Raster data and coordinate systems

Like vector data, raster datasets are not very useful unless they are registered (aligned) to one another through a common coordinate system. Before you can perform even basic operations like merging or overlaying, you need to tie the raster dataset to the landscape. ArcGIS offers two methods for changing various parameters of your data's coordinate system, including the map projection.

On-the-fly coordinate system change
ArcGIS Spatial Analyst can combine multiple rasters in different projections (coordinate systems) and output the result in yet another projection by performing a “fast” projection-on-the-fly. You may specify the projection that should be used to create the output raster as one of the analysis properties set in the Options dialog. Projection-on-the-fly won’t work for rasters that have no projection information stored with them.

Permanent coordinate system change
ArcToolbox provides tools for permanently changing the map projection and other coordinate system parameters for a source dataset. For example, if you receive a dataset that doesn't match the map projection of your database, you can use the ArcToolbox Project Wizard to change the new dataset's map projection to match your database.