THEORY OF CARTOGRAPHY
The title, given to the overall work by Ptolemy, isGewgrajikhV YjhghsewV and it is very important that its true interpretation be made clear. His aim would appear to be more as a guide to map–making rather than a scientific treatise on geography per se;'A Map–Making Guide' rather than 'A Guide to Geography'. In this context there is a matter of concern. This work is assumed by historians to be the work on Geography as forecast in the Maqhµauikh SunuaxiV or the Arabicised form called the Almagest published with observations up to AD.141. On this reasoning a date is attributed to the GewgrajiknV YjhghsewV of about AD.145. Since this work uses old nomenclature, referring, for instance, to Britain as 'Albion' and Ireland as 'Hibernia', whereas the Maqhµaukh SunuaxiV refers to them more correctly as Megalhn Breuuania and Mikra Breuuania respectively and it also lacks many of the locations known to have existed at that date, it must give rise to doubts as to whether this work is indeed the promised scientific work on Geography. It could be merely what it seems; an early, youthful work on map–making that only required the editing of the extant data available.
Therefore, in Section 1, when Ptolemy distinguishes between Geography and Chorography, 'Hgewgrajia µiµhsiV esui diagrajhV uou kaueilhµµenou uhV ghV µerouV olou.... kai diajerei uhV cwrograjiaVhe .... kaua µeroV uopouV cwriV ekasuon...', he may not be dismissing the latter in order to promote the former as a science in itself. He is merely distinguishing between two methods of map–making, the 'global' containing outlines and large distinctive features to scale, as opposed to the 'regional' containing as much detail as possible and where relationships are more important that accurate scaling. It is most important to consider this possibility at the outset because it will make so much more sense of what Ptolemy has to say in his following text of Book I and also the sequence of his co-ordinates, in Books II to VII, as distinguishing first the shape of a province followed, quite separately, by its inland features. These are meant to serve the two separate approaches to map–making.
Having tabled these two approaches, Ptolemy goes on to emphasise the unique skills that are required for each method, what strengths and weaknesses are exposed by their requirements. In no context does he express a preference for one over the other or indicate that he is pursuing the theme of one as opposed to the other. Thus in 1.§5. he underlines their respective qualitative and quantitative natures. In 1.§6/7. he states that the one requires a scientific ability while the other an artistic ability, 'Dia uauua ecein µen ouden ui dei µeqodou µaqhµauikhV, enuauqa de uouuo µalisua prohgeiuai uo µeroV.', emphasising in 1.§8. that a knowledge of astronomy is also required with the former. In 1.§9. he compares the mapping of the earth, as opposed to that of the heavens, as a desirable end in itself, since, unlike the earth, the heavens are easily viewed in situ. So, mankind requires maps in order to comprehend the earth in its totality. 'uhn de ghn dia uhV eikonoV, oui uhn alhqinhn kai µegisuhn ousan kai µh periecousan hµaV, ouue kaua µeroV, upo uwn auuwn ejodeuqhnai dunauon.'
In §1, having distinguished geographical map–making from chorographical map–making, as a means of describing the whole earth or portions thereof, Ptolemy, in §2, urges reference to what has been learned before from scientific study and from the accounts of travellers. He divides this study into two streams, The study of the land itself and the study of the heavens – the 'geometry' and the 'meteorology'. He introduces the names of two instruments as aids to the respective studies, the Astrolabe (asurolaßwn), to fix the elevation of heavenly bodies, and the Gnomon (skioqhron), to indicate the angle of the sun casting a shadow on the ground. He indicates that the gnomon is the more useful of the two. 'µeuewroskopikon de uo dia uwn jainoµenwn apo asurlaßwn kai skioqhrwn organwn.'
Next, in §,3/4 he lays down some basic rules on the configuration of points on the surface of the earth and how this may be used to determine the distance of any journey. In §5 he introduces the concept of fixed lines of longitude, or meridians (µeshµßrinoi), running north/south from pole to pole, and fixed lines of latitude, or parallels (parallhloi) running east/west but maintaining a parallel relationship and how the co-ordination of the two can precisely indicate a point on earth, 'oi grajoµenoi .... kukloi parallhloi ue kai µeshµßrinoi, uouuesuin oi µen parallhloi uaV µeuaxu pipuousaV auuwn ue kai ishµerinou perijereian uwn µeshµßrinwn'. In §6/7, he suggests that, rather than have a multitude of such lines criss–crossing the map to cater for individual points of reference, such lines should be formally arranged at predetermined, measured intervals and co-ordinates measured from the nearest. In §8, he concludes by assuring us that the earth is a sphere revolving around a central point in the heavens and that any point on earth travels in a complete circle in relation to this heavenly body throughout twenty–four hours. Further, that by observation of this phenomenon, it should be possible to ascertain the circumference of the earth.
In this section Ptolemy discusses the positioning of a point on earth by means of astral observation, claiming, in §1, that before his time men had used the Pole Star to measure distances on earth and, by plotting the angle of the pole star at the start and finish of a journey, had successfully measured the distance travelled. However, this was only if the distance was along the same parallel of latitude. Presumably meaning that the angle of elevation would therefore remain constant and the angle of traverse only would be measured. He asserts also that, by this means, they also were able to compute the circumference of the earth.
In §3 he continues by claiming that any distance can thus be measured in any direction, and therefore not on the same parallel, by the use of the astrolabe and the gnomon; the former able to fix the longitude, the latter to fix the latitude. Next, in §4/5, he states how the circumference of the earth can be better measured by observation of a proportion of the equatorial circle passing under a fixed point in the heavens over a known time and that proportion extrapolated over a full day.
It would appear that, in this section, Ptolemy is claiming that both longitude and latitude can be found from astral observation alone using the pole star as a reference point for the one and the sun's shadow for the other. If this is so then the mathematics needed would have been complex and the astrolabe used able to measure very accurately indeed the degree of elevation and traverse. He would seem to be referring, in a round–about way to the works of other scientific geographers, notably Aristotle, Dicaearchus, Eratosthenes and Hipparchus but giving them little credence. This, more than ever, would seem to indicate that Ptolemy is writing a guide to map–making only.
In this short section Ptolemy laments that such facilities were not available in his time but that only limited readings of the Pole Star had been taken by Hipparchus and these were quite insufficient for astral observation charts. He notes than a lack of proper, simultaneous observations at the time of eclipses had aggravated matters, quoting the simultaneous observations of an eclipse from both Arbela and Carthage occurring three hours apart. '– wV uhn µen en ArßhloiV peµpuhV wraV janeisuan, en de Karchdoni deuueraV –'
Again, in a similar short section, Ptolemy indicates that he has had to depend upon reports from travellers that have an unscientific basis simply because no serious scientific studies have been carried out. He must therefore make use of the latest of these.
It seems quite clear in these initial sections that Ptolemy is approaching his subject from a practical rather than a scientific viewpoint. His object, if not to personally map the whole world, is to lay down practical rules for others to achieve precisely this. He makes passing reference to the scientific geographers that have gone before but rather than discuss their work in any scientific context, is prepared to take into account the reports of recent travellers and navigators and to accept their observations, subject to some critical reasoning, as the basis of fact. Thus what seems to be emerging is a treatise on practical cartography rather than on theoretical geography.