This art involves a great deal, of which the actual measuring is only the concluding part: on the one hand, for the bodily shapes of rivers, mountains, buildings, etc., which as a rule lack strictly determining concepts and names, it must create such concepts first for their forms (in terms of pictured similarity), and then for their magnitudes and relations of magnitude, and also for the determinations of position, through the measurement of distances and angles related to known places and directions which are presupposed as being fixed.