dimensions. Direct measures of latitude were technically possible by measuring the elevation of the sun at noon or the height of the North Star at night. However, Ptolemy does not seem to have had much data of this type available. As to longitude, there were no reliable direct estimates at all - this did not become possible until well into the eighteenth century, following the invention of a reliable chronometer.28 If Roman road engineers had nothing better than Ptolemy's maps with them when they landed, which seems unlikely, then something more accurate would have been needed, and quickly.

At the opposite end of the mapping scale from Ptolemy's world atlas was the work of the land surveyors, the Agrimensores, who were concerned with the location and marking out of

parcels of land, allocated to individuals or towns by the Roman state. This was an important job, in which technical expertise was needed to avoid disputes and ensure that tax liability was clear. Training was regarded as important, not only to give a good overall knowledge, but also to gain very specific skills in geometry and measurement.29 Land was mostly set out using a

22 Dilke 1987b, 249. 23 Rivet 1970. 24 Pagani 1990. 25 Jones and Keillar 1996. 26 This is not strictly possible because the road was almost certainly built before Ptolemy's time. However, the

following analysis is worthwhile nevertheless, because any world map, available to the early Roman occupiers, is unlikely to have been any more accurate.

27 Ordnance Survey 1994; Ptolemy, Geography 2.2. 28 E.G.R. Taylor 1971, 245; Sobel 1995. 29 Dilke 1971.

DESIGNING ROMAN ROADS 9

rectangular, usually square, grid, but the surveyor needed to be able to deal with other shapes, such as triangles and hexagons. Roads are referred to in the Corpus Agrimensorum, but mainly as boundaries, limites, between the plots, centuriae, with the width of any particular road being appropriate to its relative importance.30 Roads could also provide a geographical reference system, with a place being located by referring to a specific milestone along a particular road, together with the required distance, left or right.31

A wide range of surveying instruments is known to have been available, including the groma, already mentioned, the chorobates along with various staffs and rods for measuring levels, cords or rods for measuring distance, geometric compasses, etc. For the present discussion, an important instrument would be an equivalent of the modern theodolite, which was capable of measuring angles. The mechanical engineer Heron of Alexandria described the use of just such an instrument, the dioptra.32 This could have been used for measuring either horizontal angles, for plan surveying, or vertical angles, for levelling. Vitruvius recommends use of the dioptra for designing water-supply systems33 and Pliny the Elder mentions its use for astronomy. In chs I to 20 of his Dioptra,34 Heron himself shows examples of the device in use for measuring levels, using its vertical adjustment. However, where bearings are required, he usually suggests methods based on right-angled triangles, which would need an instrument such as the groma. FIG. 4 shows an example of his approach to the design of a tunnel.35

Note that the method allows the location of an imaginary point M, which completes a right- angled triangle, with the line of the tunnel itself forming the hypotenuse. The length of each of the other two sides, BM and DM, is established by adding (or subtracting) a succession of shorter lengths (known as traverses in modern surveying). Having drawn up a scale plan, triangle BDM can now be constructed. To obtain the bearing, angle MBD, it would have been possible in principle for Heron to have used trigonometry, by employing Hipparchus' or Menelaus' tables of chords of circles.36 Instead he suggests a geometrical method, based on similar triangles, to set up points O and P on the ground, which can then be used to indicate the direction of the tunnel. These same constructed triangles could also be used to calculate the length of the tunnel, BD, by measuring OB on the ground and then scaling up by the proportion MD:ON. Alternatively the tunnel length could easily be calculated directly, using Pythagoras' Theorem of right-angled triangles, to obtain BD2 = BM2 + MD2. The vertical angle of the tunnel must also be known before construction work can begin. Provided that the surveyors took measurements of level as well as distance along the individual traverses, the difference in level between the ends of the tunnel can be worked out, and hence the required slope calculated. Thus Heron's method locates the line of the tunnel precisely, so that teams could start boring from both ends simultaneously. The orientation of the traverse lines is arbitrary, but it seems likely that they would have been aligned on north-south/east-west axes, since this would make it easier to relate the plan to operations on the ground.

Whether or not Heron's method was standard practice for tunnelling is not known, but there are many tunnels in the Roman world, the construction of which would have needed proper planning of this sort rather than trial and error methods. Two examples are the crypta Neapolitana, a road tunnel between Pozzuoli and Naples, and the water tunnel at Saldae. Though the former is referred to by Strabo and others,37 no information on its planning is

30 Campbell 1996, 84. 31 For example, Frontinus used this approach to specify the locations of sources of water feeding into the aqueducts

of Rome (Frontinus, Aqueducts 1.5.27). 32 Heron provided a diagram of the instrument in his Dioptra (see Sch6ne 1903, 193 and Dilke 1971, 75). 33 Vitruvius 8.5.1. 34 Schdne 1903, 190-253; Heath 1921, 345-6. 35 Heron, Dioptra 15 (SchOne 1903, 239). 36 Heath i92I,245. 37 Strabo 5.4.7; Seneca, Ep. adLuc. 57. I-2.

