Build Geographic Lookup Tables (GLTs) from Input Geometry

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Lesson content

Lesson 1 of 1

Build Geographic Lookup Tables From Input Geometry

In this quick guide, you will:

•

Use the Dataset Browser to read a scientific dataset in NetCDF-4 format. * •

Extract latitude and longitude grids from the dataset. * •

Build a GLT from the latitude and longitude grids. * •

Georeference a brightness temperature image using the GLT.

Sample Data

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

[ATMS_BrightnessTemps.zip

5 MB

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

Background

A Geographic Lookup Table (GLT) is a type of spatial reference used to map image pixels to ground coordinates. The location information needed to create a GLT comes from input geometry. Data vendors typically provide input geometry data with their Level-1 non-georeferenced imagery, for users who want to georeference the imagery themselves.

For example, some vendors provide Input Geometry Map (IGM) datasets from which you can build GLTs. Structured remote sensing data in HDF or NetCDF-4 format may include separate latitude (Y) and longitude (X) grids in addition to the primary images. These are a type of input geometry.

The following diagram shows how input geometry is used to create a GLT:

Once a GLT has been created, it can be used to georeference the associated Level-1 imagery. See the Georeference Images Using Geographic Lookup Tables (GLTs) quick guide for more information.

GLT georeferencing is not the same as orthorectification. The latter is a more accurate mapping process that accounts for terrain distortions and relief displacement in imagery.

Open an ATMS Dataset

For this exercise, you will open an Advanced Technology Microwave Sounder (ATMS)(opens in a new tab) dataset in NetCDF-4 format. The ATMS instrument is part of the NOAA-20, or Joint Polar Satellite System-1 (JPSS-1) platform. The ATMS brightness temperature image that you will work with has not been georeferenced to a standard map projection. Latitude and longitude grids are provided so that you can build and apply a GLT to georeference the brightness temperture 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 ATMS_BrightnessTemps_2024-06-25.nc. The file opens in the Dataset Browser but is not displayed. The primary dataset of interest here is "antenna_temp." It contains calibrated brightness temperatures, in Kelvin. The values in parentheses indicate the number of bands (22), the number of columns (96), and the number of rows (135).

Before we look at the "antenna_temp" dataset, let's build a GLT from the input geometry that is provided.

Build a GLT

1

Scroll through the list of datasets and locate "lat" and "lon." These are the latitude and longitude grids.

2

Select the lat dataset, then click the Add selected item... button to copy it to the Raster Builder section.

3

Select the lon dataset, then click the Add Selected Item button. The Raster Builder section assigns the "lat" and "lon" datasets as Bands 1 and 2, respectively.

4

Click the Open Rasters button at the bottom of the Dataset Browser. The Dataset Browser is dismissed, and the "lat" band is displayed in the Image window. 2. 5

In the Layer Manager, right-click on the ATMS_Brightness... layer and select Remove. 3. 6

In the search window of the Toolbox, enter GLT. 4. 7

Double-click the Build GLT tool that appears in the search results. The Input X Geometry Band dialog appears. 5. 8

Select the lon band.

9

Click OK. The Input Y Geometry Band dialog appears. 2. 10

Select the lat band.

11

Click OK. The Geometry Projection Information dialog appears. 2. 12

In the Output Projection for Georeferencing section, select Geographic Lat/Lon.

13

Click OK. The Build Geometry Lookup File Parameters dialog appears. 2. 14

Keep the default value of 0.145177 degrees for Output Pixel Size and the default value of 81 degress for Output Rotation. 3. 15

Enter an output filename of ATMS_GLT.dat. 4. 16

Click OK. When processing is complete, the first GLT band is displayed. The GLT image itself is not too interesting. You will remove it from the display next. 5. 17

In the Layer Manager, right-click on ATMS_GLT.dat and select Remove.

Now you have successfully built a GLT dataset. Let's use the GLT to georeference the brightness temperature dataset in the next section.

Georeference the Brightness Temperature Dataset

1

Select File > Open Recent from the Menu bar, and select the file ATMS_BrightnessTemps_2024-06-25.nc. The file opens in the Dataset Browser 2. 2

In the Available Datasets list, select antenna_temp. 3. 3

Click the Quick Open button at the bottom of the Available Datasets list.

A color composite of the brightness temperture dataset is displayed in the Image window. It is not yet georeferenced to a map projection.

4

In the Toolbox, double-click the Georeference from GLT tool. The Input Geometry Lookup File dialog appears. 2. 5

Select ATMS_GLT.dat and click OK. The Input Data File dialog appears. 3. 6

Select ATMS_Brightness_Temps_2024-06-25.nc at the top of the list, and click OK. The Georeference from GLT Parameters dialog appears. 4. 7

Keep the default parameter values. 5. 8

Enter an output file name of ATMS_BrightnessTemps_Georef.dat and click OK. When processing is complete, the georeferenced brightness temperature image is displayed in the Image window. 6. 9

In the Layer Manager, right-click on ATMS_BrightnessTemps_2024-06-25.dat and select Remove. 7. 10

Click the North Up button in the Toolbar. This rotates the image so that North is toward the top of the display.

11

From the Menu bar, select File > Open World Data > Coastlines. 2. 12

Zoom out of the display until you can see the coastlines more clearly. The image is located in eastern Africa and covers a large area.

13

This concludes the exercise.

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