Registration

Registration assigns georeferencing information to images or vector files by transforming an unregistered image to align to a registered one. This is done by selecting control points on the unregistered file and matching them to the corresponding locations on the registered file.

Transformation Type	Description
Affine	The affine transformation (or 1st order polynomial) is a six parameter transformation. It accounts for rotation in X and Y, shift in X and Y and differential scaling in X and Y. The affine transformation is useful in registering all types of raster imagery and vector data, though the results may not be as good as when using a higher order polynomial.
Similarity	The similarity transformation is two-dimensional. Similarity transformations are conformal (that is, angles are preserved between intersecting linear features) and account for rotation in X and Y, shift in X and Y and scale. The similarity transformation is useful for registering files which were digitized from media with little distortion, such as mylar, or where the distortion is uniform throughout.
Polynomial	The higher order polynomials are useful in registering vector data with varying localized distortions. The greater the local distortions, the higher the polynomial order (degree) to be used. The higher the order of the polynomial, the greater the amount of computer time required to register the image.

Transformation Type

Description

Affine

The affine transformation (or 1st order polynomial) is a six parameter transformation. It accounts for rotation in X and Y, shift in X and Y and differential scaling in X and Y.

The affine transformation is useful in registering all types of raster imagery and vector data, though the results may not be as good as when using a higher order polynomial.

Similarity

The similarity transformation is two-dimensional. Similarity transformations are conformal (that is, angles are preserved between intersecting linear features) and account for rotation in X and Y, shift in X and Y and scale.

The similarity transformation is useful for registering files which were digitized from media with little distortion, such as mylar, or where the distortion is uniform throughout.

Polynomial

The higher order polynomials are useful in registering vector data with varying localized distortions. The greater the local distortions, the higher the polynomial order (degree) to be used.

The higher the order of the polynomial, the greater the amount of computer time required to register the image.

The registered geographic coordinates (control points) for the image or vector file are stored in an XML file. This is in the same location as the file and has the same file name with the extension .rxl. If necessary, files can be re-registered with the new control points overwriting the old ones.