Some environmental changes can occur very slowly over very long time periods, such as changes in groundwater levels, some others occur very rapidly over short periods, such as pollution being washed into a river after a rainstorm, or at times when it is difficult for an observer to be there such as in the middle of the night. With these situations using a sensor to record changes is a useful technique. Sensors can be used for a range of environmental changes, for example to detect changes in air or water quality, changes in water levels and the presence of wildlife that trigger a sensor as they walk past. Sensors can be set to regularly record data such as every hour or they can be set to record something only when they are triggered to do so. Data from a sensor is often presented in a table or a graph.
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