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Derive Coefficients of Slope and Intercept

Source: R/optram_wetdry_coefficients.R
optram_wetdry_coefficients.Rd

Derive slope and intercept coefficients for both wet and dry trapezoid lines. Write coefficients to a CSV file (as input to optram_calculate_soil_moisture function)

Usage

optram_wetdry_coefficients(full_df, output_dir = tempdir())

Arguments

full_df,

data.frame of STR and NDVI values

output_dir,

string, directory to save coefficients CSV file

Value

rmse_df, data.frame, RMSE values of fitted trapezoid edges

Note

The vegetation index column is named "VI" though it can represent several vegetation indices, such as SAVI, or MSAVI.

The trapezoid method option (see optram_options) allows to choose one of three models for creating the edge coefficients of the trapezoid.

  • "linear" prepares a simple OLS regression line along the wet and dry edges of the trapezoid. Four coefficients are returned: intercept and slope for both edges.

  • "exponential" creates an exponential curve fitted to the intercept and slope, following: Ambrosone, Mariapaola, Alessandro Matese, et al. 2020. “Retrieving Soil Moisture in Rainfed and Irrigated Fields Using Sentinel-2 Observations and a Modified OPTRAM Approach.” International Journal of Applied Earth Observation and Geoinformation doi:10.1016/j.jag.2020.102113 .

  • "polynomial" fits a second order polynomial curve to the wet and dry edges of the trapezoid, following: Ma, Chunfeng, Kasper Johansen, and Matthew F. McCabe. 2022. “Combining Sentinel-2 Data with an Optical-Trapezoid Approach to Infer within-Field Soil Moisture Variability and Monitor Agricultural Production Stages.” Agricultural Water Management 274 (December): 107942. doi:10.1016/j.agwat.2022.107942 This curve fitting function returns six coefficients: alpha, beta_1, and beta_2 for both wet and dry edges

Examples

full_df <- readRDS(system.file("extdata", "VI_STR_data.rds",
  package = "rOPTRAM"))
rmse_df <- optram_wetdry_coefficients(full_df, tempdir())
#> VI series length:105
print(rmse_df)
#>    RMSE.wet  RMSE.dry
#> 1 0.1642536 0.1018425
optram_options("trapezoid_method", "polynomial")
#> 
#> New option for trapezoid_method applied.
#> [1] "SWIR_band = 11"
#> [1] "edge_points = TRUE"
#> [1] "feature_col = ID"
#> [1] "max_cloud = 12"
#> [1] "max_tbl_size = 1e+06"
#> [1] "period = full"
#> [1] "plot_colors = no"
#> [1] "remote = scihub"
#> [1] "rm.hi.str = FALSE"
#> [1] "rm.low.vi = FALSE"
#> [1] "trapezoid_method = polynomial"
#> [1] "veg_index = NDVI"
#> [1] "vi_step = 0.005"
rmse_df <- optram_wetdry_coefficients(full_df, tempdir())
#> VI series length:105
print(rmse_df)
#>    RMSE.wet   RMSE.dry
#> 1 0.1628496 0.07835765