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Mosaic Tutorial: Simple Workflow

Mosaic Tutorial: Simple Workflow

This tutorial demonstrates how to use the Seamless Mosaic tool to mosaic six overlapping digital aerial scenes. You will learn about displaying footprints and image data and how to set data-ignore values in the final mosaic. The scenes have a consistent color variation since they were acquired along a single flight path on the same day, so you will not perform color balancing in this tutorial. For instructions on performing color balancing and feathering, see Mosaic Tutorial: Advanced Workflow.

Refer to the following sections:

Select Input Files

This tutorial uses six scenes, named airphoto1.dat through airphoto6.dat. Each has an accompanying header file (.hdr). These are down-sampled versions of digital aerial photographs acquired from an Intergraph DMC camera system on 10 March 2012 over northern Kentucky. Images are courtesy of Quantum Spatial.

These scenes are in a State Plane projection, Zone 1601 (Kentucky North), NAD 83 datum, US Survey Feet.

  1. Start ENVI.
  2. From the Toolbox, select Mosaicking > Seamless Mosaic. The Seamless Mosaic panel appears. You will set all mosaic options within this single panel. Click the Help button in the lower-left corner of the panel if you have further questions throughout this tutorial.
  3. Click the Add Scenes button .
  4. Click the Open File icon in the File Selection dialog. A file selection dialog appears.
  5. Navigate to the folder that contains the tutorial data, and use the Ctrl or Shift key to select the files airphoto1.dat through airphoto6.dat. Click Open. The File Selection dialog lists all six scenes.
  6. Click the Select All button, then click OK.

Display Scenes and Footprints

Displaying a scene in ENVI requires a pyramid to be built for that scene if a pyramid does not already exist. For high-resolution images with large dimensions and file sizes, building pyramids can take a long time. The scenes for this tutorial are relatively small in size, so it will only take a few seconds to display them.

Tip: A pyramid file (.enp) contains copies of an image file at various reduced resolutions. It is used to speed image display by reducing the resampling required when displaying large portions of an image at low resolution. See the Pyramids Background topic in ENVI Help for more information.

A question dialog appears: "6 scenes need pyramids. Do you want to proceed?" Click Yes. The Process Manager in the lower-right corner of the ENVI interface shows the process of building pyramids for all six scenes.

All six scenes are displayed with their footprints (colored magenta). The footprints are initially displayed as bounding boxes that cover the background pixels plus valid image pixels.

The Show Footprints button is toggled "on" in the Seamless Mosaic panel, and the currently selected scene (airphoto6.dat) is highlighted with a border whose color is defined by the Default Selection Color preference. You can select a scene name in the table, and the corresponding footprint will be highlighted in the display. Or if you click in the display, the footprint for the top-most scene nearest the cursor is highlighted, and the corresponding scene name is selected in the table.

Click the Show Filled Footprints button to fill each footprint with a semi-transparent color. This option provides a better way to visually discern the boundaries of each scene, or it can be useful for detecting gaps in data coverage. Click the button again to hide the filled footprints.

Set the Data Ignore Value

The black areas are border (fill) pixels that should be excluded in the output mosaic. You can set a data ignore value for each scene by identifying the pixel values to exclude.

  1. Right-click in the left-most segment of the Status bar (at the bottom of the ENVI application), and enable the Raster Data Values option.
  2. Zoom into a corner of an image until you can clearly see the black area, then move the cursor over some black pixels.
  3. Look for the Data: line in the Status bar. This shows the pixel value for all three bands of the image. In this case, the value is 0. This will be the data ignore value.
  4. Notice the Data Ignore Value table column in the Seamless Mosaic panel. If the value differs for each scene, you can set it individually by double-clicking in the appropriate table cell. Since the value will be the same (0) for all scenes in this example, you can set it in one place using the following steps.

  5. Click on the Data Ignore Value column heading so that all rows are highlighted.
  6. Right-click in the column heading and choose Change Selected Parameters.
  7. In the Update Mosaic Parameters dialog, enter 0 for the Data Ignore Value. Click OK.
  8. Click on any table cell to clear the selection.
  9. Click the Recalculate Footprints button in the Seamless Mosaic dialog. The footprints will now snap to the valid image pixels. This process may take several seconds to complete.
  10. Zoom out until you can see all scenes. The display should look similar to the following:

Tip: You can also set the data ignore value in the accompanying header files, if the scenes are in ENVI raster format. You can perform this step before starting ENVI and the Seamless Mosaic tool. Open the .hdr file for each scene using a text editor, and add the following line:

data ignore value = 0

Export the Mosaic

  1. To select an output area for the final mosaic, click the Define Output Area button . (You may need to click the Hide Footprints button if the footprints interfere with your selection.)

  2. Click and drag the cursor to draw a rectangle around the area that you want to subset. After releasing the mouse button, you can adjust the sides and corners of the rectangle as needed, then right-click and select Accept Subset Area. The output area is displayed with a light-grey dashed rectangle. Pixels outside of this rectangle will be discarded when you export the mosaic in the steps that follow. To start over, click the Define Output Area button again, right-click inside the selection, and choose Clear Subset Area. The following figure shows an example of selecting a rectangular area of valid image pixels:
  3. Click the Export tab.
  4. Leave the Output Format selection as ENVI.
  5. Select a folder and filename for the output mosaic.
  6. Leave the Display Result check box enabled.
  7. The Data Ignore Value is the pixel value that will be used to fill areas where no valid image data appear in the output raster. When creating a mosaic in ENVI format, this value is automatically set to the input data-ignore value. In this example, the Data Ignore Value will be 0, so you do not need to enter a value. For more information on the Data Ignore Value, click the Help button in the Seamless Mosaic panel, and scroll to the section, "Export the Mosaic."
  8. Leave the Resampling Method selection as Nearest Neighbor.
  9. Click Finish. When processing is complete, the mosaic is displayed at full resolution, and ENVI builds a pyramid file for the mosaic.
  10. Exit ENVI when finished.

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