There are three possible approaches to using a paper map to correct an image. The first, and best, is to perform an Image to Map tiedown using a digitizing table. The second method is to scan the map, and geocode the scanned map; then use the image of the map to perform an Image to Image tie down with the original image. The third, and least accurate method, is to perform an Image To User coordinates tie down, estimating (or calculating by hand) the locations of features on the map based on their position on the map grid.
Since the first approach is the best, we will go through the steps for it. Before even starting GCPWorks, it is necessary to find and connect a supported digitizing table or tablet to the serial port on the computer where GCPWorks will be run. The tablet installation instructions provided by the manufacturer, and those supplied by PCI should be followed carefully.
Start GCPWorks and select Full Processing, Polynomial Model, and Hardcopy Map on Digitizing Table, as the setup. Accept the setup and select the Spot scene as the uncorrected image file.
The ``Tablet Setup and Map Tie-down'' panel will be launched to initialize communication with the digitizing table, and to establish the positioning of the map on the table.
GCP Collection is the next step. It is necessary to move back and forth between the digitizing table, and the on-screen uncorrected image for each GCP. Note that when map tiedown is being performed, the GCP guessing is only useful if the point is selected on the digitizing table before the point is selected on the uncorrected image. The GCP guessing moves the cursor to the approximate corresponding location, but of course it is not possible for GCPWorks to move the tablet pointing device.
Once GCPs have been collected the user should proceed to final registration. At this stage an output file may be created using the ``New'' entry on the GCPWorks ``File'' pulldown menu. Because the GCPs have already been collected, GCPWorks is able to determine the appropriate georeferenced bounds for an output file that will hold the entire input image after registration. However, it may be desirable to alter the pixel size of the output file, depending on the desired resolution of the final product. Finally perform the registration.
Note that GCPWorks will save the tablet communication and tiedown information between executions in the GCPWorks.aux file, so it may not be necessary to re-enter this information in the tablet setup step; however, it is always necessary to enter the panel and activate the tablet communications.
If it is desirable to mosaic several images based on tiedown to a map, it may be best to collect, and save GCPs for each input image using the above steps, and then exit GCPWorks. Re-enter GCPWorks, create the final mosaic file, and mosaic each input image into it using the GCPs collected earlier.
See Also: Tablet Setup and Map Tie-down