Use Georeference from Input Geometry to georeference data that have an associated file containing map locations for every pixel. Georeferenced images have null values around their edges that must be masked in processing; these are often inflated by replicated pixels. Therefore, it is suggested that you process the data first, then apply the georeferencing to derive final products.

See the following sections:

Build GLT Files

Use Build GLT to build a Geographic Lookup Table (GLT) image (also called a geometry lookup file) from an input geometry file.

The GLT image contains map locations for every pixel of the associated input image. Specifically, the GLT image consists of two bands:

  • Sample Lookup: Pixel values indicate the column (sample) number of the pixel in the input geometry file that belongs at the given Y location in the output image.
  • Line Lookup: Pixel values indicate the row (line) number of the pixel in the input geometry file that belongs at the given X location in the output image.

Follow these steps:

  1. From the Toolbox, select Geometric Correction > Build GLT . The Input X Geometry Band dialog appears.
  2. Select the band that contains the x geometry coordinates.
  3. Click OK. The Input Y Geometry Band dialog appears.
  4. Select the band that contains the y geometry coordinates. Click OK. The Geometry Projection Information dialog appears.
  5. In the Input Projection of Geometry Bands list, select the projection type.
  6. In the Output Projection for Georeferencing list, select the projection for the georeferencing.
  7. Click OK.

A default output pixel size and rotation angle are calculated, and it appears in the Build Geometry Lookup File Parameters dialog. The default output pixel size is calculated based on the map coordinates in output space. The default output rotation angle is used to minimize the output file size. If the rotation angle is set to 0 then north will be up in the output image. If it is set to another angle, then north will be at an angle and will not be “up” in the output image. The rotation angle is stored in the ENVI header and is used when overlaying grids, so the grid lines appear at an angle.

  • To change the output pixel size, replace the value in the Output Pixel Size field.
  • To change the output rotation angle, replace the value in the Output Rotation field.
  • If you change a non-zero rotation angle to 0 so north is up, your resulting image may contain a lot of background fill and may become very large.

If the GLT values are positive, then an exact match from the input image was found; in other words, the input image actually contained a pixel for this map location. If the GLT values are negative, then a nearest-neighbor pixel is being used because the input image does not actually contain a pixel for the corresponding map location. If the GLT value is 0, then there was no nearest neighbor within seven pixels of this map location in the input image.

The GLT image has a fixed pixel size projected into a rotated UTM system. It uses one-based coordinates (the upper-left pixel is located at 1,1).

Georeference from GLT

Use Georeference from GLT to georeference an image using a GLT file.

Note: ENVI currently does not allow a geographic lookup table (GLT)-based image to be displayed along with an image georeferenced to a standard map projection.

  1. From the Toolbox, select Geometric Correction > Georeference from GLT. The Input Geometry Lookup File dialog appears.
  2. Select a GLT file and click OK. The Input Data File dialog appears.
  3. Select an input file and perform optional spectral subsetting, then click OK. The Georeference from GLT Parameters dialog appears.
  4. If you used a subset of the original data as the input file, click the Subset to Output Image Boundary toggle button to select whether to output only the warped subset region or whether to output that subset warped within the entire output boundary.
  5. In the Background Value field, enter the DN value to use as the background value around the edges of the warped data.
  6. Enter an output filename.
  7. Click OK.

Georeference from IGM

Use Georeference from IGM to georeference data using an Input Geometry (IGM) file. An IGM file contains the map information of the image, stored in two bands: one for x coordinates (longitude or easting) and one for y coordinates (latitude or northing). Some datasets come with specific latitude and longitude bands, which make up the IGM file. Additionally, ENVI generates IGM files for various sensors such as SeaWiFS and AVHRR. The IGM file itself is not georeferenced, but it does contain the georeferencing information for each original raw pixel. The IGM file is used to create (on-the-fly) a geographic lookup table (GLT) file that contains the information about which original pixel occupies which output pixel in the final product.

Perform the following steps to georeference an image using an IGM:

  1. From the Toolbox, select Geometric Correction > Georeference from IGM. The Input Data File dialog appears.
  2. Select an input file and perform optional spectral subsetting, then click OK. The Input X Geometry Band dialog appears.
  3. Select the band that contains the x geometry coordinates and click OK. The Input Y Geometry Band dialog appears.
  4. Select the band that contains the y geometry coordinates and click OK. The Geometry Projection Information dialog appears.
  5. In the Input Projection of Geometry Bands list, select the projection type.

