Geographic Web Services

The Open Geospatial Consortium (OGC) has created a number of open specifications for web services including web feature services (WFS), web map services (WMS), Web Map Tile Service (WMTS), web coverage services (WCS), and web processing services (WPS). On this page, we will look at how to start working with these functions.

library(httr)
url <- "https://shiny1.envs.vcu.edu/geoserver/wfs"

q <- list( request = "GetCapabilities" )
res <- GET( url = url,
            query = q )

res

## Response [https://shiny1.envs.vcu.edu/geoserver/wfs?request=GetCapabilities]
##   Date: 2019-11-21 20:10
##   Status: 200
##   Content-Type: application/xml
##   Size: 102 kB
## <BINARY BODY>

If we need to, we can look at the response as an XML format

library(xml2)
txt <- content( res, "text" )
xml <- read_xml(  txt )
xml

## {xml_document}
## <WFS_Capabilities version="2.0.0" schemaLocation="http://www.opengis.net/wfs/2.0 http://shiny1.envs.vcu.edu/geoserver/schemas/wfs/2.0/wfs.xsd" updateSequence="269" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.opengis.net/wfs/2.0" xmlns:wfs="http://www.opengis.net/wfs/2.0" xmlns:ows="http://www.opengis.net/ows/1.1" xmlns:gml="http://www.opengis.net/gml/3.2" xmlns:fes="http://www.opengis.net/fes/2.0" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:cite="http://www.opengeospatial.net/cite" xmlns:tiger="http://www.census.gov" xmlns:nurc="http://www.nurc.nato.int" xmlns:sde="http://geoserver.sf.net" xmlns:it.geosolutions="http://www.geo-solutions.it" xmlns:topp="http://www.openplans.org/topp" xmlns:sf="http://www.openplans.org/spearfish" xmlns:envs421="ces.vcu.edu" xmlns:baja="dyerlab.com/baja">
## [1] <ows:ServiceIdentification>\n  <ows:Title>GeoServer Web Feature Serv ...
## [2] <ows:ServiceProvider>\n  <ows:ProviderName>Center for Environmental  ...
## [3] <ows:OperationsMetadata>\n  <ows:Operation name="GetCapabilities">\n ...
## [4] <FeatureTypeList>\n  <FeatureType xmlns:tiger="http://www.census.gov ...
## [5] <fes:Filter_Capabilities>\n  <fes:Conformance>\n    <fes:Constraint  ...

Grabing a Feature

On this server, there exists a feature called bio7 in the baja workspace. It is a raster image. We can grab that directly from the server as:

library(sf)

query <- list( request="GetFeature",
               typeName = "topp:states" )
file <- tempfile(fileext=".gml")
httr::GET( url=url, 
           query = query, 
           httr::write_disk(file) )

## Response [https://shiny1.envs.vcu.edu/geoserver/wfs?request=GetFeature&typeName=topp%3Astates]
##   Date: 2019-11-21 20:10
##   Status: 200
##   Content-Type: application/gml+xml; version=3.2
##   Size: 310 kB
## <ON DISK>  /var/folders/ty/1tdv5mw56p33qx86hnk5h8l80000gn/T//Rtmp72DZ9d/file33e73d5c9b82.gml

states <- read_sf( file )

then we can translate it into an sf object with the following attributes.

class(states)

## [1] "sf"         "tbl_df"     "tbl"        "data.frame"

names(states)

##  [1] "gml_id"     "STATE_NAME" "STATE_FIPS" "SUB_REGION" "STATE_ABBR"
##  [6] "LAND_KM"    "WATER_KM"   "PERSONS"    "FAMILIES"   "HOUSHOLD"  
## [11] "MALE"       "FEMALE"     "WORKERS"    "DRVALONE"   "CARPOOL"   
## [16] "PUBTRANS"   "EMPLOYED"   "UNEMPLOY"   "SERVICE"    "MANUAL"    
## [21] "P_MALE"     "P_FEMALE"   "SAMP_POP"   "the_geom"

print( states, n=3)

## Simple feature collection with 49 features and 23 fields
## geometry type:  MULTISURFACE
## dimension:      XY
## bbox:           xmin: -124.7314 ymin: 24.95597 xmax: -66.96985 ymax: 49.37174
## epsg (SRID):    NA
## proj4string:    NA
## # A tibble: 49 x 24
##   gml_id STATE_NAME STATE_FIPS SUB_REGION STATE_ABBR LAND_KM WATER_KM
##   <chr>  <chr>      <chr>      <chr>      <chr>        <dbl>    <dbl>
## 1 state… Illinois   17         E N Cen    IL         143987.   1993. 
## 2 state… District … 11         S Atl      DC            159.     18.0
## 3 state… Delaware   10         S Atl      DE           5062.   1385. 
## # … with 46 more rows, and 17 more variables: PERSONS <dbl>,
## #   FAMILIES <dbl>, HOUSHOLD <dbl>, MALE <dbl>, FEMALE <dbl>,
## #   WORKERS <dbl>, DRVALONE <dbl>, CARPOOL <dbl>, PUBTRANS <dbl>,
## #   EMPLOYED <dbl>, UNEMPLOY <dbl>, SERVICE <dbl>, MANUAL <dbl>,
## #   P_MALE <dbl>, P_FEMALE <dbl>, SAMP_POP <dbl>, the_geom <MULTISURFACE>

The geom feature scolumn is:

attr( states, "sf_column")

## [1] "the_geom"

And we can plot it as:

library(ggplot2)
ggplot() + geom_sf( data=states, aes(fill=P_MALE))