Examples

Conversion from Geodetic to Projected Coordinates

This example is ported from Proj’s quickstart guide. See proj.org/development/quickstart.html

require 'bundler/setup'
require 'proj'
# Create a context
context = Proj::Context.new
# Create a projection
crs = Proj::Crs.new("+proj=utm +zone=32 +datum=WGS84 +type=crs", context)
# Get the geodetic CRS for that projection
geodetic_crs = crs.geodetic_crs
puts geodetic_crs.to_proj_string
# +proj=longlat +datum=WGS84 +no_defs +type=crs
# Create a transformation from the geodetic to projected coordinates
transformation = Proj::Transformation.new(geodetic_crs, crs, context)
puts transformation.to_proj_string
# +proj=pipeline +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=utm +zone=32 +ellps=WGS84
# Create a coordinate for Copenhagen in degrees
coordinate_geodetic = Proj::Coordinate.new(lon: 12.0, lat: 55.0)
puts "lon: #{coordinate_geodetic.lon}, lat:  #{coordinate_geodetic.lat}"
# Transform the coordinate
coordinate_projected = transformation.forward(coordinate_geodetic)
# lon: 12.0, lat:  55.0
puts "east: #{coordinate_projected.e}, north: #{coordinate_projected.n}"
# east: 691875.632137542, north: 6098907.825129169
puts "x: #{coordinate_projected.x}, y: #{coordinate_projected.y}"
# x: 691875.632137542, y: 6098907.825129169
# Apply the inverse transform
coordinate_inverse = transformation.inverse(coordinate_projected)
puts "lon: #{coordinate_inverse.lon}, lat:  #{coordinate_inverse.lat}"
# lon: 12.0, lat:  55.0
puts coordinate_geodetic == coordinate_inverse
# true

Pipeline Operator

This example is ported from the Rust Proj documentation (see github.com/georust/proj#convert-from-nad-83-us-survey-feet-to-nad-83-meters-using-the-pipeline-operator

The pipeline operator makes it easy to create complex operations by daisy-chaining operations together. For more information refer to proj.org/operations/pipeline.html.

In this example, a coordinate from NAD 83 US Survey Feet to NAD 83 Meters. It has two steps:

• Step 1 as an inverse transform, yielding geodetic coordinates

• Step 2 as a forward transform to projected coordinates, yielding metres

  conversion = Proj::Conversion.new(<<~EOS)
                  +proj=pipeline
                  +step +inv +proj=lcc +lat_1=33.88333333333333
                  +lat_2=32.78333333333333 +lat_0=32.16666666666666
                  +lon_0=-116.25 +x_0=2000000.0001016 +y_0=500000.0001016001 +ellps=GRS80
                  +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs
                  +step +proj=lcc +lat_1=33.88333333333333 +lat_2=32.78333333333333 +lat_0=32.16666666666666
                  +lon_0=-116.25 +x_0=2000000 +y_0=500000
                  +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
                EOS
  # The Presidio, approximately
  coordinate_1 = Proj::Coordinate.new(x: 4760096.421921, y: 3744293.729449)
  coordinate_2 = conversion.forward(coordinate_1)
  assert_in_delta(1450880.2910605003, coordinate_2.x)
  assert_in_delta(1141263.01116045, coordinate_2.y)
  

Operation Factory Context

Operation Factory Contexts are used to build coordinate operations between two CRSes. This examples finds the best available conversion between EPSG 4267 and 4269.

source = Proj::Crs.create_from_database("EPSG", "4267", :PJ_CATEGORY_CRS)
target = Proj::Crs.create_from_database("EPSG", "4269", :PJ_CATEGORY_CRS)
factory_context = Proj::OperationFactoryContext.new
factory_context.spatial_criterion = :PROJ_SPATIAL_CRITERION_PARTIAL_INTERSECTION
factory_context.grid_availability = :PROJ_GRID_AVAILABILITY_IGNORED
operations = factory_context.create_operations(source, target)
coord = Proj::Coordinate.new(x: 40, y: -100)
index = operations.suggested_operation(:PJ_FWD, coord)
assert_equal(2, index)
operation = operations[index]
assert_equal("NAD27 to NAD83 (1)", operation.name)

Operation Factory Contexts have many additional attributes that can be set to control how conversions should be constructed.