ATCOR_T

Description

Parameters

Parameter descriptions

Input: Input thermal image

Specifies the name of the input thermal image from one of the supported thermal sensors (ASTER, Landsat 4 TM, Landsat 5 TM, Landsat 7, and Landsat 8).

Source background options

Specifies, potentially with Source Background Values, which pixels in the source image are to be considered background (NoData) pixels. In general, if a pixel is considered NoData, the application handles the pixel in a special manner.

See the Source Background Values parameter for specific examples.

Source background values

Sensor

Optionally specifies the name of the sensor used to capture the input image.

If this parameter is not specified, the function checks for the PlatformName metadata tag at the file level.

If this parameter is not specified and the PlatformName metadata tag is not found, the function will error.

Calibration file name

Optionally specifies the path and name of the text file that contains, for each band, the calibration coefficients for the specified sensor. This information is typically provided with the data set as an offset and a gain for each channel.

7      c0       c1 [mW/cm2 sr micron]
1   -0.0500   0.0778700
2   -0.2600   0.0798800
3   -0.3000   0.0621600
4   -0.5500   0.0639700
5   -0.1150   0.0126200
6    0.3200   0.00372047
7   -0.0420   0.00438900

The atcor\cal\ folder in the CATALYST Professional installation directory contains sample calibration files for every sensor supported by ATCOR_T. Each row in a calibration file lists the calibration coefficients for each sensor band; the bands are listed in increasing band order.

Ensure that the offset (c0) and gain (c1) values correspond to the scene. These values must be provided in the expected units for the specific sensor (shown at the top right of the calibration file).

If this parameter is not specified, ATCOR will read the RadiometricTransOffset, RadiometricTransGain, and RadiometricTransUnits metadata tags for each channel in the input file. If the Calibration File (CFILE) parameter is not specified, or if the coefficients cannot be derived from the metadata, ATCOR_T will use the calibration files stored in the CATALYST Professional atcor\cal\ folder for the specified sensor.

For more information, see Preparing data for atmospheric correction.

Output: Surface temperature map

Specifies the file name for the output surface temperature map scene.

The output file will contain as many channels as the input file for Landsat-4, Landsat-5, and Landsat-7. It will contain only one channel for Landsat-8 and ASTER. The georeferencing of the output file will be the same as the input file and the bit depth of the output data is determined by the Output Units parameter.

The specified file must not already exist.

InputTerr: Input terrain derivatives file

Optionally specifies the name of the input file that contains the DEM, slope, aspect, and sky view produced by TERSETUP for the input image.

If the terrain file is not provided, the atmospheric correction will be applied assuming a constant elevation value (see the MEANELEV parameter).

InputVis: Input visibility layer file name

Optionally specifies the name of the input file that contains the visibility map for the input image.

Approximate elevation

Optionally specifies, in meters, an approximated average elevation value used to apply the atmospheric correction.

If a Terrain Derivative file (TERFILE) is provided, this parameter may be left unspecified. If TERFILE is not provided, and this parameter is empty, an approximated elevation value is calculated from the system DEM file gmted2010. If both a Terrain Derivative file (TERFILE) and an Approximate Elevation value (MEANELEV) are specified, the MEANELEV value is ignored.

Atmospheric condition

Optionally determines the level of humidity used to estimate the water vapor column.

The water vapor may be inferred based on the season (summer, fall, winter).

Tropical applies to latitudes between 0 and ±15 degrees and subarctic to latitudes between 45-60 (or -45 to -60). By default, ATCOR_T will estimate the atmospheric condition based on the center of the scene and the acquisition date, available from the metadata. If the atmospheric condition cannot be estimated and this parameter is not defined, ATCOR_T will error.

For more information about how ATCOR_T defines the atmospheric condition, see the Details section in the ATCOR help.

Ground visibility

Optionally specifies, in kilometers, the ground visibility required to describe the atmospheric conditions at the time of image acquisition.

By default, a constant visibility of 30 km is used for the entire scene, which describes clear conditions.

Surface temperature offset

Optionally specifies, in degrees Celsius, a surface temperature offset; the default value is 0.0. This value is specified when atmospheric water vapor information is not available, but lake temperature measures (or some reference surface temperature value) exist. This offset is used to match the satellite-derived temperature at the water body (or reference point) temperature.

