Resolve a Query URL via ArcGIS FeatureServer Metadata

Checks that an ArcGIS FeatureServer layer still exists and returns a query URL that downloads every feature of the layer as GeoJSON. FeatureServer layers back the Toronto Police Service open-data portal and many municipal portals used across the MORIE family.

Usage

rslvarc(prov)

Arguments

prov

A provenance list as returned by loadprov(), or NULL. It must have the field dataset$arcgis_layer_url, a FeatureServer layer root. An example is
https://services.arcgis.com/.../
followed by Assault_Open_Data/FeatureServer/0.

Value

The layer query URL as a length-1 character string, or NULL if the field is missing, the request fails, or the layer metadata reports an error.

Details

Trailing slashes are dropped from dataset$arcgis_layer_url, the URL is checked (public https only), and one request is made to <layer>?f=json. If the answer is JSON with a name and no error element, the function returns <layer>/query?where=1%3D1&outFields=*&f=geojson: every row (where=1=1), every field, as GeoJSON. ArcGIS servers return at most their maxRecordCount features per request, so a large layer needs paging (resultOffset) by the caller; the URL is not fetched here.

NULL is returned when prov is NULL, when the field is missing, when the request fails or when the metadata reports an error. Errors: a prov that is not a list; a layer URL that is not public https.

Examples

# Missing fields return NULL rather than erroring:
rslvarc(list())
#> NULL

# only https layer URLs are accepted
try(rslvarc(list(dataset = list(
  arcgis_layer_url = "ftp://example.org/FeatureServer/0"))))
#> Error : arcgis_layer_url must be an https:// URL, not ftp:// ('ftp://example.org/FeatureServer/0'); options(rmoriebricklayer.allowhttp = TRUE) admits a trusted plain-http mirror
try(rslvarc("not a list"))
#> Error : rslvarc: `prov` must be a provenance list (loadprov())
if (FALSE) { # interactive()
# needs the network
prov <- list(dataset = list(arcgis_layer_url = paste0(
  "https://services.arcgis.com/S9th0jAJ7bqgIRjw/arcgis/rest/services/",
  "Neighbourhood_Crime_Rates_Open_Data/FeatureServer/0"
)))
rslvarc(prov)
}