1.1 Basic idea and main task of photogrammetry If you want to measure the size of an object, let's say the length, width and height of a house, then normally you will carry this out directly at the object. Now imagine that the house didn't exist anymore - it was destroyed, but some historic photos exist. Then, if you can determine the scale of the photos, it must be pos- ble to get the desired data. Of course you can use photos to get information about objects. This kind of - formation is different: So, for example, you may receive qualitative data (the house seems to be old, the walls are coloured light yellow) from photo interpre- tion, or quantitative data like mentioned before (the house has a base size of 8 by 6 meters) from photo measurement, or information in addition to your background knowledge (the house has elements of the "art nouveau" style, so may be c- th structed at the beginning of the 20 century), and so on.