Spatial Referencing - Georeferencing

Concepts
   georeferencing
   georectifying
   transformation
   referencing toolbars

Georeferencing
Georeferencing refers to the process of assigning a spatial reference to something.  In most cases, it refers to assigning spatial reference to a raster.  The georeferencing information is in addition to the projection system, as it describes the coordinates of the upper-right corner of the raster and size of each pixel.  If you have scanned in a map, such as a tax map or historic map, you will need to georeference the map.  Georeferencing is accomplished by linking points in the raster to points in a known data layer, then updating the georeference information.  Georeferencing does not alter the raster, it only updates the reference information.



Georectifying
Georectifying takes georeferencing a step further and warps the raster (creating a new raster in the process) as necessary to fit the links created in georeferencing.  In some cases, such as a scanned-in map, the original raster should fit without any stretching or warping (because it is a correct 2-d representation, which is known as planimetrically correct).  In other cases, such as an aerial photo, portions of the photo are taken at oblique angles, are thus not planimetrically correct, and require warping in the georectification process.

Transformation
Transformation is the process used to change the reference system of GIS data.  This occurs when reprojecting data between NAD27 and NAD83, and it also is used when rectifying a raster.  During the rectification process, you have to choose one of 3 transformation methods:

1.)  Affine (first order) - a simple transformation which just shifts the raster and scales it.  Three or more links are required.  No stretching or warping occurs.  This method should be used when referencing planimetrically correct raster images.

2.)  Polynomial (second order) - a more complex transformation which will stretch or warp the raster as required.  Six or more links are required.

3.)  Polynomial (third order) - the most complex transformation with the highest degree of stretching or warping.  Ten or more links are required.



Referencing toolbars
In ArcMap, there are three toolbars to use for adjusting spatial reference:

1.)  Georeferencing - tools for georeferencing/georectifying rasters


2.)  Spatial Adjustment - tools for transforming vector data


3.)  CAD Transformation - tools for georeferencing CAD files