"Creating a Mosaic of Aerial Photos"
Creating a Mosaic of Aerial Photos by Jennifer Sieracki 6/05/01 The purpose of this how-to document is to describe in detail the steps that can be taken to create a mosaic of color-infrared aerial photos. The steps below do not describe the only possible method that can be used, but these steps have been found to be very affective. Scan Aerial Photos Once the aerial photos are obtained, they must be scanned to be placed in digital format. Below I describe how to scan using Adobe Photoshop 5.0.2. Make sure the scanner glass is clean. 1. Select the Import/Twain_32… command from the File menu. 2. Set Scan Type to Color Photo. 3. Choose a dpi of 400-600. 4. Set Scan Quality to Fine. 5. Choose the Preview button. When the photo shows up on the input screen to the right, resize the dotted box to choose the area to be scanned. 6. Choose the Scan button. The image will then appear in a window in Photoshop. Close the Twain 32 dialog box. Preparing the Image The image must be radiometrically and spectrally enhanced in Photoshop prior to registration. 1. Choose Adjust/Variations… from the Image menu. Click on each of the scanned images to adjust the current pick. Once happy with the current pick, hit OK. 2. Choose Adjust/Auto Levels from the Image menu. The contrast of the image should improve. 3. Repeat these two steps until all images are clear and match one another. 4. Save the image as a Tiff file. Registering the Image In order to register the Image, we will use Imagine 8.4. This program will create an error message when trying to register a .tif, so it must first be imported as an .img file. ERDAS Imagine is preferred for registration over the ArcView Image Analysis Extension. An error message will occur on some occasions when trying to mosaic images that were registered in ArcView. 1. Click the Import button at the top of the screen. 2. Set the the type to TIFF and the media to file. 3. Search for the input file, and place the output file on the proper drive by clicking on the buttons with the yellow folders on them to the right of each line or by using the menu bars below. Click the OK button. 4. Another window will open. Hit the OK button again.Once complete, click OK. Now, the registration of the image may begin. When registering photos, it is best to use recent DOQs since they have a high spatial resolution, and are comparable to aerial photos. 5. Open a Raster Layer in the Viewer window. 6. Single click on the .img file you will be registering. Before hitting OK, click on the Raster Options tab. Make sure your Red band is 1, and your Green band is 3. Click OK. 7. Right click on the viewer and choose Fit Image To Window. 8. Choose Data Prep from the top of the screen. 9. Choose Image Geometric Correction… from the Data Prep menu. 10. From the Set Geo Correction Input File window, click the Select Viewer button. 11. Click inside of the Viewer containing your image. 12. Set Geometric Model to Polynomial. Click OK. 13. Close the Polynomial Model Properties window that pops up. 14. Choose an Image Layer to collect reference points from. 15. Find the DOQ you will be using. Make sure the Reference Map Information is correct. 16. Find similar points on the aerial photo and DOQ. Use the cross-hair icon in the GCP Tool window at the bottom of the screen to pick reference points on each photo. For the first three points, the location will have to be marked on both the aerial photo and the DOQ. For all other point, once a point is placed on the aerial photo, it will automatically be calculated on the DOQ. Use the black arrow to move the points on the DOQ to the appropriate place once it has been calculated. Try to keep the overall RMS error within 15 m. Place from 8-12 points on the photos. Points may be placed in the smaller viewers as well. 17. Once satisfied with your results, choose the resample icon (a tilted grid of four pixels) from the Geo Correction Tools window at the top of the screen. Place the new image in the correct folder, and rename it. Click OK. Once resampling is complete, click OK. 18. Click Exit in the Geo Correction Tools menu. It is not necessary to save any of the information. Manipulating Spectral Bands As you may have noticed, when opening your image, Imagine defaults to a spectral band order with Red being 3 and Green being 1. This can be switched. 1. Choose Interpreter from the top of the screen. 2. Choose Utilities from the menu that pops up. 3. Choose Layer Stack… from the next menu. 4. Select the correct input file. Name an output file and place it on the correct drive. 5. Select the Layer to be 3. Click Add. Next add 2, then 1. 6. Click OK. Once complete, click OK. Mosaicking the Aerial Photos Once all of the aerial photos have been processed as described above, you may begin to mosaic. 1. Choose Data Prep from the top of the screen. 2. Choose Mosaic Images… from the menu. 3. Choose Add Images from the Edit menu. Find your aerial photos, and click Add for each one. Make sure they are in the desired order. 4. At this point you may or may not choose to change some of the option found in the Edit menu. You may adjust the overlay process and image matching, but I have found that these features only enhance the mosaic when the photos are already well matched. 5. Choose Run Mosaic... from the Process menu. Enter the location and name of the ouput mosaic. Encoding the Mosaic Mr. Sid is a geospatial encoder that allows one to maintain high-resolution imagery without using a lot of space. This is not a simple compression file; therefore, it should not be labeled as a compression file in the metadata, and should be considered a Mr. Sid file. 1. First, you must obtain permission to use Mr. Sid. The lab admin will change your profile accordingly. 2. Mr. Sid 1.4 is currently on Sensor. It is found in the startup menu. Open it using the green icon. 3. To add an image, go to File>Add Image. A browse menu will appear. Choose the correct file type. And browse to find the image of interest. 4. Select Tools>Options. Choose the Encode Settings tab. For all USFWS images, set the Encoding Ratio to 20. Set the Zoom Level to 6. Hit OK. 5. Check the Image Information Display in the upper-right-hand quarter of the screen. Make sure that all settings are correct. You can also change the Encoding Ratio here. 6. Finally, make sure the image you want to encode is highlighted and hit the Encode button. The output image will be placed in the same file as the input image. You must have enough space to contain the input image, intermediate files, and the output image. If an error occurs while encoding images other than .tif files, try converting your image to a .tif file in another program, then try to encode again.