Rectification or "rubber sheeting” changes the geometry of a raster image using multiple control points that provide georeference control. Depending on the chosen geometric transformation (referencing) method, the correct process can decrease geometric distortions, reorient and rescale an image so that its lines are parallel to the axes of a specified geographic coordinate system.
In this example, you’ll take a historic map of an unknown projection and georeference and rectify it to the WGS 84 coordinate system. The map has intersecting lines of longitude and latitude which will serve as good locations for control points. A sufficient number of ground control points distributed evenly across an image will need to be specified. To rectify the image, the Quadratic transformation method will be used.
1.In Adobe Photoshop, open Dakotas.jpg from the Tutorials Data folder.
Notice that the image is not georeferenced. Next, you’ll import existing ground control points, move several of them to the correct position, and add two additional points to complete referencing.
2.In the Geographic Imager panel, click the Georeference button.
Next, you'll add some control points.
3.Click the Import Additional Control Points button. Browse for dakotas_gcp.csv, then click Open to add them to the Georeference dialog box.
Six ground control points appear in the image view as well as the control point list. For the purpose of this tutorial, four of the points have already been placed in the correct location. Next, you’ll move two control points (Point 1 and Point 3) to their correct positions. Notice that Point 1 has a world X and Y coordinate of -104.049 and 49.000 decimal degrees.
4.On the image view, zoom into Point 1 (top-left area of the image). Enable the Select Points button, then click and drag Point 1 to where the 49° line first perpendicular line intersect (see the following graphic).
Point 1 is now in the correct position of approximately -104.000 and 49.000 decimal degrees.
In the reference list, notice that Point 3 has a world coordinates of -97.000 and 49.000 decimal degrees. You'll move Point 3 to the correct position as well.
5.On the image view, zoom into Point 3 (top-right area of the image). Move Point 3 to where the 49° line intersects with the 97° line.
Point 3 is now in the correct position of approximately -97.000 and 49.000 decimal degrees. Next you’ll add two more ground control points and then choose the appropriate reference method.
6.Click the Add Point button. Pan to the top-center area of the map. Click where the lines of 101° and the 49° intersect. (If necessary, use the Select Points tool to move it into the proper position.) In the control point list, double-click the WX or WY cell.
7.On the Edit Location dialog box, enter 49 into the Lat box and -101 into the Long box, then click OK.
Point 7 has its world coordinates updated.
8.With the Add Point button still enabled. Pan to the bottom-center area of the map. Click where the lines of 101° and the 43° intersect. For Point 8, double-click the WX or WY cell. Enter the world coordinates of 43 into the Lat box and -101 into the Long box, then click OK.
With eight control point pairs, you can use a higher order method to rectify the image.
Having a sufficient number of control points does not guarantee the result will be a plausible solution; a trial-and-error approach may be appropriate to obtain the best results. See Using Georeference for more information about georeferencing and polynomial methods.
Next you'll specify WGS 84 as the image coordinate system and then choose a method to complete the rectification.
9.Click the Image Coordinate System Specify link.
10. On the Choose Coordinate System dialog box, click the [No Coordinate System Specified] link.
11. In the Specify Source Coordinate System dialog box, expand the Coordinate Systems > Geodetic > World category, select the WGS 84 coordinate system and click OK.
12. Click OK to close the Choose Coordinate System dialog box.
13. To rectify the image, click the Rectify using check box. Click Proceed to rectify the image when the message dialog box appears.
14. From the Rectify using drop-down list, select Quadratic Polynomial (min 6 points). Click OK.
The Quadratic Polynomial method uses at least six ground control points to calculate better georeferencing. The pixel and world error values will have minimized (horizontally scroll the reference list to see error values.) WGS 84 is being used as the image coordinate system because of the easily recognizable longitude and latitude lines as reference. Using a combination of the quadratic polynomial method and WGS 84 coordinate system will help rectify this image into a useable georeferenced map.
15. To finish the image rectification, click Transform. (Your pixel size may vary slightly due to how precise your control points were.)
The image uses the Transform tool to complete the rectification. After the Transform tool finishes, the image is rectified to the WGS 84 geographic coordinate system. Notice that the lines of longitude and latitude on the map are straight. To verify the georeference, use the Validate tool in the Georeference dialog box.
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