THE UNIVERISTY OF CALIFORNIA AT BERKELEY
ESPM 271
SPATIAL DATA GEOREFERENCING LAB
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Purpose: A geometric correction is usually performed on images that are not georeferenced in a geographic coordinate system (utm, lat/long, etc.). These data have x,y coordinate values but these values relate to nothing on the ground. This exercise will help you to georeference an image to a base map. The map that you will use is a local map of the area (scale 1:24,000). Take some time to study the map and become familiar with certain land marks. You will need to pick out from the map at least 6-8 good ground control points that you can find on the image.
Materials: A topograhic map of the Oakland West, California; the 1990 TM image of the Berkeley area.
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NOTE: Be sure and use the Help provided for you in the PCI software for each program that you use. This will help you to further understand the programs.
GEOREFERENCING AN IMAGE TO A MAP
A flowchart for the georeferencing process
STEP 1: GEOMATRIC CORRECTION SET UP
¨Click on icon "GCPWorks", a GCPWorks Setup panel will appear. What you need is to select options in the three portions of the panel. Under "Processing Requirements" choose "Full Processing", use "Polynomial" as Mathematical Model, under "Source of GCPs", select "User entered coordinates". In this lab, we are going to geometrically correct a portion of a TM image using the corresponding map as reference. At last, you can preview your "corresponding Processing Steps" which guide you to go through all the procedures. Then click "Accept".
STEP 2: SELECT UNCORRECTED IMAGE
¨Begin with "Select Uncorrected Image." Choose file "berkeley.pix" under your own directory, notice that you are not allowed to go to other steps without completing the respective step in order, highlight the image file you want to carry out geometric correction, click "ok".
¨Under the window of "Database Channels", assign three channels to the three image planes. Then "Load & Close". Two image windows, "Uncorrected Image Overview" and a smaller one with full resolution named "Uncorrected Selection" will show up. Notice that while you put the cursor and click on either of the images, the corresponding cursor on the other image will ram to corresponding places. Try to apply different image operation for each image window, e.g. using different zooming and filter technique to distinguish and target the ground control points in the image.
STEP 3: DEFINE GEOREFERENCING UNITS
¨Choose "Metre" as georeferencing system unit, click on "Accept".
STEP 4: COLLECT GCPS
Here are some GCPs for your reference. (Attached Page)
¨Choose "Collect GCPs", look for GCPs that are easy to locate on both the image and the map such as road intersections, building corners, end points of land interfacing with the ocean, and other obvious land marks that can be found from both the image and the map. You can lunch the "GCP Selection and Editing" task for this purpose. Click the cursor on the right pixel you choose, uncorrected image coordinates "uncorr" will be shown in the window with coordinates in units of "pixels" and "lines". Try to move the cursor around, notice that the coordinates are changing, until you consider the point as a desirable GCP. Find the corresponding point on the map, read the UTM map coordinates from your map, and type them into "georef" with coordinates of Eastings and Northings. Click on "Accept as GCP" once you decide to treat it as your ground control point. Check your "uncorrected image overview", your first GCP has been labeled as number "1", using the same method to find more points.
¨You can also select a point, but not treated as a GCP. Instead you want it to be your checking point. Click on "Use as check pt", it will not be included in the computation for the spatial transformation model, but used as a test point for georeferencing accuracy. Note that the "Accept " button will move the GCP down into the "Accepted GCP" list, the "Use as Check Pt" button will move the point down into the Check GCP list, and the Delete button will remove the point. After an action button is used, a new blank GCP with a unique Id will be generated and placed in the GCP Editing area.
¨Notice that from the 4th GCP you choose, the software is able to predict the georeferenced coordinates, since you are using the 1st order polynomial spatial transformation model. After you finish choosing your 4th GCP, errors will be calculated. You can edit your GCPs by adding or deleting GCPs until your are satisfied with them. Usually the GCPs with large errors are deleted, but they may not be the worst GCPs. After you have finished your 6th GCP, the model will be able to use the "2nd order" polynomial to fit, naturally the 2nd order button will be activated.
¨ The GCP Scatter Plot Report can be activated by selecting "GCP Scatter Plot" from the "Reports" menu on the GCP Selection and Editing panel. This report shows the X and Y residual errors for each GCP point on a cross hair graph. This report will be updated each time the GCP list is changed. It may be popped down by selecting the `Close' entry from the window frame's pulldown menu. The Image Fit Report can be activated by selecting "Image Fit" from the "Reports" menu. The Image Fit Report shows a graphical representation of the outline of the georeferenced image area, and GCP points. The uncorrected image outline is transformed according to the current GCP model. The Model Coefficients report is simply a textual display of the forwards and backwards transformation equations currently being used. This report will be updated each time the model is updated.
The difference between a "first order polynomial" (left) and a "2nd order polynomial" (right).
¨Pull down "File" menu to "Save GCPs".
STEP 5: Perform Registration to Disk
¨Click on "Perform Registration to Disk" from the main GCPWorks control panel, a panel of "Disk to Disk Registration" will appear. It is used to perform final registration from the uncorrected image file to a final output (geocoded) image file. Any number of channels may be registered at once, to any desired output file with the appropriate type of resampling. Choose "New output file" to create a "PCIDSK" format file. To make sure that pixels in your new dataset are proportional to your original image, adjust the number of pixels and lines according to the right size of the image pixel (30 m X 30 m in both dimensions for this example image). Select "Output file image
channels" by clicking the corresponding channels in the "uncorrected image channels", and change it by using "Reselect". Choose a "resampling mode". Then, click on "Perform registration".
¨Load your georeferenced image from "ImageWorks" to look at your result.
After georeferencing
UTM Coordinates for some selected GCPs
Point # / Easting(m) / Northing(m)GCPs / #1 / 560106 / 4190618
#2 / 559055 / 4185990
#3 / 564650 / 4191715
#4 / 560370 / 4187785
#5 / 559490 / 4191910
#6 / 564484 / 4186378
#7 / 563540 / 4186835
#8 / 561160 / 4190240
Check Pts / #9 / 561061 / 4191475
#10 / 561440 / 4188695
#11 / 565920 / 4191680