ATCOR_T

Description

ATCOR_T takes a thermal band in scaled radiance, the DEM and derivatives produced by TERSETUP, and, optionally, a visibility map to compute a surface temperature map for the thermal image. This function supports only ASTER and Landsat 4 TM, Landsat 5 TM, Landsat 7, and Landsat 8 sensors.

Parameters

Name	Type	Caption	Length	Value range
FILI*	String	Input thermal image	1 - 192
DBIC	Integer	Input thermal channels	0 - 5	1 -
SRCBGD	String	Source background value	0 - 192	Default: FILE
ASENSOR	String	Sensor name	0 - 16
CFILE	String	Image calibration coefficients	0 - 192
FILO*	String	Surface temperature map	1 - 192
TERFILE	String	Input terrain derivatives file	0 - 192
MEANELEV	Float	Mean elevation value	0 - 1
VISTYPE	String	Ground visibility	0 - 192	Default: constant,30
ATMCOND	String	Atmospheric conditions	0 - 10
STOFF	Float	Surface temperature offset	0 - 1	Default: 0.0
EMISSIV	Float	Emissivity value	0 - 1	Default: 0.98
OUTUNITS	String	Output units	0 - 26	Default: Temperature
FTYPE	String	Output file type	0 - 4	Default: PIX
FOPTIONS	String	Output file options	0 - 64

* Required parameter

Parameter descriptions

FILI

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).

DBIC

Optionally specifies the list of input thermal channels to be processed.

Specified channels must already exist. If this parameter is not specified, all thermal bands from the input file are processed for Landsat; for ASTER, the function processes the channel that holds band 13.

Ranges of channels or segments can be specified with negative values. For example, {1,-4,10} is internally expanded to {1,2,3,4,10}. When you are not specifying a range in this way, only 48 numbers can be specified explicitly.

SRCBGD

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

Available options are:

FILE: reads the NoData value from the input file's NO_DATA_VALUE metadata. The file-level metadata tag is used as the default for every channel; channel-level tags, when available, override the default. When metadata does not exist, every pixel in the input is considered valid.
0: every pixel, in every channel, having a value of 0 is considered NoData
NONE: every pixel, in every channel, is considered valid
<value>: every pixel, in every channel, having the specified value is considered NoData.
FILE [, <value>]: same as FILE, but defines the NoData value for every channel, when metadata does not exist.

For example, the default "FILE, 0" specifies that the function uses metadata. If the metadata is unavailable, pixels that have a value of 0 are considered NoData>.

Note: To specify multiple values, use a comma-delimited list. The first value is applied to the first channel, the second value to the second channel, and so on. If fewer values are specified than the number of input channels, the last value is repeated for all remaining channels. If more values are specified than the number of input channels, the extra values are ignored.

ASENSOR

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

Supported sensors are:

Landsat-4 TM
Landsat-5 TM
Landsat-7 ETM+
Landsat-8
ASTER

Note: Define this parameter using the exact spelling from the list above.

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.

CFILE

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.

The specified text file should contain the band number, offset and gain, and the units for each. The following shows an example of a calibration file for Landsat images:

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.

FILO

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

The output file will contain as many channels as the input file (FILI) 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 OUTUNITS parameter.

The specified file must not already exist.

TERFILE

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).

MEANELEV

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 file gmted2010.pix stored in the etc subdirectory of the CATALYST Professional installation folder. If both a Terrain Derivative file (TERFILE) and an Approximate Elevation value (MEANELEV) are specified, the MEANELEV value is ignored.

VISTYPE

Optionally specifies the ground visibility and atmosphere type required to describe the atmospheric conditions at the time of image acquisition.

Supported values are:

An existing visibility file, previously computed from the input scene. For example, C:\Data\scene01_visibility.pix.
A keyword and, optionally, a visibility value.
For example:
- Constant,[Ground_visibility_in_kilometers]
where [<Ground_visibility_in_kilometers>] indicates the farthest distance (in km) from which a person could see a prominent object.

The following websites provide historical empirical measurements of visibility in the United States and Canada:

USA: http://www.noaa.gov/wx.html
Canada: https://www.weather.gc.ca/canada_e.html

In general, a constant atmosphere type should be used for images captured over flat areas with a single land cover type dominating the scene.

ATMCOND

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).

Supported values are:

dry: (w.v. = 0.3-0.8 cm^10)
summer or midlatsummer: mid-latitude summer (wv=2-3 cm)
fall or spring: fall/spring (wv=1-1.5 cm)
winter or midlatwinter: midlatitude winter
subarctic or subarticsummer: sub-arctic summer
tropical: tropical (w.v.= 3.5 cm or g cm-2)
US or UStandard: US Standard (default)

Tropical applies to latitudes between 0 and \u00c2\u00b115 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.

STOFF

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.

EMISSIV

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 (\u00ce\u00b5) 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 \u00ce\u00b5 = 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.

OUTUNITS

Optionally specifies the output units for the atmospherically corrected image.

Available options are:

Temperature: absolute values of temperature, in degrees Celsius, will be stored in a 32R channel (default).
Scaled_Temperature,[<scaling factor>]: 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.

FTYPE

Specifies the output file format type, represented by a three- or four-letter code. The format type must be a GDB-supported file type.

Supported formats include, among others:

PIX: PCIDSK format (default)
TIF: Tagged Image File format
JP2: JPEG 2000 format

For a complete list of GDB-recognized file types and their codes, see GDB file formats in the Technical Reference section of the CATALYST Professional Online Help.

FOPTIONS

Specifies the file creation options to be applied when creating the output file. These are specific to the file format; in each case, the default of no options is allowed. This parameter can be used to specify compression schemes, file format subtypes, and other information.

Different options are available for different file types (see the FTYPE parameter). The options are described in GDB file formats in the Technical Reference section of the CATALYST Professional Online Help.

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.

The atmospheric correction on the thermal band is handled through a separate function for the following reasons:

the resolution of thermal bands is different from the multispectral or panchromatic bands, which means it is unlikely to be included with the image file
it applies only to Landsat and ASTER sensors
the purpose of using the thermal band is to create a surface temperature map, which produces a different output than the reflectance image produced by ATCOR

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.

Example

Generate a surface temperature map of thermal band 6 on alpmon.pix from a Landsat 4/5 TM sensor.

The atmosphere condition is summer and visibility type is constant 15 km.

EASI>fili	=	"alpmon.pix"
EASI>dbic	=	6	! input thermal band
EASI>srcbgd	=	
EASI>asensor	=	"Landsat-4/5 TM"
EASI>cfile	=	"tm_standard.cal"
EASI>filo	=	"alpmon_thermal.pix"
EASI>terfile	=	"terfile.pix"
EASI>meanelev	=	
EASI>vistype	=	"Constant,15.0"
EASI>atmcond	=	"summer"
EASI>stoff	=	0.0
EASI>emissiv	=	
EASI>outunits	=	""
EASI>ftype	=	
EASI>foption	=	  

EASI>r ATCOR_T

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.

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