2*

IO HUGH E.H. DAVIES

P

/ K

// D Q L

G

O

FIG. 4. Heron of Alexandria's method for digging a tunnel through a hillside. The instructions specify that a tunnel is to be dug from both sides through a hill (in plan) ABCD. From B, draw a random line BE, a perpendicular EF, and so on round the hill to K. Construct a line from D to be perpendicular to JK at L. Measure lengths around the hill. Calculate the lengths BM and DM, being sides of the right-angled triangle BDM. Produce EB to a random point N, and construct O such that BN:NO = BM:MD. Likewise Q and P are located such that DQ:QP = BM:MD. The direction of the

tunnel follows sightings along OB and PD (after Dilke 1987a, 232).

known. However, the building of the latter is the subject of a long inscription.3" This describes how the tunnel had to be rebuilt, by Nonius Datus, because of the failure of the first attempt, in which the builders started from each end but failed to meet in the middle. Nonius Datus claims, in the inscription, to have produced a plan of how the two parts could be linked up underground. The form of the plan is not known, but the fact of its production demonstrates that plans were used for design work, and thus supports the case for the use of maps in road design. According to the inscription, Nonius Datus marked out the line of the tunnel, on the ground above it, with a line of posts. Such marking is still normal practice in tunnel design, in case intermediate shafts are needed for excavation39 or ventilation. It should be noted that, nowadays, as no doubt was the case in Nonius Datus' time, the marking is done after a plan has been prepared; it is not part of the planning process itself.

38 CIL 8.2728. 39 During the railway-building era it was claimed that, if tunnelling work was to be completed in under one year,

tunnelling should commence from intervening shafts, which were no more than 200 yards apart, as well as from the ends (Simms 1896, 35). No doubt many tunnellers in Antiquity would have been under similar time pressure and so would also have used excavation at intermediate shafts.

DESIGNING ROMAN ROADS I I

Though Heron does not describe the groma, it would be an appropriate instrument to use in marking out the traverses for a tunnel, and both Adam40 and Hargreaves41 have verified its potential accuracy using replicas.

VII. ROMAN MAP-MAKING FOR ROAD DESIGN

It has been shown above that previous notions about how the Romans set about fixing alignments for their roads rely on cumbersome trial-and-error methods, whose effectiveness has yet to be demonstrated. It is suggested here that a more likely alternative is the use of maps or plans as a design medium. Unfortunately, as Nicolet points out, there is little evidence of maps between the small scale of world maps like Ptolemy's and the local cadastra maps of towns showing the land they owned,42 and it is this gap into which road-design plans fall. However, there would appear to be no shortage of either relevant knowledge or equipment, so it is possible to make an informed guess as to how road plans could have been produced.

The first question to answer is how the designers knew in what direction to plan the road, assuming, as is usually the case, that the ends are not within sight of each other. There are two candidates, direct observation and dead-reckoning. In the former, the latitude and longitude of the ends must be established independently, for example by astronomic observation, so that the bearing of a line joining them can be worked out. With dead-reckoning, a series of links, of known length and direction, must be followed between the ends. In the former case, the position of the ends is known absolutely, while in the latter, one end is known relative to the other. As has already been noted, a major obstacle to position location was the fixing of longitude. Ptolemy acknowledged this by pointing out that the distance between two places of known latitude, but not lying on a meridian (in other words not having the same longitude), can be found if their relative bearing is known, or vice versa.43 The difficulty for the road designer, trying to link two distant points, is that neither their bearing nor distance apart is known. An example of Ptolemy's problems with longitude has already been noted in his figures for Londinium and Noviomagus; they could not have formed the basis for road design between the two towns. It therefore seems much more likely that Roman road designers would have used dead-reckoning, both to provide the required bearing and information about the intervening landscape. All this information would have been used to produce maps, of true scale, from which road alignments would have been designed.

Some known Roman maps, such as the Peutinger Table and the Dura Shield, are based on existing roads and are not necessarily drawn to scale, so they are not suitable for designing new roads. We need to look at the work of the land surveyors, and, though the published techniques do not relate directly to road building, there would have been little difficulty in adapting them for this purpose.

