In this section:
Small errors can exist in the latitude and longitude of each pixel when a satellite's true orientation and position are not known exactly. In this case the data view image may not line up correctly with coastline overlays. The image may appear to be shifted, rotated, or sheared in comparison to the coastlines. You can use the navigation correction tool to write a correction value to the data file so that the data view image lines up correctly with actual geographic features. Navigation correction is not likely to be necessary if the data file has been produced recently, but may be needed for older data from CoastWatch IMGMAP (.cwf) files for example, or from experimental data products and satellites.
The following figure shows the navigation correction controls, with visual correction mode, manual correction fields, and data variable list:
You can use the visual correction modes to apply corrections by clicking and dragging on the data view. The Translation mode corrects the image by shifting the origin by some number of rows and columns. The Rotation mode corrects the image by rotating it about the view center. Analogous manual corrections may be applied using the manual controls by setting the field values and clicking Perform. The cumulative set of corrections may be reset by clicking Reset correction to identity option. The manual correction options also include a general affine option, which is applied using matrix multiplication:
|r'| |a c e| |r| |c'| = |b d f| |c| |1 | |0 0 1| |1|
Navigation corrections should only be applied to sensor and sensor-derived data variables in a data file -- not earth location, graphics, or viewing angle data. You must select which specific variables to correct. When corrections are performed either visually or manually, they are applied only to the selected variables in the list. Examples of data variables that may require correction include AVHRR channel data, SST and cloud. The following figure shows an example of an uncorrected and corrected SST image: