We can structure time by events (moments) or periods (intervals). When we represent intervals by a start and an end event, we can derive temporal relationships between events and periods, such as “before”, “overlap”, and “after”.
Valid time (or world time) is the time when an event really happened, or a string of events took place. Transaction time (or database time) is the time when the event was stored in the database or GIS. Note that the time at which we store something in a database is typically (much) later than when the related event took place.
Process models in the Earth sciences describe the evolution of geo(bio)physical surface properties in time, independently from remote sensing observations. Examples of such process models on various time scales are, for instance, numerical weather prediction models (NWPs), vegetation growth models, hydrological models, oceanographic models and climate models.
Processes on the planet Earth are complex phenomena that are taking place in space and in time, i.e. in four dimensions.
In many of these processes, differences in one dimension (e.g. height above the geoid) can be disregarded, so that two spatial dimensions and the dimension time remain. Despite this simplification, the physical description of the phenomena remains a difficult task. To better understand the processes it often helps if the same geographic region is viewed repeatedly and, if possible, also from different directions and in different wavelength regions. Integration of data from a variety of sources can be a means to retrieving information about processes that would otherwise remain undetected.