Time

As time is the central concept of the temporal dimension, a brief examination of the nature of time may clarify our thinking when we work with this dimension:

• Discrete and continuous time: Time can be measured along a discrete or continuous scale. Discrete time is composed of discrete elements (seconds, minutes, hours, days, months, or years). For continuous time, no such discrete elements exist: for any two moments in time there is always another moment in between. We can also 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 and transaction time: 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.
• Linear, branching and cyclic time: Time can be considered to be linear, extending from the past to the present (‘now’), and into the future. This view gives a single time line. For some types of temporal analysis, branching time - in which different time lines from a certain point in time onwards are possible - and cyclic time - in which repeating cycles such as seasons or days of the week are recognized - make more sense and can be useful.
• Time granularity: When measuring time, we speak of granularity as the precision of a time value in a GIS or database (e.g. year, month, day, second). Different applications may obviously require different granularity. In cadastral applications, time granularity might well be a day, as the law requires deeds to be date-marked; in geological mapping applications, time granularity is more likely to be in the order of thousands or millions of years.
• Absolute and relative time: Time can be represented as absolute or relative. Absolute time marks a point on the time line where events happen (e.g. “6 July 1999 at 11:15 p.m.”). Relative time is indicated relative to other points in time (e.g. “yesterday”, “last year”, “tomorrow”, which are all relative to “now”, or “two weeks later”, which is relative to some other arbitrary point in time.).