Rectification is necessary in cases where the pixel grid of the image must be changed to fit a map projection system or a reference image. There are several reasons for rectifying image data:
- comparing pixels scene to scene in applications, such as change detection or thermal inertia mapping (day and night comparison)
- developing GIS data bases for GIS modeling
- identifying training samples according to map coordinates prior to classification
- creating accurate scaled photomaps
- overlaying an image with vector data, such as ArcInfo
- comparing images that are originally at different scales
- extracting accurate distance and area measurements
- mosaicking images
- performing any other analyses requiring precise geographic locations
Before rectifying the data, you must determine the appropriate coordinate system for the data base. To select the optimum map projection and coordinate system, the primary use for the data base must be considered.
If you are doing a government project, the projection may be predetermined. A commonly used projection in the United States government is State Plane. Use an equal area projection for thematic or distribution maps and conformal or equal area projections for presentation maps. Before selecting a map projection, consider the following:
- How large or small an area is mapped? Different projections are intended for different size areas.
- Where on the globe is the study area? Polar regions and equatorial regions require different projections for maximum accuracy.
- What is the extent of the study area? Circular, north-south, east-west, and oblique areas may all require different projection systems (Environmental Systems Research Institute, 1992).
When to Georeference Only
Rectification is not necessary if there is no distortion in the image. For example, if an image file is produced by scanning or digitizing a paper map that is in the desired projection system, then that image is already planar and does not require rectification unless there is some skew or rotation of the image. Scanning and digitizing produce images that are planar, but do not contain any map coordinate information. These images need only to be georeferenced, which is a much simpler process than rectification. In many cases, the image header can simply be updated with new map coordinate information. This involves redefining:
- map coordinate of the upper left corner of the image
- cell size (area represented by each pixel)
This information is usually the same for each layer of an image file, although it could be different. For example, the cell size of band 6 of Landsat TM data is different than the cell size of the other bands.
Use Image Metadata dialog to modify image file header information that is incorrect.
Disadvantages of Rectification
During rectification, the data file values of rectified pixels must be resampled to fit into a new grid of pixel rows and columns. Although some algorithms for calculating these values are highly reliable, some spectral integrity of the data can be lost during rectification. If map coordinates or map units are not needed in the application, then it may be wiser not to rectify the image. An unrectified image is more spectrally correct than a rectified image.
Some analysts recommend classification before rectification, since the classification is then based on the original data values. Another benefit is that a thematic file has only one band to rectify instead of the multiple bands of a continuous file. On the other hand, it may be beneficial to rectify the data first, especially when using GPS data for the GCPs. Since these data are very accurate, the classification may be more accurate if the new coordinates help to locate better training samples.
Nearest neighbor is the only appropriate resampling method for thematic files, which may be a drawback in some applications. Available resampling methods are discussed in detail later in this chapter.
Registration and rectification involve similar sets of procedures. Throughout this documentation, many references to rectification also apply to image-to-image registration.
Usually, rectification is the conversion of data file coordinates to some other grid and coordinate system, called a reference system. Rectifying or registering image data on disk involves the following general steps, regardless of the application:
- Locate GCPs.
- Compute and test a transformation.
- Create an output image file with the new coordinate information in the header. The pixels must be resampled to conform to the new grid.
Images can be rectified on the display (in a Viewer) or on disk. Display rectification is temporary, but disk rectification is permanent, because a new file is created. Disk rectification involves:
- rearranging the pixels of the image onto a new grid, which conforms to a plane in the new map projection and coordinate system
- inserting new information to the header of the file, such as the upper-left corner map coordinates and the area represented by each pixel