1.1 Basic idea and main task of photogrammetry If you want to measure the size of an object, lets 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 didnt 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.