calibrating images tutorial

Calibrating Images TutorialIn this tutorial, you will calibrate a QuickBird Level-1 image to spectral radiance andreflectance while learning about the various metadata fields that ENVI® uses toperform calibration.

Files Used in This TutorialThe tutorial data files are available in a single ZIP file from the Exelis VIS website.Extract this file to a local directory. Go to the folder named rigorous_ortho\005606990010_01_P008_MUL.

Files Description005606990010_01_P008_MUL\05JUL*.TIF

QuickBird Level-1 multispectral imagery forPhoenix, AZ from 11 July 2005

QuickBird files are courtesy of DigitalGlobe and may not be reproduced withoutexplicit permission from DigitalGlobe.

BackgroundCalibrating imagery is a common pre-processing step for remote sensing analystswho need to extract data and create scientific products from images. Calibrationattempts to compensate for radiometric errors from sensor defects, variations inscan angle, and system noise to produce an image that represents true spectralradiance at the sensor.

ENVI's Radiometric Calibration tool provides options to calibrate imagery to radiance,reflectance, or brightness temperatures. See the "Radiometric Calibration" topic inENVI Help for more information on how each option is computed.

The available calibration options depend on what metadata is included with theimagery. Most vendors distribute a metadata file or ephemeris data along with theimage data.Note: It is important to select the correct metadata file (using the File > Openmenu option) whenopening data from various satellite sensors so that ENVI reads the required calibration parameters. Referto the following table:

of 14© 2015 Exelis Visual Information Solutions, Inc., a subsidiary of Harris Corporation. All Rights Reserved. Thisinformation is not subject to the controls of the International Traffic in Arms Regulations (ITAR) or the Export

Administration Regulations (EAR). However, this information may be restricted from transfer to various embargoedcountries under U.S. laws and regulations.

Calibrating Images Tutorial

http://www.exelisvis.com/Learn/Resources/Tutorials.aspx

Sensor

Calibration OptionsMetadata File to

OpenRadiance

Reflectance

BrightnessTemperatur

eALOS AVNIR-2 and PRISMLevel-1B2data

• • HDR*.txt

AlSat-2A • .dim

DMC DIMAP • • .dim

EO-1 ALI • • Use the File > OpenAs > EO-1 > HDFmenu option and selecta *_HDF.L1G file. A *_MTL.L1G file must be inthe same directory.

EO-1Hyperion

• • Use the File > OpenAs > EO1 > HDFmenu option and selectan .L1R file. Calibrationmetadata is hard-coded into theapplication and notread from anymetadata files.

Gaofen-1 • • Use the File > OpenAs > CRESDA > GF-1menu option and selectan .xml file.

GeoEye-1 • • .til

IKONOS • • metadata.txt

KOMPSAT-3 • *_aux.xml

Landsat TM,ETM+, andLandsat-8OLI/TIRS

• • • *_MTL.txt, *WO.txt,*.met




Sensor


OpenRadiance

Reflectance


edataOrbView-3 • • .pvl

PleiadesPrimary orOrtho (singleor mosaic)

• • DIM*.xml

QuickBird • • .til

RapidEyeLevel-1B

• • *_metadata.xml

A NITF/NSIF license isrequired to open thesefiles.

ResourceSat-2

• .h5

SPOT DIMAP • • DIM*.xml

SSOT DIMAP • METADATA.DIM

WorldView • • .til

Ziyuan-1-02C

• • Use the File > OpenAs > CRESDA > ZY-1-02C menu option.Select from thefollowing files:*.orientation.xmlopens the MUX-PANdata product withmetadata, *-MUX.xmlopens multispectraldata with metadata, *-PAN.xml openspanchromatic data withmetadata, and *.xmlopens the HRC dataproduct (images with




Sensor


OpenRadiance

Reflectance


etwo parallel cameras)with metadata.

Ziyuan-3A • • Use the File > OpenAs > CRESDA > ZY-3menu option. Selectfrom the following files:*.orientation.xmlopens the TLC dataproduct (images withnadir, forward,backward view) withmetadata, *.xml opensmultispectral data withmetadata, *-NAD.xmlopens TLC nadir-viewdata with metadata.

To open QuickBird or WorldView data, select the image file. ENVI will read thenecessary metadata from the accompanying *.IMD file.

Open a QuickBird Image and View Its Metadata1. Start ENVI.

2. From the menu bar, select File > Open. A file selection dialog appears.

3. Navigate to the folder where you saved the tutorial data and select the file05JUL11182931-M1BS-005606990010_01_P008.tif.

4. In the Layer Manager, right-click on the filename and select View Metadata.




5. Click the Spectral category on the left side of the Metadata Viewer. This showsseveral metadata fields related to calibration. ENVI needs gain and offsetvalues in units of W/(m2 * µm * sr) to calibrate imagery to radiance. You cansee these values under the Gains and Offsets columns. The gains and offsetsare already in the correct units in this image. If they are not in the correct units,you can use the Scale Factor field (discussed in the calibration steps below) toscale the calibrated image to the correct units.

6. Select the Image Parameters category. You can see the values for SunAzimuth and Sun Elevation that were derived from the QuickBird metadata.




