The Romans had now acquired more detailed information regarding the number and position of the tribes of Caledonia, their names, the situation of their towns, and the leading geographical features of the country. These are preserved to us, as they existed at this time, by the geographer Ptolemy, and his account of the north part of the island has apparently been compiled from the itineraries of the Roman soldiers, the observations made from the fleet in its circuit round the island, and the reports of those who had penetrated into the interior of the country. From these and other sources of information he lays down the position of the prominent features of the coast—the headlands, bays, estuaries, and mouths of the rivers, and the position of the towns in the interior, by giving the latitudes and longitudes of each. These degrees of longitude, however, are subject to a double correction. First, he places the island in too northern a latitude; and secondly, his degrees of longitude are less than the true degree, and therefore the number of degrees stated between two places is greater than they ought to be. Besides this, he has fallen into the extraordinary error of turning the country north of the Firths of Forth and Clyde to the east instead of to the north. This error mainly affects that part of the country between the Solway and the Clyde on the west, and the Wear and the Forth on the east—the coast on the west being unduly expanded, and that on the east proportionably contracted. Beyond the Firths of Forth and Clyde the effect of this strange error is to alter the points of the compass, and to substitute north for west, east for north, south for west, and west for south. The former error does not much affect the accuracy of the relative distances of places near each other. The latter, with the distortion of the distances and relative position of the localities which it creates, can be corrected without difficulty, and that part of the map reconstructed as if this error had not been fallen into. Where the country is unaffected by these mistakes, his accuracy is so great, when compared with the face of the country, that his localities can be laid down, with some rare exceptions, with considerable confidence.[55]