Frequently Asked Questions about the Landsat Missions
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If gap mask files are not provided with the data, can I create my own?
Yes. Since pixels with missing data have digital number (DN) values of 0, you can differentiate between valid and missing data pixels using a number of different remote sensing software packages. A gap mask can then be created to show where valid and missing data pixels are located.
The process used to create a gap mask using ENVI is shown below. (Please note that this does not signify an endorsement by USGS EROS, as we cannot endorse any specific remote sensing software. Other remote sensing software packages may also contain similar functionality.)
- Open ENVI
- Click File -> Open External File -> Landsat -> GeoTIFF
- Select the file for which a gap mask is to be created and click Open
- From the Available Bands List select the file and click Load Band
- Within the band display, click Tools -> Color Mapping -> Density Slice...
- Select the band in the dialog box that is displayed and click OK
- The Density Slice box will be displayed and show eight defined density slice ranges
- Click Delete Range until only two ranges remain (it does not matter which ranges are deleted)
- Once there are only two ranges displayed, highlight the first range and click Edit Range
- Set the Range Min and Range Max values at 0
- Select a color you would like to use to display the missing data pixels and click OK
- Highlight the second range and click Edit Range
- Set the Range Min at 1 and the Range Max at 255
- Select a color you would like to use to display the valid pixels and click OK
- To view the output, click Apply
- If you would like to make changes to the colors used, select one of the ranges and click Edit Range
- To save the output, click File -> Output Ranges to Class Image...
- Click Choose and navigate to the location where you would like to save the file
- Name the file and click Open, then click OK
- The gap mask file will be added to the Available Bands list
- To view the gap mask and the original image at the same time, open two displays and load the images
- Within the band display of one of the images, click Tools -> Link -> Geographic Link...
- Click the arrow buttons next to Display #1 and Display #2 so that the box next to each changes to "On" and then click OK
- The gap mask and original data are now linked