ARC GIS (Geografical information system)

A valuable feature of ArcMap is something called “on-the-fly re-projection”, which means that the user can add datasets that are in different projections and ArcMap will make them line up in the same coordinate space. A critical element of this on-the-fly re-projection process is that each of the layers has its projection defined (i.e., that you, the user, tell the software how each layer is projected). Some of the datasets you come across will already have their projection defined. However, if when adding a dataset to ArcMap you receive a warning message or the layer does not line up with other layers, the dataset’s projection is either undefined or defined incorrectly. The solution is to properly define the projection using ArcToolbox (to tell the software how your data are projected).