It is well known that by employing aero-photogrammetric methods and instruments one is able to produce maps in considerably shorter time and at much lower cost than is possible when the work is done according to the old terrestrial methods This saving of time and money will, however, vary according to whether a sufficiency of terrestrial bases is already known or not For it is, indeed, a fact that in the hitherto practised aero-photogrammetric methods most of the time was spent in finding at least three fixed points for every plate-pair which were recessary to make an exact setting into the plotting instrument possible If such terrestrial measurements have to be carried out in difficult country, for instance in primeval forest areas or extensive swamps, it cannot but happen that the expenditure of time and money will be so greatly augmented that in the end the whole advantage of aerial survey becomes illusory