How accurate and detailed were geographical maps created and updated by the Romans?

Question

Knowledge is power, and having superior geographical knowledge vs. your imperialistic enemies and competitors was probably a big advantage in the middle ages. Some specific examples would be knowledge of fast and easy ways for traveling long distances, knowledge of water sources, places for animal hunting, agriculture, etc.

What techniques did the Romans use to measure geographical distances/topography, and how detailed were their maps from a qualitative and quantitative point of view? Could they use this knowledge to quantify how much provisions and men they needed for distant conquests and travels?

Answer 1

Well one way to answer would be to just show you some Roman maps. While they are not a perfect as modern maps, I would say that, for Europe at least, they are not bad. Remember that until recent times It was not possible to gauge longitude accurately. So for example, on this map copied in the 15th century from Ptolemy's 150AD work, you can see allot of east to west distortion but the north south seems to be reasonable to my eye.

Note that the map makes seas and rivers prominent but doesn't show land features like mountains. It is really a map of routes. Like a modern subway map, it is the line and the names that are important. The scale, less so.

Large sections of this map have been drawn from imagination or stories. Notice that the area east of Maysia is quite incorrect. An enlargement of this East Asian area is shown below.

_{(source: wikimedia.org)}

Other Roman maps I have seen, such as this other map of the world, look odd to our eyes. Several conventions that we take for granted have not been followed. North is not at the top. The Mediterranean, the centre of the Roman world, is in the centre, and the area between Asia and Africa has been guessed and so no Americas. However, once you get past those issues, It does seem to be a usable depiction of much of Europe, Northern Africa and near Asia.

Many more Roman maps can be found by in google's image search.

Answer 2

One basic instrument seems to have been the odometer, described in Vitrivius' book, Chapter 9. Similar mechanisms could also measure miles by sea, by using a paddled wheel.

Thus, as the wheel proceeds, it acts on the first drum- wheel, the tooth of which, in every revolution, striking the tooth of the upper wheel, causes it to move on; so that when the lower wheel as revolved four hundred times, the upper wheel has revolved only once, and its tooth, which is on the side, will have acted on only one tooth of the horizontal wheel. Now as in four hundred revolutions of the lower wheel, the upper wheel will only have turned round once, the length of the journey will be five thousand feet, or one thousand paces. Thus, by the dropping of the balls, and of the noise they make, we know every mile passed over; and each day one may ascertain, by the number of balls collected in the bottom, the number of miles in the day's journey.

In navigation, with very little change in the machinery, the same thing may be done. An axis is fixed across the vessel, whose ends project beyond the sides, to which are attached wheels four feet diameter, with paddles to them touching the water.

In modern robotics, wheel encoders are used which also measure axis rotations, and the accuracy of such systems is much dependent on how much the wheel slips. If the wheel has good surface friction in a planar surface, it is possible to get good accuracy. But if the wheel slips (low friction, mud, water, too high acceleration, etc), or if the terrain is irregular so that the wheel jerks and goes up and down, or if the trajectory has many curves, then accuracy is poor.

I expect the accuracy in Roman times would be good in regular paved Roman roads, specially in flat areas and straight lines.

Some of the Roman maps (or other ancient maps) were not maps in the modern sense but what the medievals called Portolans i.e., accurate shapes were not the objective, the point was to get right the sequence of ports, cities, natural features (mountains, harbors, etc), along the coasts and/or roads, and the local distances between them.

Therefore, even if the ship misses her target after crossing the sea, which was common due to navigation innacuracy, after recognizing a city or natural feature, the portolan could tell how far was the desired destination. One such Roman (or medieval, I do not remember) portolan of Italy is in the cover of this famous atlas.

PS1: Do not underestimate the importance of portolans. The point for them was to find the destination, not to decorate a room with beautiful maps. To have a precise map is not so relevant if you do not have precise localization for yourself. Even during the discovery era, the Portuguese had 'roteiros' which were as secret and important as maps. Roteiros (rutters, I think, in english) are just texts describing the sequence of land features and navigational hazards between points A and B.

PS2: I have seen a 1-meter diameter globe from ~1937. It was still noticeably wrong if you looked closely. perfect accuracy, even for the naked eye, came only with satellites.