Image rectification is the process of transforming an image from an image coordinate system into a map coordinate system. You can also rectify an image to a different satellite or pixel coordinate system. This process is also known as georeferencing or geometric correction.
Rectification is accomplished by matching corresponding image and map points. These points, known as 'control points', are then used to compute the best fit polynomial transformation to align the image to the target map or image. ArcView's Image Analysis extension includes a technique to perform image rectification. The tool is called the Align tool.
This exercise is broken into two parts. Part 1 consists of downloading a 1997 Digital Orthophoto Quad (DOQ) of an area of interest in Delaware and a Digital Line Graph (DLG) reference map. Part 2 involves the actual process of performing image rectification by walking you through the main steps in doing this preprocessing technique. The end result is a georeferenced image in map coordinates.
Part 1. Download Image and Reference Layer
Download a 1997 Digital
Orthophoto Quad (DOQ) image of an area of interest in Delaware. The
images are found on Spatial
Anlaysis Lab web site (Department of Food Resources and Economics,
University of Delaware). Just click on the 1997 orthophotos and then
select an image to download. The orthophotos are 1997 greyscale digital
orthophotographs at 5 meter resolution in Delaware State Plane coordinate
system and NAD83 projection.
Next, download Digital Line Graph (DLG) coverage of the same area as the image you downloaded. Note that the DLGs cover a larger geographic extent than the DOQs. The DLG theme will serve as the reference map for rectification of the image. The Delaware DLGs are found at the Delaware State DRG and DLG Server homepage. First, review the information in selecting the data and the file format information. Once you have reviewed this information, click on the county and follow the instructions in making your selection. Select the ArcView shapefile as the file format.
The DLGs are downloaded
in a WinZip compressed format. You'll need to uncompress the DLG
using WinZip loaded on PCs in Training Center. Notice that the DLGs
are composed of numerous themes. The two themes that will probably
be most useful in this exercise are the:
Rd*.shp - Roads layer
Hy*.shp - Hydrography layer (streams and lakes)
Part 2. Image Rectification using ArcView Image Analysis Extension
ArcView's Image Analysis extension provides an Align tool to perform image rectification. Online documentation on Aligning Images is found by:
- clicking on the Help menu, select Help Topics and click on the Contents tab
- select Extensions --> Image Analysis --> Aligning Images.Follow the steps below to guide you through the process of image rectification.
Start ArcView and view the DOQ image along with DLG theme
Start ArcView and load the Image Analysis extension along with JPEG image support extension.You can change the alignment defaults of the image by making the image active and selecting properties under the Theme menu. This Alignment panel assigns the default parameters for use with the Align tool . Notice what features you can change - link color, snap links to points enable, snapping tolerance. For now, let's leave these features as the defaults.
Add the image as an Image analysis data source and the DLG theme (Feature data source) to a new View.
The Align tool can be used any time two themes are misaligned (themes only slightly misaligned or in totally different coordinate systems) and the theme to be aligned is an Image Analysis theme.
Zoom to the full extent of the image and DLG theme. Notice that the themes disappear and appear only as small points in the View. This occurs because the DOQ and DLG are in different coordinate systems.To begin the alignment process, simply make the DOQ image theme you want to rectify active and click on the Align tool. The first click of the Align tool rescales the image and moves it within the extent or bounding box of the other themes in the view. The Align tool is then used to select 'control points' that may be seen in both themes.
Collecting From and To Points ('control points' pairs). The Align tool is designed to collect control points in an image that correspond to a feature theme or another image. The points located in the image you wish to rectify are referred to as the From points. Points located in the feature theme (DLG theme) or rectified image are called To points. A 'control point' pair is make up of a From point (from DOQ image) and its corresponding To point (DLG theme) and is represented in the view as a graphic link.
Select control point pairs. Look for a ground location (feature) you can find in the DOQ image and on the DLG theme. Click to select a From point in the image. After you select the From point, a rubber-banding line appears indicating to select the corresponding To point in the DLG theme. At this time, the map coordinates are being shown in the status area of the ArcView GIS window. Continue to select pairs of points in this manner until you are pleased with the alignment between the image and the feature theme.
Once you collect at least 4 sets of 'control point' pairs between the DOQ image and DLG theme (best to collect a minimum of 6 evenly distributed 'control point' pairs throughout the image), a root mean square error (RMSE) is displayed in the ArcView status area. You can use the 'Pan to Next Link', 'Zoom to Selected Links', or 'Display Link Error' on the Align popup menu accessed by clicking on the right mouse button in the view to see the RMSE for individual 'control point' pairs.
As the 'control points' are created, the RMSE is calculated and reported in pixels in the ArcView status area. You can view the RMSE for any link by clicking that link and selecting 'Display Link Error' from the Align popup menu that is accessed by clicking the right mouse button in the view. View the other links' errors by selecting 'Pan to Next Link'. The result is displayed in the ArcView status area. The lower the RMSE error the more accurate the rectification.
QUESTION: Define RMSE? Report the RMSE for each link and the total RMSE. What is an acceptable RMSE? Describe how the orientation of the image changes as you add link1, link2, link3 and so forth.Edit 'control point' pairs. The primary tool for editing existing 'control points' is the Pointer tool.
Control points are stored in a graphic layer on top of the image and you can use the pointer tool to adjust their position. To move a From or To point, use the Pointer tool to select it, then drag the point to the new location. When you release the mouse button, it is in its new locations. The RMSE error is recalculated and displayed by clicking the right mouse button in the view and selecting 'Display Link Error'.QUESTION: Edit any links you may have done incorrectly or any with a high RMSE. Report the new RMSE for each edited link and the new total RMSE.To delete a control point pair, use the Pointer tool to select it and then press the DELETE key on your keyboard or you can click the 'control point' pair and choose Delete Graphics from the Edit menu.
What happens behind the scenes? After the first 'control point' is taken, the image is shifted so the From and To locations line up. After the second control point is taken, the scale of the image is adjusted. With the third point, an affine transformation is calculated and the image is altered so that all three points line up. Starting with the fourth point, the RMSE is calculated and displayed for each 'control point' pair. The Image Analysis extension uses first and second order transformations only. You need three sets of 'control point' pairs for a first order transformation and six sets of 'control points' for a second order transformation.
QUESTION: What is an affine transformation? An affine transformation corrects for what three parameters?
As each 'control point' pair is created, the polynomial transformation is calculated based on the number and distribution of the 'control point' pairs in the image and the feature theme. As you collect more 'control point' pairs, the order and complexity of the polynomial transformation may increase. The calculation is automatic and the image is redisplayed in the view.
Saving rectified image and control point pairs. Once you collect all the control points, the RMSE is acceptably low, and the image is rectified to a map coordinate system, you have two options of how to save the image. You can either calibrate or resample the image.Save your rectified image and your links as a point coverage.QUESTION: Explain the difference between calibrating or resampling an image. What type of resampling method is used?That's it for this exercise! Turn in answers to the above questions along with a printout of the corrected image overlaid with the DLG theme and the link point coverage. Zoom in on a particular area of the image so features are actually visible and print the view. If you had any particular problems, please make a note of them in your writeup.
Return to Geog474 Homepage
Last revised on October 27, 1999 by Tracy DeLiberty