General Configuration

Inputting the General LUT Configuration parameters is the first step to define the configuration of a LUT in ALG, independently of the selected atmospheric RTM. This step appears when the user press the New LUT configuration button () or, alternatively, through the menu bar by clicking on FileNewLUT config..

The user starts by introducing the LUT file path/name in the corresponding Save LUT file(s) text box or by pressing the button (1). All LUT-related files and folders will have the introduced name but different extensions (e.g., .h5, .hdr, .bat, .xml). By default, ALG sets the LUT path to the LUTs_folder folder but this can be edited in the software preferences.

Then, the number of parallel CPU cores used to run the RTM is specified in the # parallel runs pop-up menu (2). This number is automatically limited by the maximum CPU cores of the PC where ALG is installed. By selecting a value of the N parallel runs, ALG will automatically distribute all the RTM simulations in N different sub-folders with the path/name introduced in the Save LUT file(s) text box and a suffix _i (with i from 1 to N).

Next, the user level (Basic, Medium or Advanced) is selected in the radio-button group at the bottom-left of the General LUT Configuration panel (3). This parameter allows users to have access to more advanced LUT configuration parameters and options such as the key input variables and the LUT sampling strategy.

The general LUT configuration follows with the selection of the RTM used to generate the LUT data and its operation mode (4). The available atmospheric RTMs are described in the Third-party Software section of this Help guide. With respect to the operation mode, the following options are currently implemented:

  • The Transfer Functions option allows generating the so-called atmospheric transfer functions for the selected RTM. These functions include typical standard outputs of atmospheric RTMs such as path radiance, transmittance or at-surface irradiance. Use the transfer Functions mode for applications such as forward simulation or atmospheric correction.
  • The At-sensor Radiance option runs the selected RTM in a forward mode, calculating the radiance reaching the sensor. In this mode, in addition to atmospheric, geometric and spectral configuration, users will also define the surface properties (e.g., reflectance, fluorescence, temperature, emissivity) and cloud properties. Not yet implemented
  • The Transmittance option runs the selected RTM in transmittance mode and stores the direct target-to-sensor transmittance for available gasses.

Notice that not all these operation modes are available for all atmospheric RTMs. Particularly, the transmittance mode is only available for MODTRAN5 and MODTRAN6.

Once selected the RTM and its operation mode, the user must choose the sampling strategy on its pop-up menu (5). This parameter defines the distribution of the LUT nodes in the input variable space. In ALG, the current available options are:

  • Manual (gridded) sampling: The key input variables' values are combined all-with-all, giving as a result a n-dimensional rectangular grid (n=dimensionality of the input variable space). This option is available for Basic, Medium and Advanced users.
  • Automatic (gradient) (Vicent et al., 2018): With this option, LUT nodes are automatically distributed in order to bound the error in linear interpolation below an user-input value. Note that this method only works for a LUT containing continuous variables and thus it does not accept various values of discrete variables. This option is available for Basic, Medium and Advanced
  • Latin Hypercube (McKay et al., 1979): The input variables are pseudo-randomly and homogeneously distributed in the input variable space, resulting in a set of scattered LUT nodes. This option is available for Medium and Advanced users.
  • Sobol/Halton sequence (Bratley et al., 1988,Halton et al., 1964): The input variables are pseudo-randomly and homogeneously distributed in the input variable space, resulting in a set of scattered LUT nodes. This option is available for Advanced users.
  • Read distribution: With this option, users will select an pre-calculated nodes distribution from a previous LUT (in .h5 file) or stored in a .txt file. In the case of a .txt file, this file should contain a matrix of N input variables (in rows) and M combinations (in columns), each column being separated with a tabulation. Note that, with this option, an alternative Key Input Variables window will be opened. This option is available for Medium and Advanced users.

Depending on the chosen sampling strategy, different options will then be available to evaluate the stop condition, i.e., to determine the number of nodes included in the LUT (6):

  • To input the maximum number of input samples/nodes. This option is fixed when the sampling is manual (gridded) or for a Latin Hypercube sampling. It is also available for Sobol and Halton sequence distribution.
  • To evaluate the maximum linear interpolation error of an atmospheric LUT, in TOA radiance, through "leave-one-out" cross-validation. With this option, ALG will automatically stop adding new LUT nodes when 95% of the cross-validation errors are below a given relative error threshold. This method is only available for Sobol and Halton sequence.

When the user selects the cross-validation error option to evaluate the stop condition, two additional parameters will be visible.

This first of them allows users to introduce the relative error threshold (in %) (7). The second, allows selecting the surface reflectance spectral (8). In combination with with the atmospheric transfer functions in a LUT, this parameter allows generating the TOA radiance used to evaluate the interpolation error through cross-validation. Two options are available: Vegetation (conifers) (Baldridge et al., 2009) and Sea water.

Once introduced all these parameters, the user should click on the Next button at the bottom-left corner to proceed introducing the key input atmospheric RTM variables (or the alternative version when the Read distribution option is selected in the sampling strategy). This will create a LUT configuration .xml file with the selected file path/name (e.g., C:\ALG\LUTs_folder\LUTfile.xml), storing these parameters for their later use in the execution of the atmospheric RTMs and generation of input data.

References