Transformation is the task of moving data between different geographic coordinate systems. A developer may, for example, have data in WGS84 that was collected from a GPS reading. However, the map may be in a different spatial reference, such as British National Grid, which is based on geographic coordinate system: OSGB 1936. To convert the data from WGS84 to British National Grid, the developer must apply a transformation as well as a projection. A number of transformations are available, depending on the geographic area the data comes from. Transformations are performed using equation-based transformation methods or grid-based transformation methods.

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