Flaash atmospheric correction

Flaash atmospheric calibration (IRSP6-08.3.24)

The purpose of Atmospheric correction is to eliminate the influence of air and light on the reflection of objects, and obtain real physical model parameters such as the reflectivity, radiation rate, and surface temperature of objects, it is used to eliminate the influence of water vapor, oxygen, carbon dioxide, methane and ozone on the reflection of Earth objects, and to eliminate the influence of atmospheric molecular and aerosol scattering. Flaash can process any hyperspectral data, satellite data, and aerial data (860nm/1135nm), which is composed of hymap, AVIRIS, CASI, hydice, epoch (EO-1) aisa, Harp, Dais, probe-1, TRWIS-3, sindri, mivis, OrbView-4, Nemo and other sensors. Flaash can also calibrate vertical imaging data and lateral view imaging data.

The flaash atmospheric correction uses the code of the modtran 4 + radiation transmission model. Based on Pixel-level correction, it corrects the joint effect caused by diffuse reflection, including the classification diagrams of volume clouds and opaque clouds, it can be used to adjust the spectral smoothing caused by artificial stop.

Flaash can be used for Landsat, Spot, AVHRR, Aster, MODEMU, MERIS, AATSR, rapid atmospheric correction analysis is performed on hyperspectral data, aerial images, and custom-format hyperspectral images, such as IRS. It can effectively eliminate the influence of air, light and other factors on the reflection of the object, and obtain real physical model parameters such as the reflectivity, radiation rate, and surface temperature of the object.

Correction Process

Click ENVI -- basic tools-preprocessing-calibration utilities-flaash

Spectral-flaash. Or click envi-Spectral-flaash.

1. The input data must be the data after the radiation correction, and the radiation correction data must be converted to the BIL or BIP format (basic tools -- convert data );

2. Edit the header file of the input data. Edit the wavelength Wavelenth (the wavelength center value of each band) and wavelength width FWHM (the wavelength range of each band. FWHM cannot be edited for non-Hyperspectral Data. (ENVI -- file -- edit ENVI header)

3. After entering the data, the following dialog box is displayed:

There are two options. If the input image has different conversion factors in different bands, select the first one, and vice versa. I used the same factor for all the bands of IRS images, so I chose the second one. The factor value is based on the unit of the input data and the ENVI standard.

Unit conversion scale.

Radiance scale factors is a unit conversion factor. If your radiance (spectral sensitivity) is a standard unit W/m2 * Um * rad, if the flaash parameter is set to UW/cm2 * sr * NM, this factor is 10.

1 m = 103 Mm = 106 μm = running Nm = 1012 PM (skin meter)

1 W = 103 Mw = 106 μW 1 MW = 106 mW

Rad plane angle radian Sr three-dimensional angle spherical degree

4. Set output parameters, including output reflectance file., output directory for flaash files, and output

Directory for flaash files

5. Input imaging and sensor parameters

The longitude and latitude of the center of the scene center lacation image. You can open the image and view the longitude and latitude of the center (you can enter the row and column numbers of the center in the following window)

Sensor altitude sensor height (orbital height). After selecting the correct sensor, it will be displayed.

The average elevation of ground elevation (in the selected region) is measured in KM.

6. atmospheric model Earth atmospheric model and aerosol model

Six standard atmospheric models

Select the atmospheric model of the corrected region according to the following table.

The selected atmospheric model based on the data longitude and latitude and acquisition time

Water Retrieval)

Water vapor inversion settings, using two methods to remove water vapor

A. Use the Moisture Removal Model to restore the water volume of each pixel in the image

Using the Moisture Inversion Model, the data must have a spectral resolution of more than 15 nm and cover at least one of the following spectral ranges: 1050-1210nm (preferred), 770-870nm, 870-1020nm. For most sensors, water vapor inversion displays no by default, because most sensors do not have proper bands to compensate for the effect of water vapor.

B. A single moisture factor is used for the overall image. The default value is 1. The Moisture Inversion Model is used for the multi-spectral data and can be set in the multi-spectral data.

Manually set the moisture band

Aerosol retrieval)

Aerosol models require data bands covering both 660nm and 2100nm.

A. Four standard modtran aerosol models are provided

Rural (country), urban (city), Maritime (Ocean), tropospheric

B. Two aerosol inversion methods

2-band (K-T) method (similar to the fuzzy reduction method), if the adapted black value (usually shadow area or water) is not found, the system will use the visibility value for calculation; therefore, you must give this option.

Relationship between weather conditions and visibility

7. spectral polishing (hyperspectral) Spectral Polishing

Spectral polishing (hyperspectral data)

Make the spectral curve more similar to the spectral curve of real objects

Fine-tune the spectral Curve

8. Multi-spectral data parameter settings

When the water vapor Inversion Model and aerosol model are set in the Basic settings, the parameters are set in the modify multi-spectral settings box.

Moisture Removal Model Parameters

The parameter settings of the aerosol model (the data band is required to cover the 660nm and 2100nm spectra using the aerosol model) are shown in the following table:

 

9. Hyperspectral Data parameter settings

Automatically select Channel Definition (recommended)

Set Channel Definition

10. Advanced Settings

Spectral definition file: built-in AVIRIS, hymap, hydice, Hyperion, CASI, and Aisa

Aerosol height

CO2 mixing rate: 390ppm

Use domain correction

Calculation results using the previous modtran Model

Set the spectral resolution of the modtran model (recommended value 5 cm-1)

Set modtran multi-scattering model

Corner of the sky "azimuth (for non-star sensor)