Whilst they were thus engaged in fixing one part of the geography of the interior, the Viceroy turned to account the temporary sojourn at Buenos Ayres of some of the officers attached to the commission for laying down the boundaries under the treaty of 1777 with Portugal, and employed them in mapping other portions of the territory under his immediate jurisdiction.
In 1794 M. Sourreyere de Souillac, the astronomer of the third division of that commission, laid down the line of road from Buenos Ayres to Cordova, and fixed the latitude of that city in 31° 26´ 14".
In 1796 Azara, with Cerviño and other officers employed on the same service, made a detailed survey of the frontiers of the province of Buenos Ayres, in the course of which they fixed the positions of all the towns and forts of any importance between Melinqué, its north-western extremity, and the most southern bend of the river Salado, beyond Chascomus. That river they found to have its origin in a lake in latitude 34° 4´ 45", longitude from Buenos Ayres 3° 36´ 32"; it is an insignificant stream, of trivial importance till joined by the Flores.
Thus materials were collected for laying down a considerable portion of country upon the very best authorities; but, like the surveys of the coast, many years were suffered to elapse ere they were made available to the public. Bauza's map was not published till 1810, and it was only in 1822 that the positions fixed by Azara in 1796 appeared for the first time as his in the "Statistical Register," published that year at Buenos Ayres. De Souillac's might have remained unknown for ever, had not Señor de Angelis lately brought them to light; as well as Malaspina's "Fixed Points on the Shores of the River Plate."
But, after all, however valuable were these data in perfecting a knowledge of the country already occupied, they led to no new discoveries, and by far the greater part of the interior of the continent, to the south of the Plate, remained unexplored, till Spain becoming involved in the general war carried on between the great powers in Europe, her colonial subjects on the shores of the Pacific began to experience more or less inconvenience from the stoppage of their ordinary trade. They found that the ships which used to visit them direct from Europe for the most part ran into the river Plate, rather than encounter the increased risk of capture in the longer voyage round Cape Horn; and it became therefore to them an object of considerable importance to shorten, if possible, the over-land journey from thence to the opposite side of the continent, and particularly to the southern parts of Chile.