Change detection
Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 6 "Introduction to Remote Sensing"; Class 12, Ch 4 "Spatial Information Technology"
Meaning
Change detection means comparing images or maps of the same area taken on different dates to find out what has changed. It is possible because satellites return to the same place again and again. Their revisit time is called temporal resolution. In GIS, the same idea is used through overlay analysis, where maps from two years are stacked on top of each other to make a new map of the changes. Change detection is used to track seasons, disasters, forest loss and the growth of cities.
Example
NCERT shows IRS images of the Himalaya taken in May and November. They show how conifers, deciduous trees and crops change with the season. In GIS, overlaying the urban land use of Aligarh, Uttar Pradesh, for 1974 and 2001 shows urban sprawl, which is the spread of the city into nearby land.
Don't confuse with
- Buffer analysis: finds the areas within a set distance of a feature, such as the land around a hospital. It does not compare two dates.