dem_derivatives: Convenience functions for simple DEM derivatives

In gdalraster: Bindings to 'GDAL'

Convenience functions for simple DEM derivatives

Description

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).

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)

Value

Numeric vector of transformed values.

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))

# 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)
deleteDataset(f_asp)

gdalraster documentation built on May 4, 2026, 9:06 a.m.