Introduction
Through this lab, students will experiment with images of the Eau Claire and Chippewa area in western Wisconsin, to learn about different image processing and visual interpretation techniques offered in ERDAS IMAGINE 2013. Students will explore image subsetting, image fusion, radiometric enhancement, resampling, and linking images with Google Earth.
Methods
Image subsetting:
When analyzing aerial imagery, the entire image is commonly not needed and focus is given to a specific area. This area is called the area of interest (AOI). Creation of an AOI can be accomplished by either utilizing the inquire box or a shape file. The inquire box is much more simple but restricts the shape of the AOI to a square or rectangle. Therefore, utilizing a shape file to create an AOI is more versatile and practical in most situations.
Creation of AOI through use of the Inquire box
Figure 1: An inquire box can be added to an image opened in ERDAS by right-clicking anywhere in the view and selecting "Inquire Box."
Figure 2: After "Inquire Box" is selected a white outlined rectangle appears on the screen along with the inquire box viewer. The inquire box can be moved by clicking within the box and dragging it. The size of the inquire box can be changed by clicking a corner of the box and dragging it to the desired shape and size. The inquire box viewer displays the location of the inquire box.
Figure 3: Once the inquire box is in the proper location, click the "Raster" tab, then "Subset & Chip", and then "Create Subset Image" to bring up the Subset window.
Figure 4: The Subset window. Here the input and output files are selected and the button "From Inquire Box" is clicked. Once the "From Inquire Box" is clicked the Subset Delineation should be the same as in the inquire box viewer. Clicking "OK" will yield an image of only the area inside the inquire box.
Creation of AOI through use of a shape file
Figure 5: Once the desired image and the shape file are added to the same view in ERDAS, the shape file needs to be selected to create the AOI. After selecting the shape file, navigate to the "Home" tab and click "Paste form selected object" and a dashed line should appear around the area. Go to "File", "Save As", and "AOI Layer As" to save the area within the dashed lines as a aoi file.
Figure 6: Once the aoi file is created, go to the "Raster" tab and select "Subset & Chip", then choose "Create Subset Image". This time instead of clicking the "From Inquire Box" button, the "AOI..." button on the bottom of the screen is selected. Choose the aoi file that was just created and click "OK" to yield an image of only the AOI.
Image Fusion:
Sometimes more spatial detail is needed for analysis than what is available in the imagery. When this happens, a solution is to use a technique like pan sharpen to merge two images of the same area together. By "pan-sharpening" an image with course spatial resolution with an image of finer spatial resolution in the panchromatic band, the image of coarse resolution will adopt the finer resolution allowing for easier visual interpretation.
Pan Sharpen
Figure 7: With the coarse spatial resolution image opened in ERDAS, go to the "Raster" tab and select "Pan Sharpen" and then "Resolution Merge" to open the Resolution Merge window.
Figure 8: The Resolution Merge window. Here the inputs and output images are selected as well as the method and resampling technique. When the settings are correct, clicking "OK" will yield a new pansharpened image.
Radiometric Enhancement Techniques:
When atmospheric conditions cause imagery to become unclear, techniques like haze reduction can be used to eliminate the disturbance.
Haze Reduction
Figure 9: With the desired image opened in ERDAS, go to the "Raster" tab and select "Radiometric" and then "Haze Reduction" to open the Haze Reduction window.
Figure 10: The Haze Reduction window. Here the input and output files are selected as well as other settings. With the settings correct, clicking "OK" will yield a clearer image devoid of atmospheric disturbance.
Resampling:
Resampling an image simply means changing the pixel size. Depending on the analysis the pixel size can be reduced (resample up) or increased (resample down).
Figure 11: With the desired image opened in ERDAS, go to the "Raster" tab and select "Spatial" and then "Resample Pixel Size" to open the Resample window.
Figure 12: The Resample window. Here the input and output files are selected, the resample method is chosen, and the new size of the pixels is entered into the "Output Cell Size" option. Clicking "OK" will yield a new image with different pixel sizes (assuming the values were changed in the "Output Cell Size" option).
Linking Views with Google Earth:
At times, aerial imagery can be difficult to interpret visually when zoomed in to a large extent. One possible way around this would be to use Google Earth as a selective key. Because the imagery in Google Earth tends to be much more detailed, features can be distinguished more readily. Not to mention the supporting selectable icons and text to help further distinguish features on the landscape. New to this version of ERDAS is the ability to link views with Google Earth to compare the imagery with ease.
Figure 13: With the desired image opened in ERDAS, go to the "Google Earth" tab and select "Connect to Google Earth" this will open Google Earth in a separate window and allows the rest of the options in the "Google Earth" tab to be selected.
Figure 14: Next, click "Match GE to View" to show the same spatial extent in Google Earth as what's present in the imagery. Then by selecting "Sync GE to View", zooming and panning around
the image in ERDAS will move the imagery in Google Earth in the same fashion allowing for quick and easy visual interpretation.
Results
Each screenshot features the original image on the left and the enhanced image on the right.
Figure 15: The image subset created through use of the inquire box.
Figure 16: The image subset created through use of a shape file.
Figure 17: The pansharpened image. The reflective image had a spatial resolution of 30m and was pansharpened with an image in the panchromatic band with a spatial resolution of 15m. The pansharpen gave the image deeper, richer colors and more texture.
Figure 18: The haze reduced image. Notice the absence of the "white film" on the new image and the reduction of clouds in the lower right corner of the image.
Figure 19: The nearest neighbor resampled image. There is not much of a difference between the quality of the images. The only noticeable difference is the arrangement of pixels.
Figure 20: The bilinear interpolation resampled image. Here there is a noticeable difference between the images. The resampled image has smaller pixels than the original across the entire image and results in a higher quality image.
Figure 21: ERDAS with imagery of UWEC synced to Google Earth imagery. Notice how in ERDAS, only pixels can be seen and visual interpretation is difficult to impossible. However, linking the views with Google Earth allows an analyst to distinguish features that were undistinguishable in ERDAS.
Data Sources
UWEC Department of Geography and Anthropology