  6. In the Output Projection for Georeferencing list, select the projection for the georeferencing.

  7. Click OK.

    A default output pixel size and rotation angle are calculated, and they appear in the Build Geometry Lookup File Parameters dialog.

    The default output pixel size is calculated based on the map coordinates in output space. The default output rotation angle is used to minimize the output file size. If the rotation angle is set to 0, then north will be up in the output image. If it is set to another angle, then north will be at an angle and will not be “up” in the output image. The rotation angle is stored in the ENVI header and is used when overlaying grids, so the grid lines appear at an angle.

    • To change the output pixel size, replace the value in the Output Pixel Size field.

    • To change the output rotation angle, replace the value in the Output Rotation field.

      Note: If you change a non-zero rotation angle to 0 so north is up, your resulting image may contain a lot of background fill and may become very large.

  8. In the Georeference Background Value field, enter the DN value to use as the background value around the edges of the georeferenced data.
  9. Enter an output filename.
  10. Click OK.

Reproject GLT with Bowtie Correction

Use Reproject GLT with Bowtie Correction to georeference GOES-16, GOES-17, MODIS, NPP VIIRS, PRISMA, and Sentinel-3 datasets that contain a geographic lookup table (GLT) and to correct for "bowtie" visual artifacts in MODIS imagery. See GLT and Bowtie Correction Background to learn how how these methods are applied.

You can also write a script to perform GLT reprojection using the ReprojectGLT task.

  1. Use the File > Open menu option to open a dataset.
    • For GOES-16, open a .nc file.
    • For MODIS, open a .hdf or .hdfeos file.
    • For Sentinel-3, open a .xml file.
  2. From the Toolbox, select Geometric Correction > Reproject GLT with Bowtie Correction.
  3. In the Data Selection dialog, select the dataset that you want to process.
  4. Perform optional spatial and spectral subsetting.
  5. Click OK in the Data Selection dialog. The Reproject GLT with Bowtie Correction dialog appears.
  6. The default Projection Type is Geographic WGS-84. Choose Polar Stereographic WGS-84 if the imagery covers the polar regions.
  7. Select an Interpolation Method: The GLT algorithm establishes a regular output grid and finds the best location for each source image pixel. Any output grid point for which a source pixel could not be found will be filled with one of the following methods.
    • Distance Weighted (default): A distance-weighted average of the surrounding valid values. Use this method to create an image with the fewest visible interpolation artifacts.
    • Nearest Neighbor: Select this method when the set of valid pixel values cannot be modified, for example, when values represent the integral values of a classification.
  8. Click OK to begin processing.

NPP VIIRS

The Reproject GLT with Bowtie Correction tool is meant for users who previously selected Load Raw Image in the NPP VIIRS Parameters dialog and now want to georeference the imagery and correct for bowtie effects.

Build Super GLT Files

Use Build Super GLT to build super GLT files from IGM files. The super GLT file is not an image file; it contains information about how many and which input pixels contribute to the output pixel.

The Super GLT process involves optimal radial resampling, where every input pixel within a given radius that contributes to the value of an output pixel is considered in a weighted fashion. In comparison, bilinear resampling arbitrarily averages a group of 2 x 2 neighbors, and cubic convolution resampling averages a group of 4 x 4 or 16 x 16 neighbors. Super GLT averages every relevant pixel in the file within a given radius to determine the best possible output pixel value. Super GLT, therefore, is much slower than a normal GLT process.

The Super GLT method is particularly useful for aircraft data where the sensor may scan a given point on the ground multiple times, due to the roll, pitch, or yaw of the aircraft. This means an output pixel might be best represented by an average of three different input pixels that are located in three different locations in the input file. This example illustrates why the Super GLT process can take a longer time to process.

Follow the steps below to build a Super GLT.

Note: You can use Super Georeferencing from IGM to combine building the super GLT and georeferencing from it into one step.

  1. From the Toolbox, select Geometric Correction > Build Super GLT. The Input X Geometry Band dialog appears.
  2. Select the band that contains the x geometry coordinates and click OK. The Input Y Geometry Band dialog appears.
  3. Select the band that contains the y geometry coordinates and click OK. The Geometry Projection Information dialog appears.
  4. In the Input Projection of Geometry Bands list, select the projection type.

  5. In the Output Projection for Georeferencing list, select the projection for the georeferencing.

  6. Click OK.

    A default output pixel size and rotation angle are calculated, and they appear in the Build Geometry Lookup File Parameters dialog.