7. Select the Time category. The Acquisition Time for this scene is listed inCoordinated Universal Time (UTC). The Sun Azimuth, Sun Elevation, andAcquisition Time are used in combination with the various fields under theSpectral category when calibrating the image to reflectance.

8. Close the Metadata Viewer.

Calibrate the Image to Radiance1. From the Toolbox, select Radiometric Correction > RadiometricCalibration. The File Selection dialog appears, with the QuickBird file alreadyselected.

2. Click OK. The Radiometric Calibration dialog appears.

3. Leave the default options as-is. You will create a floating-point radiance image(BSQ interleave) in units of W/(m2 * µm * sr). Keeping the Scale Factor at1.00 ensures the units will remain the same as the original gain and offsetvalues.

4. The Appy FLAASH Settings button is for users who will subsequently performatmospheric correction using the FLAASH® tool. You can skip this step for thetutorial, but here is some background information if you plan to use FLAASH inthe future:

The use of FLAASH requires a separate Atmospheric Correction Module: QUAC®and FLAASH® license. FLAASH requires input imagery to meet the followingcriteria:

n The image must be calibrated to radiance in units of µW/(cm2 * nm* sr).

n The input image can be floating-point, long integer (4-byte signed), orinteger (2-byte signed or unsigned).

n The image can be in band-interleaved-by-line (BIL) or band-interleaved-by-pixel (BIP) format.

Clicking the FLAASH Settings button will create a radiance image in BIL,floating-point format. It will apply a scale factor of 0.1 to the radiance image toget it in units of µW/(cm2 * nm* sr). Clicking this button prevents you fromhaving to separately convert the interleave of the radiance image and figuringout the appropriate scale factor for use with FLAASH.




When you start FLAASH, select the radiance image that you just created withthe Radiometric Calibration tool. When the Radiance Scale Factors dialogappears, leave the default value of 1 for the Single Scale Factor field.

5. Select an output folder for the radiance image, and name it qb_radiance.dat.

6. Ensure that the Display Result check box is selected.

7. Click OK.When processing is complete, the calibrated radiance image isdisplayed.

8. To visually compare the original and calibrated images, toggle the qb_radiance.dat layer off and on in the Layer Manager.

9. With both layers selected in the Layer Manager, click the Cursor Value iconin the main toolbar.

10. In the Cursor Value dialog, look for the "Data" values for each image. Theoriginal image has integer pixel values, while the calibrated image has floating-point values. The following figure shows an example where the calibrated imageis displayed in true color:

n Band 3 is assigned to the red channel

n Band 2 is assigned to the green channel

n Band 1 is assigned to the blue channel

For the current pixel location, the calibrated image has a radiance value of126.556503 W/(m2 * µm * sr) in the red band, while the original image has araw DN value of 709 in the red band.




11. Close the Cursor Value dialog.

Another way to quickly verify the radiance values is to display a spectral profile:

1. Uncheck the original QuickBird image (05JUL*) in the Layer Manager so thatonly the calibrated image is displayed.

2. Click the Spectral Profile button in the main toolbar.

3. Click anywhere inside the image to display a plot of radiance values for theselected pixel location. You can use a spectral profile to help identify features ofinterest:

The following example shows a pixel that represents soil. The radiance valuespeak in the red wavelength region (~ 650 nm).




The next example shows a pixel that represents water. The radiance valuespeak in the blue wavelength region (~ 485 nm).




The next example shows a pixel that represents vegetation. The radiancevalues peak in the near-infrared wavelength region (~ 900 nm).

5. When you are finished, close the Spectral Profile dialog.

6. Right-click on each layer name in the Layer Manager, and select Remove.

Calibrate the Image to ReflectanceNext, you will calibrate the QuickBird image to top-of-atmosphere reflectance. Thisimage has all the metadata needed to calibrate to reflectance:

n Gains

n Offsets

n Solar irradiance

n Solar elevation

n Acquisition time

Follow these steps:




1. From the Toolbox, select Radiometric Correction > RadiometricCalibration.

2. In the File Selection dialog, select the original QuickBird image (05JUL*), thenclick OK.

3. In the Radiometric Calibration dialog, change the Calibration Type toReflectance.

4. Leave the other options as-is.

5. Select an output folder for the reflectance image, and name it qb_reflectance.dat.

6. Ensure that the Display Result check box is selected.

7. Click OK. When processing is complete, the reflectance image is displayed.

8. Click the Cursor Value icon in the main toolbar.

9. Look at the "Data" values for each band in the Cursor Value dialog and verifythat the values are less than 1.0.

10. Click the Spectral Profile button in the main toolbar.

11. Click anywhere inside the image to display a plot of reflectance values for thecurrent pixel location. The following figure shows an example of a pixel thatrepresents water. Reflectance values range from 0.04 to 0.09 across all fourbands, with the lowest value (0.04) in the near-infrared wavelength region:




12. When you are finished, exit ENVI.

For more information on the topics covered in this tutorial, see the "Radiometric

Calibration" topic in ENVI Help, or click the help button in the RadiometricCalibration tool.

Copyright Notice:

ENVI is a registered trademark of Exelis Inc., a subsidiary of Harris Corporation.

QUAC and FLAASH are registered trademarks of Spectral Sciences, Inc.




calibrating images tutorial

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