There is no reason in principle why maps should not have been produced by triangulation, the method used by the early Ordnance Survey and described in Section v above. Heron's dioptra was designed to do the same job as the Ramsden Theodolite, and knowledge of trigonometry was adequate to the task of calculating the distances to points whose bearings had been measured. However, the dioptra was not equipped with a telescopic sight, which would have restricted the length of the sides of triangles that could be sighted, and in any case it has been suggested that the device was too cumbersome for general use in the field.44 A

40 Adam 1994, 14. 41 Hargreaves I990. 42 Nicolet 1993, 72. 43 Ptolemy, Geography I.3 (Stevenson 1991, 28). 44 Dilke 1971, 79.

12 HUGH E.H. DAVIES

Contour

.B ">

-._.... .......... ..................

gnary line

Imaginary line •:

FIG. 5. Proposed survey method for road planning. The diagram shows an area of countryside with a river running through it. Points A and B are hills to be linked to a survey. The survey team first marks off a section AC due east, then a section CD due north, and so on until B is reached. These sections are called traverse lines. The length of each traverse is measured, along with a change in level. Lengths such as FG, which crosses the river and so may be difficult to measure, can be estimated using Nipsus' method. The distance between A and B, and the bearing of B relative to A

can be worked out by adding the easterly and northerly lengths and then constructing the imaginary triangle ABK.

simpler instrument, the groma, certainly was used, but, because it was limited to treating right- angles, it would not have been suitable for estimating distances by means of triangulation. We must therefore consider procedures which rely on all distances being measured directly.

The method described by Heron of Alexandria for tunnel design, noted above, is a likely candidate. The only adaptation needed is to direct the survey so that it links areas of high ground, rather than going round them. Such an approach is illustrated in FIG. 5. The figure shows an imaginary area of countryside lying approximately along the line of a required new road. The first task for the surveyors is to select relevant high points. These will lie within the region through which the road will pass, but it is not necessary, at this stage, to know whether any particular hill lies on the direct line between the end points. Next, pairs of hills are linked with surveys, by using the groma to establish a series of traverses, which are straight lines, each at right-angles to its neighbours. In FIG. 5, these are shown orientated north-south/east-west, though other orientations could be used. A record of the length of each traverse section is made, so that the total distances east and north, between any two high points, can be worked out. From these two figures, the distance apart and bearing of the two hills can be calculated, using the properties of right-angled triangles. Changes in level can also be recorded along each traverse section, to provide information on the intervening topography. When obstacles, such as rivers, make distances difficult to measure directly, a method of estimating short distances, ascribed to Nipsus, can be used.45 The survey continues, establishing bearings between all suitable pairs of high points in the area, and includes the start and end points of the road. The

45 Dilke 1971, 61. The method relies on sighting an object, such as a tree, on the opposite side of the obstacle, and then setting up similar triangles on the ground, marked by posts. The size of these triangles is related to the distance to be estimated, so the method is not suitable for estimating long distances between hills.

DESIGNING ROMAN ROADS 13

straight line between each pair of hills is called a survey line, and the survey needs to continue until there is at least one continuous series of survey lines linking the start and end points of the road. Finally, by adding the total distance east and the total distance north of the end of the road, relative to its beginning, the length of the straight line between the end-points, and its bearing, can be established. The position of important features such as rivers, streams, forests, existing roads and towns, can then be filled in, again using the groma, relative to points whose positions are already known.

There are several methods available for measuring distances. Vitruvius describes a hodometer, a wheeled device using cogs to record rotations and so indicate distance travelled,46 but it may have been too elaborate and cumbersome for use across country. Rods, cords, and chains may well have been the basic devices for short lengths, but simply pacing out distances may well have given sufficient accuracy, and could be done far more quickly.47 Although any orientation can be used for the survey lines, the advantage of using north-south and east-west is that checks for accuracy can be made regularly, using a portable sundial. It also aids the linking together of different surveys. This points to an advantage of the proposed approach, because several teams can be in the field at once, covering different areas. Their work does not need to be co-ordinated - unlike the teams who have previously been proposed as seeking a straight road alignment - all that is needed to link the work of the various teams together is for one or more high points to be common to adjacent surveys.

It is now appropriate to consider how the survey data would have been recorded and displayed. The surveyors in the field would record the information on the length of each traverse line, and the corresponding change of level, in tabular form, probably on wax tablets or scratched on wood. The same method would be used to produce sketch plans of the local area in which they were working. Having completed a section of the survey, involving one or more pairs of hills, each survey team would return to the base, probably the local military headquarters, where their work would be transferred to the design plan. This would be a map covering the area of country encompassing the entire length of the road. The map would need to be of true scale, and large enough to contain all the relevant data in graphical form. However, it would not need to be produced on a permanent medium. The most likely form would be a layout on the floor of a large room, possibly using tiles as markers, with lines being scratched, painted or marked in sand on the floor itself. The hills involved in the survey, and other prominent features, would need to be labelled using names or numbers, and ground levels shown relative to a convenient datum, such as the military base itself.

