For example, aerial photographs taken in 1985 might show corn growing in a field. An aerial photo taken of the same field in 1995 might show oranges. With only these data one can do no more than hypothesize how the change actually took place. Soil salinization and mass wasting are examples of each respectively. There are two levels of measurement.
Incidental observation is the crudest.
It involves events which were recorded because an observer just happened to be there or because it was regarded as an unusual happening. A classic example of a massive and infrequent event would be the person who fortuitously has a video recorder handy when a tornado hits. An example of understanding slow continuous changes involves the corn to orange case mentioned above. In 1990 I happened to see a field in eastern Mexico that had orange saplings planted between the rows of corn. As the trees matured and the canopy cover increased, there would be less sunlight for the corn.