Add ONS Open Geog API example

Add ONS Open Geog API query example. Update README with additional details.

open-geog-API/ONS-open-geography-example.R

+# get feature layer data from ONS open geography API
+
+# Really useful example of using ONS open geography portal with R
+# https://medium.com/@traffordDataLab/pushing-the-boundaries-with-the-open-geography-portal-api-4d70637bddc3
+
+library(sf)
+
+# Load Local Authority geography from ----
+# https://geoportal.statistics.gov.uk/datasets/ons::local-authority-districts-december-2020-uk-bgc/api
+
+# Construct query URL from elements
+geo_endpoint <- "https://ons-inspire.esriuk.com/arcgis/rest/services/"
+geo_boundarylayer <- "Administrative_Boundaries/Local_Authority_Districts_December_2020_UK_BGC/"
+geo_server <- "FeatureServer/0/"
+geo_search <- "LAD20NM IN "
+
+# Construct a vector of local authorities to load
+# the following local authorities are the 'Solent' region
+las_to_load <- c("Southampton","Portsmouth","Winchester",
+                 "Eastleigh","Isle of Wight","Fareham",
+                 "Gosport","Test Valley","East Hampshire",
+                 "Havant","New Forest","Hart","Basingstoke and Deane")
+
+geo_where <- las_to_load # sometimes we don't want all boundaries
+geo_outfields <- "*" # returns all fields
+#geo_outfields <- c("LAD20CD","LAD20NM","LONG","LAT") # use in place of line above to return selected fields only
+geo_outSR <- "4326"
+geo_format <- "json"
+
+# Assemble the full URL for the query from elements above
+geo_query_string <- paste0(geo_endpoint,geo_boundarylayer,geo_server,
+                           "query?where=",geo_search,"(",paste(paste0("'",geo_where,"'"), collapse = ","),
+                           ")&outFields=",(paste(geo_outfields, collapse = ",")),"&outSR=",geo_outSR,"&f=",geo_format)
+
+
+# Format the URL to remove spaces
+geo_query <- URLencode(geo_query_string)
+message("Loading LA geometry from ONS Open Geography API")
+# API query
+sf_data <- st_read(geo_query)
+#plot(st_geometry(sf_data))
+
+# Useful lookup spatial reference for CRS
+# https://spatialreference.org/ref/epsg/27700/
+st_coord_sys <- st_crs(sf_data) # check coord system
+st_coord_sys # current coord system EPSG: 4326 (is what leaflet wants - good)
+
+# transform the coord system if required
+if(st_coord_sys$epsg != 4326){
+  sf_data <- st_transform(sf_data, "+proj=longlat +datum=WGS84")
+}
+
+# Create map (using leaflet) ----
+
+# create popup first (using htmltools)
+# by adding a column to sf_data object
+library(htmltools)
+sf_data$popup_text <-
+  paste("Locial authority area code: ","<b>", sf_data$lad20cd, "</b>",
+        '<br/>', 'Local authority: ', '<b>', sf_data$lad20nm, '</b>', ' ') %>%
+  lapply(htmltools::HTML)
+
+# plot map
+library(leaflet)
+leaflet(sf_data) %>%
+  addTiles() %>%  # Add default OpenStreetMap map tiles
+  addPolygons(color = "blue", fillColor = "blue", fillOpacity = 0.2, weight = 1.5, popup = ~(lad20nm), # popups clicked
+              label = ~(popup_text),                                            # define labels
+              labelOptions = labelOptions(                                      # label options
+                style = list("font-weight" = "normal", padding = "2px 2px"),
+                direction = "auto"),
+              highlight = highlightOptions(
+                weight = 5,
+                color = "#666",
+                fillOpacity = 0.7,
+                bringToFront = TRUE))