Create a Quick Mosaic
Lesson content
Lesson 1 of 1
Create a Quick Mosaic
In this quick guide, you will:
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Use the Quick Mosaic feature in the Layer Manager to build a virtual mosaic from 9 georeferenced orthophoto tiles. * •
Save the virtual mosaic to disk. * •
Optionally edit the default stretch type and band order in the associated ENVI header file. The result is a true-color image with better contrast than the original mosaic.
Sample Data
Download sample data below. Then extract the contents of the .zip file to a local directory.
[USGS_HighResOrtho.zip
890 MB
DownloadArrow down with horizontal line beneath it](assets/USGS_HighResOrtho.zip)
Background
If you will be using remote sensing imagery to analyze a large study area, one image may not cover the entire area. This is especially true with high-resolution imagery. Data vendors typically deliver imagery in small tiles so that you can only download what you need.
Instead of managing multiple images in ENVI, you can create a mosaic from them. A mosaic is one large image of a study area. A benefit of creating a mosaic is that you only have to run a process on one larger image instead of several tiles. Also, you can calculate statistics and perform spectral analysis on the entire mosaic instead of doing these processes repeatedly with each tile.
The U.S. Geological Survey (USGS) is one example of a data vendor who distributes tiled image products that are already georeferenced to a standard map projection. They also apply color balancing and radiometric quality measures so that analysts can easily create a mosaic where the visual quality appears to be seamless from one image to the next.
Let's look at an example.
Open and Display USGS Orthophotos
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Select File > Open from the Menu bar. An Open dialog appears. 2. 2
Go to the location where you saved the sample data, then go to the Quick Mosaic directory. 3. 3
Use the Ctrl or Shift key to multi-select the following files:
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Click Open. The images are added to the Layer Manager and displayed in the Image window. 2. 5
Monitor the Progress bar in the lower-right corner of the ENVI application, and wait for raster pyramids to build for all 9 tiles. 3. 6
Click the Zoom to Full Extent button in the Toolbar to see the individual images. ENVI positions them correctly because they are georeferenced.
Create a Quick Mosaic
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In the Layer Manager, right-click on the View item (located at the top) and select Quick Mosaic.
The Quick Mosaic dialog appears. You do not need to change anything here. With Remove Layers set to Yes, the individual TIFF images will be removed from the Layer Manager when the quick mosaic is created.
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Click OK. ENVI creates the quick mosaic, adds it to the Layer Manager, and displays it in the Image window.
This concludes the Quick Mosaic part of the quick guide. The process is simple and fast. However, the mosaic is relatively dark with little contrast, and its colors do not represent a true-color view. Next, you will improve the visual quality of the mosaic. These steps are optional.
Optional: Improve the Visual Quality of the Mosaic
You will use the Edit ENVI Header tool to set the Default Stretch and Default Bands to Load metadata fields in the mosaic. To do that, however, you must first save the virtual mosaic to disk.
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From the Menu bar, select File > Save As > Save Raster As (ENVI, NITF, TIFF, DTED). The Data Selection dialog appears. 2. 2
Select Quick Mosaic and click OK. The Save File As dialog appears. 3. 3
Keep the default selection of ENVI for the Output Format, and leave the Data Ignore Field empty. 4. 4
Enter an output file name of QuickMosaic.dat. 5. 5
Click OK. ENVI writes the mosaic file to disk. 6. 6
In the search window of the Toolbox, enter edit. 7. 7
Double-click Edit ENVI Header. The Data Selection dialog appears. 8. 8
Select QuickMosaic.dat and click OK. The Edit ENVI Header dialog appears. 9. 9
Click the Add button at the top of the Edit ENVI Header dialog. The Add Metadata Items dialog appears. 10. 10
Use the **Ctrl** key to multi-select **Default Bands to Load** and **Default Stretch**.
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Click OK in the Add Metadata Items dialog. Both metadata items are added to the Display tab in the Edit ENVI Header dialog.
The USGS delivers orthophotos in this format: Band 1 = Red, Band 2 = Green, Band 3 = Blue, Band 4 = Near-infared. However, the orthophotos do not contain any wavelength metadata, so ENVI does not know what wavelength each band represents. When an image does not contain wavelength metadata, ENVI displays the first band as blue, the second band as green, and the third band as red. Thus, the displayed image does not represent a true-color view. You will correct this in the next step.
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In the Default Bands to Load section, click the red color box. The Select Band dialog appears.
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Select Band 1 and click OK. Band 1 is now assigned to the red color channel. 2. 14
Click the green color box. 3. 15
In the Select Band dialog, select Band 2 and click OK. Band 2 is now assigned to the green color channel. 4. 16
Click the blue color box. 5. 17
In the Select Band dialog, select Band 3 and click OK. Band 3 is now assigned to the blue color channel.
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Click the Default Stretch drop-down list and select % Linear. The default value is 2%, which is what you want. 2. 19
Click OK in the Edit ENVI Header dialog. The quick mosaic is removed from the Image window and redisplayed with the new color combination and stretch.
It has a more natural, true-color appearance.
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This concludes the exercise.
In summary, creating a quick mosaic from multiple images allows you to apply display enhancements and run analytics on one image rather than each individual image.
Additional Resources
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Create a Color-Balanced Mosaic quick guide
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