Grids

Proj uses grid files for high-accuracy coordinate transformations (datum shifts). Grid files contain offset data that is applied during transformations. The Grid class manages downloading and querying grid files.

Checking Grid Availability

Grids referenced by transformations can be checked for availability:

grid = context.database.grid("us_noaa_alaska.tif")
puts grid.available?     # Whether grid is available at runtime
puts grid.downloadable?  # Whether grid can be downloaded
puts grid.open_license?  # Whether grid has an open license

Downloading Grids

Proj can download grid files on demand. Network access must be enabled on the context first (see Contexts).

context = Proj::Context.new
context.network_enabled = true

grid = context.database.grid("us_noaa_alaska.tif")

unless grid.downloaded?
  grid.download do |progress|
    puts "Download progress: #{(progress * 100).round}%"
  end
end

Grid Information

For grids available through the legacy API:

info = Proj.grid_info("us_noaa_alaska.tif")

Grid Cache

Downloaded grids are cached locally in a SQLite file. Configure the cache through Context#cache:

context = Proj::Context.new
cache = context.cache

cache.enabled = true
cache.path = "/custom/cache/path/cache.db"
cache.max_size = 500  # MB
cache.ttl = 86400     # seconds
cache.clear           # clear the cache

The default cache size is 300MB. See Contexts for more details.