Emissivity

Optionally specifies the emissivity value. By default, a constant value of 0.98 is used trough the entire scene, but any value in the range of 0.95 and 1.0 may be specified; values outside this range will cause the function to error.

The emissivity of a material (ε) is the relative ability of its surface to emit energy by radiation. Emissivity is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature (a true black body would have an ε = 1, which is the maximum). The more reflective a material is, the lower its emissivity. It depends on factors such as temperature, emission angle, and even wavelength. The emissivity of Earth's atmosphere varies according to cloud cover and the concentration of gases (such as greenhouse gases) that absorb and emit energy in the thermal infrared.

Output units

Optionally specifies the output units for the atmospherically corrected image.

Scaling factor

Optionally specifies scaling factor by which to multiply the temperature units.

This parameter applies only if Output Units are specified as Scaled_Temperature. By default, the scaling factor is 10 for 16-bit and 4 for 8-bit input data. If the area contains negative temperature values, an offset value is required (see STOFF) so that the negative values are not truncated to 0 in the output temperature map.

Details

ATCOR and ATCOR_T are the final algorithm in the atmospheric correction workflow. While ATCOR is used to compute ground reflectance of terrain on satellite imagery, ATCOR_T retrieves temperature values for each pixel composing the image. Both ground reflectance and surface temperature are used to get energy balance components over the scene. The computation of a surface temperature map is useful for detecting local climate variability and analyzing anthropogenic climate change.

The computed temperatures, in degrees Celsius, are converted to digital numbers (DN) stored to disk with the help of surface temperature offset (STOFF) and temperature scaling factor. The two values are defined in parameters STOFF and OUTUNITS, respectively.

The conversions between temperatures in Celsius and DNs are as follows:

      DN = (Temperature + STOFF) * (scaling factor)
      Temperature = DN / (scaling factor) - STOFF

Scaled_Temperature, scales the output temperature units to fit the input bit depth. By default, the scaling factor is 10 for 16-bit and 4 for 8-bit input data. If the area contains negative temperature values, an offset value is required (see STOFF) so that the negative values are not truncated to 0 in the output temperature map.

ATCOR_T supports only Landsat TM, ETM, and ASTER sensors. The function requires a pre-computed visibility map (created by ATCOR, using the multispectral data for the same scene) and will produce a surface map for each thermal band from Landsat and from band 13 (8.925 to 9.25 um) from ASTER.

The input visibility file must fully overlap the input file (FILI); otherwise, ATCOR_T will error. If a terrain derivatives file (TERFILE) is provided, ATCOR_T uses the intersection of the input files with the image provided in FILI to define the output extents. If TERFILE and FILI overlap by less than 33 x 33 pixels, ATCOR_T will error.

The metadata is then used to populate all ATCOR_T required parameters. ATCOR_T verifies if the values provided trough the metadata (or trough the parameters) are within the expected range; if this is not the case, the function errors. For example, the Solar Zenith values must be within the range of 0-90 degrees and the Solar Azimuth angle must be within the range of 0-360 degrees.

ATCOR_T verifies the metadata from the input file (FILI). If the function still cannot determine the sensor and spectral range for each channel, it errors and prompts the user to provide the required metadata tags in the input file.

When all parameters and metadata values had been verified, ATCOR_T verifies the visibility map, if one is provided. If it cannot overlay the visibility map with the input thermal file (FILI), ATCOR_T errors.

ATCOR_T uses the visibility value to compute the atmospheric LUT. The computed visibility map is then used with the viewing geometry and DEM-derived products to retrieve the values from the atmospheric database for surface temperature.

References

Richter, R. (1998) Correction of Satellite Imagery over mountainous terrain. Applied Optics, 37 (18), 4004-4015.

Richter, R. (1996) Atmospheric Correction of Satellite Data with Haze Removal Including a Haze/Clear Transition Region. Computers & Geosciences, 22 (6), 675-681.

Zhang, Y., Guindon, B., Cihlar, J. (2002) An Image Transform to characterize and compensate for spatial variations in thin cloud contamination of Landsat Images. Remote Sensing of Environment, 82, 173-187.

Richter, R. (2010) Atmospheric / Topographic Correction for Satellite Imagery - ATCOR2/3 User Guide). DLR - German Aerospace Center, 1-165.

Richter, R., et al (2006) An automatic atmospheric correction algorithm for visibile/NIR imagery. International Journal of Remote Sensing, 27 (10), 2077-2085.