Georeference Images Using Geographic Lookup Tables (GLTs)

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Lesson content

Lesson 1 of 1

Georeference Images Using GLTs

In this exercise, you will use a predefined GLT to georeference an Earth Surface Mineral Dust Source Investigation (EMIT) hyperspectral image.

Sample Data

Download sample data below. Then extract the contents of the .zip file to a local directory.

[EMIT_L2A_Reflectance.zip

1.3 GB

DownloadArrow down with horizontal line beneath it](assets/EMIT_L2A_Reflectance.zip)

Background

A Geographic Lookup Table (GLT) is a type of spatial reference used to map image pixels to ground coordinates. Some airborne and unmanned aerial vehicle (UAV) data providers include GLTs with uncorrected images for users who want to georeference the images.

Pixel values in a GLT indicate the sample (column) and line (row) of the pixel in the input image that belong at the given map location in the output image. A GLT maps the original image pixels to known ground coordinates.

Open and Display an EMIT Image

1

Select File > Open from the Menu bar. The Open dialog appears. 2. 2

Go to the location where you saved the sample data, and select the file EMIT_L2A_RFL.nc. 3. 3

Click Open. A true-color representation of the image is displayed in the Image window. 4. 4

Press the F12 key on your keyboard to zoom out to the full extent of the image.

5

Look at the Property Sheet in the lower-right corner of the application. The "Projection" field says *GLT* Geographic Lat/Lon. This means that the image is not georeferenced to a standard map projection. Instead, it contains GLT information that can be used for optional georeferencing.

6

Select File > Data Manager from the Menu Bar. The Data Manager appears. 2. 7

Scroll through the list of bands in the reflectance image. It contains 285 spectral bands that range from 381 to 2,493 nanometers (nm).

Below the band list is a "GLT" image. It contains two bands: "GLT X" and "GLT Y." You will use these to georeference the reflectance image.

Georeference the Reflectance Image

1

In the search window of the Toolbox, enter GLT. 2. 2

Double-click the Georeference from GLT tool that appears in the search results. The Input Geometry Lookup File dialog appears. 3. 3

Select the second EMIT_L2A_RFL.nc dataset. The "Dims" field on the right side should say 2461 x 1875 x 2 [BSQ]. The "2" indicates that the dataset contains two bands (GLT X and GLT Y).

4

Click OK. The Input Data File dialog appears. This is where you select the image to georeference. 2. 5

Select the first EMIT_L2A_RFL.nc dataset and click OK. This is the reflectance image. The Georeference from GLT Parameters dialog appears. 3. 6

Keep the default parameter settings. 4. 7

Enter an output file name of EMIT_L2A_RFL_Georef.dat.

8

Click OK to georeference the reflectance image. Processing takes several minutes. When it is finished, the georeferenced image is added to the Layer Manager and displayed in the Image window. The black pixels around the image have values of 0.

9

Look at the Property Sheet and notice that the "Projection" field says "Geographic Lat/Lon." This indicates that the image is georeferenced to a geographic coordinate system. 2. 10

From the Menu bar, select Views > Reference Map Link. A window appears, showing the geographic location of the image over central California.

This concludes the exercise.

To summarize, you can use GLT data in a structured data format (HDF or NetCDF) to georeference the associated imagery. Since the image is georeferenced, you can use it—and any derived analytics—to create a map presentation.

Not all GLT information is structured the same way. Georeferencing steps may vary, depending on the dataset. ENVI automatically reads the GLT grids in EMIT NetCDF-4 files and puts them in the Data Manager for easy access. Other NetCDF and HDF datasets that contain GLT information may require you to use the Dataset Browser to extract GLT X and Y grids and build a new raster from them. See the Build Geographic Lookup Tables (GLTs) From Input Geometry quick guide for details.

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