To specify surface temperature parameters:
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From the Atmospheric Correction workflows Tree List, select Surface Temperature.
This task is available only for the ATCOR Surface Temperature workflow.
Note: The sensor specified in the Sensor and image settings panel must be one of the supported thermal sensors. If your previous workflows specified a satellite that does not have a thermal band, the Sensor and image settings panel will be invalid (displaying a red bullet) and all the subsequent workflows will become disabled. You must select a multispectral image from a sensor supporting a thermal band to be able to work on the Surface Temperature workflow.
The Surface Temperature panel appears.
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Specify the Input Thermal Image: from the Input File list, select the file that contains the thermal band, or click Browse to select the file.
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Specify the Radiometric Information for the thermal file.
If the specified input thermal file contains calibration metadata, the calibration coefficients and wavelengths are populated from the Channel Metadata. The Band Setup window lists only the thermal bands for the specified sensor.
If the specified input thermal file does not contain the appropriate metadata, the Calibration coefficients source field defaults to Manual Entry. You must either manually enter the required information, or import the data from a text file.
Note: You must verify all calibration coefficients before proceeding to another workflow. If you skip this step and this information is not available through metadata, the calibration offset and gain are assumed to be 0 and 1 respectively. These values are acceptable for haze removal, but will produce incorrect results for the TOA, cloud masking, ground reflectance, and surface temperature calculations.
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Specify the Emissivity value.
The emissivity of a material 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 emissivity of 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.
By default, a constant value of 0.98 is used through 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.
Do one of the following:
- Select Use a constant value and specify the emissivity constant for all pixels in the image
- Select Surface cover dependent to calculate the emissivity for each pixel using specific values for Water, Vegetation, and Other material.
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In the Output section, specify whether to Scale the output reflectance units.
If selected, the calculated output will be multiplied by the specified Scale factor.
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If you select the Scale output option, specify the Scale factor by which to multiply the output.
By default, the workflow does not scale the output, and the values are absolute temperature values at ground surface.
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Optionally specify the Offset to Surface Temperature, in degrees Celsius.
If you know the temperature of a specific reference point (a lake, for example), this value can be used to correct the computed temperature for all pixels in the image. Otherwise, accept the default value of 0.0 degrees Celsius; the computed values will not be shifted, and the temperature map values will be as computed.
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Optionally select the Compute energy balance components option
Selecting this option generates the following raster layers during the atmospheric correction process:
- Thermal flux difference: calculates the difference between the emitted atmospheric radiation and the emitted surface radiation
- Ground heat flux: calculates the exchange rate of energy between the Earth's surface and the underground
- Latent heat: calculates the exchange rate of stored heat energy between the air and the Earth's surface. Latent heat flux is measures the amount of energy needed to change matter from one state to another (from solid to liquid to gas).
- Sensible heat: calculates the exchange rate of excess heat energy between the air and the Earth's surface. Sensible heat flux measures the amount of energy needed to change air temperature.
- Net radiation: calculates the difference between absorbed and emitted shortwave and longwave radiations
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Click Settings to display the Thermal Flux window.
For more information, see Specifying thermal flux settings.