Geographic Referencing (georeferencing) establishes the "image-to-world" relationship of source image pixel coordinates to the source map coordinates (known within the Geographic Calculator as the reference coordinate system). A source image must be georeferenced in order to be used within a mapping/geographic framework.
The Georeferencing tool (Georeferencer) allows you to assign coordinates to an image to locate the image in the world. Once an image has been georeferenced, it can be transformed to any other appropriate projection for display in other GIS software. The Georeferencer can be launched directly from a Transform Job or from the application’s Window menu. If an image you want to transform does not have coordinates assigned to it in the form of a reference file, you will need to Georeference the image prior to Transformation.
The Georeferencer is made up of three main panels: the Source Data Viewer, the Reference Viewer, and the Reference Point List. The Georeferencer also provides access to the Projection Recovery Tool and the ability to Create a Fitted Coordinate System for referenced data.
When you reference a raster image, a reference file (Blue Marble Reference Settings file (*.rsf), a MapInfo Table file (*.tab), an ECW Reference file (*.ers), or an Esri World file (*.tfw, *.jgw, etc)) is created which contains the relationship between source image coordinates and reference coordinates. Once this relationship is established, any source coordinate can be transformed into the corresponding reference coordinate and vice versa.
Source Image Coordinates
Image data is arranged into rows and columns of pixels. The objective of georeferencing is to establish a relationship between image pixel coordinates and a corresponding geographic coordinate system. Source image pixels are arranged exactly as they were initially digitized (or captured) during a scanning/collection process. In order to use an image in a mapping system these arbitrary image coordinates must be transformed into a geographic map coordinate system.
The georeferencing process requires that you define a series of reference points that are both clearly visible in the image and have known geographic coordinates. Examples of such points are grid intersections and ground survey control points on a scanned paper map, or a road intersection on an aerial photograph that has been ground surveyed or observed with GPS. The coordinates for reference points must all be specified in the same geographic reference coordinate system.