using Proj
# Proj.jl implements the CoordinateTransformations.jl API.
# A Proj.Transformation needs the source and target coordinate reference systems (CRS),
# or a single pipeline.
trans = Proj.Transformation("EPSG:4326", "+proj=utm +zone=32 +datum=WGS84")
# The CRS can be a string or the CRS type, which also interfaces with GeoFormatTypes.jl.
# Once created, you can call this object to transform points.
# The result will be a tuple of Float64s, of length 2, 3 or 4 depending on the input length.
# The 3rd coordinate is elevation (default 0), and the 4th is time (default Inf).
# Here the (latitude, longitude) of Copenhagen is entered
trans(55, 12)
# -> (691875.632137542, 6.098907825129169e6)
# Passing coordinates as a single tuple or vector also works.
# Note that above the latitude is passed first, because that is the axis order that the
# EPSG mandates. If you want to pass in (longitude, latitude) / (x, y), you can set the
# `always_xy` keyword to true. For more info see https://proj.org/faq.html#why-is-the-axis-ordering-in-proj-not-consistent
trans = Proj.Transformation("EPSG:4326", "+proj=utm +zone=32 +datum=WGS84", always_xy=true)
# now we input (longitude, latitude), and get the same result as above
trans(12, 55)
# -> (691875.632137542, 6.098907825129169e6)
# using `inv` we can reverse the direction, `compose` can combine two transformations in one
inv(trans)(691875.632137542, 6.098907825129169e6) == (12, 55)