The numeric companion to tissot_map(): distortion sampled on a grid, so it
can be summarised, compared or mapped rather than eyeballed. Computed by
projecting a small circle at each grid point and measuring what happens to
it, which is the definition rather than an approximation of it.
Usage
projection_distortion(
projection = "equal_earth",
measure = c("areal", "angular", "max_scale"),
spacing = 10,
max_lat = 85
)Arguments
- projection
Projection to measure (see
projection_info()).- measure
"areal"(default; projected area divided by true ground area, so 1 is undistorted),"angular"(maximum angular deformation in degrees, 0 for a conformal projection) or"max_scale"(the larger principal scale factor).- spacing
Grid spacing in degrees (default
10).- max_lat
Absolute latitude limit (default
85).
Value
A tibble of lon, lat and distortion, with the area-weighted
mean and the range attached as the "countryatlas_distortion" attribute.
Reading the numbers
An equal-area projection has "areal" distortion of 1 everywhere – that is
what equal-area means, and it is worth checking rather than trusting.
A conformal projection has "angular" distortion of 0 everywhere and
unbounded areal distortion. A compromise projection is bad at both by a
little, everywhere, which is the trade it makes.
Examples
# \donttest{
if (requireNamespace("sf", quietly = TRUE)) {
d <- projection_distortion("mercator", measure = "areal")
attr(d, "countryatlas_distortion")
}
#> # A tibble: 1 × 5
#> projection measure mean min max
#> <chr> <chr> <dbl> <dbl> <dbl>
#> 1 mercator areal 4.40 1.00 132.
# }