The wekeoR package simplifies provides a wrapper for the WEkEO Harmonised Data Access (HDA) API.
WEkEO is one of several portals which provide access to ESA Copernicus and Sentinal satellite data as well as derived products .
More information on WEkEO can be found here. Documentation and more information on the API this package wraps can be found here.
You can install the development version of the package using dev tools.
# install.packages("devtools")
devtools::install_github("wave1art/wekeoR")library(wekeoR)
hda_list = wekeoR::init(api_key, output_directory)
dataset_id = 'EO:CRYO:DAT:HRSI:FSC'
# Define the query to run on WEkEO API
query = list(datasetId = 'EO:CRYO:DAT:HRSI:FSC',
boundingBoxValues = list( list(name = 'bbox',
bbox = c( 6.9375502668826,
46.51799925662627,
9.453098610481598,
47.214513711335606))),
dateRangeSelectValues = list( list( name = 'Observed',
start = '2021-06-15T23:59:59.999Z',
end = '2021-07-09T23:59:59.999Z'),
list( name = 'Published',
start = '2021-06-15T23:59:59.999Z',
end = '2021-07-09T23:59:59.999Z')),
stringChoiceValues = list( list( name = 'productType',
value = 'FSC'))
)
# Query the WEkEO database
hda_list = hda_list |> get_access_token() |>
acceptTandC() |>
submit_wekeo_query(query) |>
get_query_results_list()
# Download files
hda_list = hda_list |>
get_order_ids() |>
download_data(hda_list, 'zip')
Before being able to search or download data from the WEkEO API you need to have registered an account.
- Find the relevant dataset of interest on WEkEO
- Query the WEkEO API to determine relevant raster tiles
- Order and download the raster tiles
WEkEO provides a simple online interface to browse the dataset catalogue and visualise datasets.
This interface also gives access to the metadata needed to construct a valid query. It can be used to generate a template query which can then be adapted to suit your specific needs.
Sometimes the query JSON in the WEkEO brower appears to be malformed so it is generally a good idea to test a query through the web interface to check that it is accepted. Already knowing whether there is a problem with the JSON generally accelerates debugging.
The exact format required of the WEkEO query is given by the dataset metadata. In practice it can be a hassle to create a correctly formatted query object. Often it is faster to use a JSON object created by the WEkEO web interface for the dataset of interest as a starting point.
The query function submit_wekeo_query expects the query argument to be in the form of a nested list. This is converted to a JSON object before submitting to the WEkEO API.
There are a couple of tricks which can ensure the submitted JSON is as expected by the WEkEO API.
Single element lists
Because httr auto-unboxes lists before converting to JSON this can result in a JSON object which WEkEO rejects for single item lists. In the example below unboxing creates a JSON dictionary where WEkEO was exepting an array of dictionaries. As a result the query will fail.
# Fails: argument gets unboxed and WEkEO rejects the query
query = list( ...
stringChoiceValues = list( name = 'productType',
value = 'FSC')
....
)
JSON:
{....
"name":["productType"],"value":["FSC"] <-- This should be an array
....
} The trick is to wrap these single item lists in a list. The output is then a dictionary inside an array, which is the format WEkEO was expecting.
# Succeeds: wrapping in an extra list ensures httr submits this as a single element JSON array
query = list( ...
stringChoiceValues = list( list( name = 'productType',
value = 'FSC'))
....
)
JSON:
{ ...
[{"name":["productType"],"value":["FSC"]}] <-- now part of an array as required
...
}