As soon as the two ends of the road had been linked with a continuous series of survey lines, the length and bearing of the direct line linking them could be calculated and marked on the plan. The engineers could then start planning their alignment in detail.48 As the survey continued, more detail would be added progressively, thereby assisting the engineers to narrow their search towards the preferred line. Individual sections of road might be marked out to a larger scale, on another part of the floor, particularly for difficult situations. The way the road itself may have been marked on the plan is shown below.

FIG. 6 shows a series of survey lines, denoted by the dashed lines, linking relevant high points in an area, with part of a road alignment being shown by the striped line. The road is defined by points along its length, the position of each point being defined by a specified distance along

46 Vitruvius 10.9.I. 47 Fischer (1975, 153) refers to 'bematists', Egyptian clerks trained to pace evenly, and Aujac (1987, 149 note 7)

refers to the 'bematistae' whose job it was to record the daily distance covered by Alexander's army. Sherk ( 1974, 557) also mentions pacing, as a means of positioning milestones once a road has been built. Also it is likely that most Roman army personnel would have been trained to pace evenly for efficient marching.

48 Of course the engineers would have been active before this stage to familiarize themselves with the countryside through which the road would be likely to pass.

14 HUGH E.H. DAVIES

Contou

Rive p

II II

s tIIII at ""ttitti II i u111111 ,

OffsetOffset linele

ALSurvey ine"-"-

FIG. 6. The diagram shows a series of survey lines, denoted by the dashed lines, linking relevant high points. The striped line shows the design for a new road alignment. Its position is defined by a series of offsets, denoted by the dash-dot lines, which are marked off at right-angles to the survey lines. Thus, each point on the road is defined by a specified distance along a survey line together with a specified distance to left or right. Calculating the position of the

road relative to the survey lines is made simpler if the road is designed as a series of straight elements.

a particular survey line, together with an offset, which is a line of specified length at right- angles to the survey line, either to left or right.49 Once the engineers had designed the road, using the plan, surveyors would be called in again to transfer the planned road to points on the ground. They would measure the required distances along each survey line, establish the right- angle with a groma, and then measure the specified offset distance. The end-point of each offset defines a point on the road. Because the road had been designed with the aid of a proper survey, it should not have been necessary to make significant modifications to the line, though local corrections could easily have been accommodated without jeopardizing the overall plan.

The procedure just described explains why Roman roads are composed of straight, rather than curved elements. The process of transferring the alignment from the plan to the landscape is computationally far simpler if linear rather than curved lengths of road are used, since the length of each offset along the same survey line and the same element of road, is a simple linear proportion of its predecessor. Each straight length of road would therefore be made as long as possible, so as to maximize this run of simple linear progression of offset calculation. Mathematical convenience can thus be seen as the factor which gives Roman roads their most well-known characteristic.

Once a particular road was built, the design plan would no longer be needed, since the road itself becomes the geographical reference datum for the area. It seems likely, though, that small-scale copies, showing the line of the new road, would be preserved. These would later form the bases for maps and itineraries such as those mentioned in Section vi. But the original

49 This is, in essence, the same method of specifying position as that which was referred to in Section vi, whereby the Romans located a place in relation to an existing road. In this case, it is the road which is being located, while the survey lines provide the position reference.

DESIGNING ROMAN ROADS 15

plan, so vital in the road design process, would have fulfilled its purpose and would probably be obliterated, allowing the room which contained it to be allocated to other functions.

VIII. CONCLUSIONS

The absence of written design manuals for Roman roads has led to the assumption that evidence for how the roads were planned must lie in the alignments themselves. The observation of long, straight sections of road, sometimes aligned on high ground, has given rise to a deceptively simple proposition, namely that alignments were designed by successively moving a series of fire beacons to bring points along them into line. The apparent simplicity of this approach is deceptive: it becomes highly complex when applied to places which are not visible from each other, and fails completely to explain how alignments as long as Fosse Way or Stane Street were planned. An alternative approach is described which assumes that a ground survey would have been conducted, to link the start and finish points of a proposed road. The survey results would enable the required bearing of the road to be worked out, and would also allow important features, such as rivers, to be located accurately. A suitable survey procedure could have been similar to that used for the design of tunnels and published by Heron of Alexandria. Information from the survey would have been displayed on a map to assist the road designers to plan a suitable alignment. While wax tablets may have been used by surveyors in the field to record their findings, the main design plan would have been a true- scale map, probably drawn on the floor of a large room. Once the road design was complete, its course could be set out on the ground without the need for successive adjustment, irrespective of its length. The method also accounts for the long straight sections, so characteristic of Roman roads: far from requiring the elaborate procedures envisaged by current theories, the design feature is shown to arise directly from the requirements of mathematical convenience.

Sandhurst

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16 HUGH E.H. DAVIES

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