[help] How to use geodata package with targets

[njtierney]

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Description

Hi folks,

I want to have a target that gets data using the geodata package, specifically the crop_spam() function - like so:

 crop_coffee_data = crop_spam(
    crop = "acof",
    var = "area",
    path = "data",
    africa = TRUE
    )

This package requires that you specify a directory to save the data to, but at the time of writing I am not 100% sure what the file names will be. I was wondering what the best practice is to be able to:

• store the output of these downloaded files
• make sure they don't run again when we run tar_make()

Let me know if I can make this clearer or provide more information!

Cheers,

Nick

[Aariq]

Since it looks like crop_spam() returns a SpatRaster object, the discussion here might be useful. In particular, I've been using the following for the format argument for targets that return SpatRasters with success.

format_geotiff <- tar_format(
  read = function(path) terra::rast(path),
  write = function(object, path) terra::writeRaster(x = object, filename = path, filetype = "GTiff", overwrite = TRUE),
  marshal = function(object) terra::wrap(object),
  unmarshal = function(object) terra::unwrap(object)
)

list(
tar_target(coffee_data, crop_coffee_data(), format = format_geotiff)
)

The other option is to write the results out explicitly as part of your crop_coffee_data() function (e.g. with terra::writeRaster()) rather than storing the resulting SpatRaster in the _targets store and use format = "file". You don't need to know the file names ahead of time, you just need a function that returns file names of the written files. Hope this helps!

[njtierney]

Woh that just works! Thanks so much! One thing I wasn't anticipating was that I need to namespace (pkg::fun) the terra functions when using tar_format.

I feel like some geospatial targets tarchetype package would be really helpful, my team does this sort of work a lot