Custom projection

Custom projections are projections that are suited for only a certain city, specific extent, or area. Many older cities keep all of their data in a custom projection. Custom projections have existed ever since historic data was created, before the invention of GIS systems. During this time the global coordinate systems (GCS) was not utilized, as it was not needed to collect local data. This led surveyors and other mapping profesionals to measure and map data from an unidentified control point. Thus the 0-0 point of a custom projection must be arbritrarily placed, and must be defined against a known projection in order to line up with modern data.

Today, custom projections continue to exist due to many factors. Some of these are a general resistance to change, while others are resistance from surveryors and profesionals who have a vested interest in keeping this system.

Steps to Creating a Custom Projection in ArcMap

 * 1) Add the custom projection to the map frame, followed by a common projection that would be suitable for the data. When selecting this data, keep in mind the extent of the data and the attributes to maintain. For cities and such, examples would include State Plane, or UTM.
 * 2) Begin manipulating the data frame's false Easting, and false Northing until the data is lined up.
 * 3) If this lines up the data, save the projection. This new file can be applied to any other data that is in a custom projection.
 * 4) If the data is rotated in any way, begin changing the azimuth of the data. More advanced users can also change the projection definition to Rectified Skew Orthometric Center. This allows one to rotate the data from a determined center point, and thus line up rotated data.