Lab 17: Creating Image Mosaics

Transcription

1 Lab 17: Creating Image Mosaics Study areas often extend spatially across more than a single satellite scene. In these situations, one can analyze each image separately or merge (mosaic) them to create a single combined image. Mosaicking is often efficient, because you can process one big image rather than several smaller ones. Problems arise because adjacent satellite images may have been acquired on different dates with different atmospheric conditions and different geometry. One way to deal with this is to carefully atmospheric correct each image first and then join them. Another is to use routines to adjust the brightness (DNs) of the images so that they match more closely. Ideally you might do both. If you mosaic two images without using one of these techniques, you may see a distinct seamline between the images, which can result in artificial differences (for example in a derived map) from one side of the seamline to another. On the other hand, if you artificially change brightness values, you are changing the spectral information (original DNs), so you need to be thoughtful about this. In today s lab you will learn how to use the mosaic tool in Erdas. We will not atmospherically correct the images first, but you should for a real project. 1. You will work with two images from the Great Divide Basin (in the lab17_mosaic folder) that include the town of Wamsutter along I-80. Both images were acquired in June 2013, but on different days. One image is called 3531_wamsutter_east_june2013.img. The other image is called 3631_wamsutter_west_june2013.img They are from adjacent Landsat paths. a. [READ ALL OF THIS INSTRUCTION BEFORE YOU OPEN THE IMAGES, OR YOU WILL HAVE TO DO IT AGAIN!!] Open the two images in a single viewer with 3531 on top of 3631 without allowing Erdas to contrast stretch them. To turn off contrast stretching, and to open two images in one Viewer simultaneously, you have to use the Raster Options tab when you open the images. Check the No Stretch box and uncheck the Clear Display box (if it is checked). Also, be sure to check the Background Transparent box, so that the black pixels around the images don t obscure the other image. Remember that you can rearrange the layers in the Contents box on the left by dragging them up or down. b. Notice the north-south seam where the two images meet. (Worksheet question #1). We ll be experimenting to combine the images so that this seam is less obvious. 2. Start the mosaicking tool (Toolbox tab/tools area à Mosaic/Mosaic Pro). This tool includes many options for creating image mosaics. I ll walk you through some of them

2 and you can experiment with others as time permits. This is a useful tool that may be needed for some of your semester projects. a. On the Mosaic Pro window, choose Edit/Add Images and add both of the images. You can do this either at once by control-clicking the two add adding, or one at a time. (See result on following page). Make sure that the 3531 (east) image is on top. If it isn t, you can select it by clicking on it (it s box should turn yellow) and then clicking the icon in the top right area of the mosaic pro that looks like a white rectangle on top of two grey rectangles (send selected image to top). The Erdas mosaic pro window with wire frame outlines of the two images displayed. b. One way to mosaic images is to just overlay them with no adjustment in pixel values. To do this, from the Mosaic Pro window choose Edit/Set Overlap Function and choose Overlay. Now from the Mosaic Pro window choose Process/Run mosaic. You will be prompted for the name of an output file (name it something so that you can remember that you used the overlay option). Also, click on the Output Option tab and check the Stats Ignore Value box, which should be set to 0. This will exclude the black pixels around the image from contributing to the image statistics. c. Display the result in a 2 nd regular Erdas 2D Viewer (keep your original unmosaicked images displayed) without allowing Erdas to contrast stretch it. Compare this to the view in which the two images are displayed on top of each other (note that you can link the viewers, zoom in on the same portion of the

3 seamline on both, and use the pan tool to examine the length of the seam. (Worksheet question #2). 3. Now we ll explore color correction as a way to reduce the visibility of the seam line. We ll do another mosaic, again using the overlay option under Set Overlap Function but this time using histogram matching as well. a. In the Mosaic Pro tool, set the overlap function to overlay as you did previously. Then choose Edit/Color corrections and choose histogram matching (at the bottom). Click on Set and for the Matching method choose For all images. Run the mosaic as you did before, naming the output something that allows you to remember what you did, and telling Erdas to ignore the black pixels (0). b. Display the result and compare to your previous mosaic. (Worksheet question #3). c. Use the Help button on the Mosaic Pro and navigate to a description of histogram mapping (you may have to allow blocked content ). (Worksheet question #4) 4. Now let s try another overlap function called feathering. Feathering adjusts the DNs of the pixels along the seam line to make them match better, reducing the amount of adjustment with distance from the seam line. a. First, turn off the color correction that you used last time by going to Edit/Color corrections and unchecking the histogram matching box. b. Now go to Edit/Set overlap function and choose Feather. Run the mosaic as you ve done previously and examine the result. How does the seam line look now? Can you find it? 5. Experiment with other mosaicking options (you might need to read more of the Help), and see if you can create a nearly seamless mosaic. This is difficult, but if you are successful, paste your result into your worksheet, along with a caption describing how you created it for 5% extra credit on this worksheet. 6. Note that in Mosaic Pro you can save a mosaic file (File/save) so that you can pick up where you left off, without having to reload all of your images, etc. This can be a timesaver.

4 Name Worksheet Lab 17: Mosaicking 1. Describe the appearance of the seamline where the two images meet. Why is there a difference in color across the seam? What are possible factors that might be causing this? 2. When you display the result of mosaicking using the overlay option, is the seam still visible? Does this image look any different than the unmosaicked images displayed together? 3. When you mosaic using the overlay method with histogram matching of the histograms from all images, what happens to the visibility of the seam line? Does this mosaic look better than the previous one? Is it seamless? 4. According to the Erdas Help, what does histogram matching do, and in what situations is it sometimes useful?

5 5. A common issue in remote sensing is the presence of clouds. Explain how you might use the mosaic tool, along with two images of the same place from two different times, to help eliminate clouds. 6. If you successfully created a nearly seamless mosaic, paste it below along with a caption describing how you made it for 5% extra credit on this worksheet.