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Convenience functions for simple DEM derivatives

Source: R/dem_derivatives.R
dem_derivatives.Rd

Functions to calculate simple DEM derivatives, currently northness() and eastness() for transforming aspect degrees into the range -1:1.

Usage

northness(asp_deg)

eastness(asp_deg)

Arguments

asp_deg

Numeric vector of aspect values in degrees (0:360).

Value

Numeric vector of transformed values.

Details

northness() is a cosine transform of aspect degrees, with any flat aspect values (-1 if present) set to 90 degrees (east) as a neutral value:

northness <- cos(asp_deg * pi / 180)

eastness() is a sine transform of aspect degrees, with any flat aspect values (-1 if present) set to 0 degrees (north) as a neutral value:

eastness <- sin(asp_deg * pi / 180)

Note

No validation is done on the input values. The caller is responsible for ensuring input has valid type and range.

See also

calc(), dem_proc()

Examples

## plot northness from slope-masked aspect for the Storm Lake AOI
f_dem <- system.file("extdata/storml_elev.tif", package="gdalraster")

# slope degrees
f_slp <- basename(tempfile(pattern = "storml_slp", fileext = ".tif"))
f_slp <- file.path("/vsimem", f_slp)
dem_proc("slope", f_dem, f_slp)

# aspect
f_asp <- basename(tempfile(pattern = "storml_asp", fileext = ".tif"))
f_asp <- file.path("/vsimem", f_asp)
dem_proc("aspect", f_dem, f_asp)

# compute masked aspect as an in-memory raster
expr <- "ifelse(SLOPE >= 2, ASPECT, -9999)"
(ds_masked_asp <- calc(expr = expr,
                       rasterfiles = c(f_slp, f_asp),
                       var.names = c("SLOPE", "ASPECT"),
                       fmt = "MEM",
                       dtName = "Float64",
                       nodata_value = -9999,
                       setRasterNodataValue = TRUE,
                       return_obj = TRUE))
#> ℹ output written to: "calc20914e518c0d"
#> C++ object of class <GDALRaster>
#>   • Driver: In Memory Raster (MEM)
#>   • DSN: "calc20914e518c0d"
#>   • Dimensions: 143, 107, 1
#>   • CRS: NAD83 / UTM zone 12N (EPSG:26912)
#>   • Pixel resolution: 30.000000, 30.000000
#>   • Bbox: 323476.071971, 5101871.983031, 327766.071971, 5105081.983031

# diverging palette for northness
# adapted from "heatmap3" in ltc-color-palettes
# https://github.com/loukesio/ltc-color-palettes
pal <- c("#d7191c", "#fdae61", "#ffffbf", "#abd9e9")

plot_raster(ds_masked_asp, legend = TRUE, col_map_fn = pal,
            pixel_fn = northness, na_col = "#2c7bb6",
            main = "Storm Lake AOI northness")


# clean up
ds_masked_asp$close()
deleteDataset(f_slp)
#> [1] TRUE
deleteDataset(f_asp)
#> [1] TRUE