    The default output pixel size is calculated based on the map coordinates in output space. The default output rotation angle is used to minimize the output file size. If the rotation angle is set to 0, then north will be up in the output image. If it is set to another angle, then north will be at an angle and will not be “up” in the output image. The rotation angle is stored in the ENVI header and is used when overlaying grids, so the grid lines appear at an angle.

  7. To change the output pixel size, replace the value in the Output Pixel Size field.

  8. To change the output rotation angle, replace the value in the Output Rotation field.

    Note: If you change a non-zero rotation angle to 0 so north is up, your resulting image may contain a lot of background fill and may become very large.

  9. Enter an output filename.
  10. Click OK.
  11. The process runs in two parts. ENVI adds the resulting GLT file to the Layer Manager. It does not add the Super GLT file to the Layer Manager.

Georeference from Super GLT

Use Super GLT Georeference to georeference data using a super GLT file. This function performs radial resampling to derive the output image using a specified kernel size. Each output pixel is calculated by weighting all the pixels in the kernel by the inverse of their radial distance to the center of the output pixel. You specify the kernel size and minimum number of pixels used in the resampling.

Note: Because super GLT functions are slow, allow a significant amount of time for processing.

  1. From the Toolbox, select Geometric Correction > Super GLT Georeference. The Input File dialog appears.
  2. Select an input file and perform optional spectral subsetting, then click OK. The Select SGL Filename dialog appears.
  3. Select the super GLT file and click OK. The Georeference from SGL Parameters dialog appears.
  4. In the Background Pixel Value field, enter the DN value to use as the background value around the edges of the warped data.

  5. In the Kernel Size Min and Max fields, enter minimum and maximum kernel sizes, respectively.

    The minimum kernel size is used in the resampling unless fewer than the minimum number of pixels to resample, or valid pixels, are contained in the kernel. If fewer than the minimum number of valid pixels are contained in the kernel, the kernel size is increased until either the minimum number of valid pixels is met or the maximum kernel size is met. If there are fewer than the minimum number of valid pixels in the maximum kernel size then the output pixel value will be set to the background value.

  6. Enter a value for Minimum Pixels to Resample.

    If fewer than the minimum number of pixels are contained in the maximum kernel size, then the output pixel value will be set to the background value.

  7. Select output to File or Memory.
  8. Click OK. ENVI adds the resulting output to the Layer Manager.

Super Georeference from IGM

The Super Georeference from IGM tool combines building a super GLT file and georeferencing from it into a single process. The process georeferences an image using an IGM file and uses radial resampling to derive the output image. Additionally, you have the option to output the super GLT that is built during this process.

The initial steps for super georeferencing from IGM files are the same as those for regular georeferencing from IGM files. However, there are additional steps required for building the super GLT file and creating output.

  1. From the Toolbox, select Geometric Correction > Super IGM Georeference. The Input File dialog appears.
  2. Select an input file and perform optional spectral subsetting, then click OK. The Input X Geometry Band dialog appears.
  3. Select the IGM band that contains the x geometry coordinates and click OK. The Input Y Geometry Band dialog appears.
  4. Select the band that contains the y geometry coordinates and click OK. The Geometry Projection Information dialog appears.
  5. In the Input Projection of Geometry Bands list, select the projection type.

  6. In the Output Projection for Georeferencing list, select the projection for the georeferencing.

  7. Click OK. A default output pixel size and rotation angle are calculated, and they appear in the Build Geometry Lookup File Parameters dialog.

  8. In the Kernel Size Min and Max fields, enter minimum and maximum kernel sizes to use, respectively.

    The minimum kernel size is used in the resampling unless fewer than the minimum number of pixels to resample, or valid pixels, are contained in the kernel. If fewer than the minimum number of valid pixels are contained in the kernel, the kernel size is increased until either the minimum number of valid pixels is met or the maximum kernel size is met. If there are fewer than the minimum number of valid pixels in the maximum kernel size then the output pixel value will be set to the background value.

  9. Enter a value for Minimum Pixels to Resample.

    If fewer than the minimum number of pixels are contained in the maximum kernel size, then the output pixel value will be set to the background value.

  10. Enter an output SGL filename.
  11. In the Georeference Background Value field, enter the DN value to use as the background value around the edges of the warped data.
  12. Select output to File or Memory.
  13. Click OK. ENVI adds the resulting output to the Layer Manager.