Improve vignette "spatial", add links

Author: Florian Mayer

vignettes/spatial.Rmd

@@ -36,8 +36,9 @@ If several spatial fields are captured, it is up to the user to choose which
 field to use as primary geometry.
 
 The second difficulty is that the parsed data from ODK Central is a plain table 
-(`tbl_df`) with some columns containing spatial data. WKT is stored in text 
-columns, GeoJSON is stored in list columns.
+(`tbl_df`) in which some columns contain spatial data. WKT is stored in text 
+columns, GeoJSON is stored in list columns. This format is not recognised as a
+standard spatial input format.
 
 ```{r, message=FALSE}
 library(ruODK)
@@ -91,11 +92,15 @@ longitude, altitude, and accuracy individually e.g. to plot a Leaflet map.
 
 ```{r, fig.width=9, fig.height=7}
 geo_sf_point <- geo_wkt %>% sf::st_as_sf(wkt="point_location_point_gps")
+# Mapview using sf
 mapview::mapview(geo_sf_point, col.regions = sf::sf.colors(10))
+# GGplot using sf
 ggplot2::ggplot() + ggplot2::geom_sf(data = geo_sf_point, ggplot2::aes(fill = device_id))
+# Leaflet using sf
 leaflet::leaflet(data = geo_sf_point) %>% 
   leaflet::addTiles() %>% 
   leaflet::addMarkers(label = ~ device_id, popup = ~ device_id)
+# Leflet using extracted coordinate components in tbl_df
 leaflet::leaflet(data = geo_wkt) %>% 
   leaflet::addTiles() %>% 
   leaflet::addMarkers(
@@ -111,9 +116,13 @@ We use `sf::st_as_sf` on a text column containing a WKT geotrace.
 
 ```{r, fig.width=9, fig.height=7}
 geo_sf_line <- geo_wkt %>% sf::st_as_sf(wkt="path_location_path_gps")
+# Mapview using sf
 mapview::mapview(geo_sf_line, col.regions = sf::sf.colors(10))
+# GGplot using sf
 ggplot2::ggplot() + 
   ggplot2::geom_sf(data = geo_sf_line, ggplot2::aes(fill = device_id))
+# Leaflet using first extracted coordinate compoments from plain tbl_df
+# Note this represents the geotrace (line) with a simple point
 leaflet::leaflet(data = geo_wkt) %>% 
   leaflet::addTiles() %>% 
   leaflet::addMarkers(
@@ -123,19 +132,46 @@ leaflet::leaflet(data = geo_wkt) %>%
     popup = ~ device_id)
 ```
 
-## Map geoshapes
+## Map geoshapes (polygons)
 Again, we'll use `sf::st_as_sf` but select a WKT geoshape column.
 
 ```{r, fig.width=9, fig.height=7}
 geo_sf_poly <- geo_wkt %>% sf::st_as_sf(wkt="shape_location_shape_gps")
+# Mapview using sf
 mapview::mapview(geo_sf_poly, col.regions = sf::sf.colors(10))
+# GGplot using sf
 ggplot2::ggplot() + 
   ggplot2::geom_sf(data = geo_sf_poly, ggplot2::aes(fill = device_id))
+# Leaflet using first extracted coordinate compoments from plain tbl_df
+# Note this represents the geoshape (polygon) with a simple point
 leaflet::leaflet(data = geo_wkt) %>% 
   leaflet::addTiles() %>% 
   leaflet::addMarkers(
     lng = ~ shape_location_shape_gps_longitude,
     lat = ~ shape_location_shape_gps_latitude,
     label = ~ device_id, 
     popup = ~ device_id)
-```
+```
+
+# Outlook
+The above examples show how to turn spatial data into an `sf` object, and give
+very rudimentary visualisation examples.
+
+See the [sf homepage](https://r-spatial.github.io/sf/) for more
+context and examples. 
+The [sf cheatsheet](https://github.com/rstudio/cheatsheets/blob/master/sf.pdf) 
+deserves a spatial mention.
+
+There are several other good entry points for all things R and spatial, 
+including but not limited to:
+
+* The [R Spatial CRAN Task View](https://cran.r-project.org/web/views/Spatial.html)
+* The [RSpatial](https://rspatial.org/) website
+* [Geospatial data in R and beyond](https://www.maths.lancs.ac.uk/~rowlings/Teaching/UseR2012/) 
+  by [Barry Rowlingson](http://barry.rowlingson.com/)
+* [GIS with R](https://www.jessesadler.com/post/gis-with-r-intro/) 
+  by [Jesse Sadler](https://www.jessesadler.com/page/cv/)
+  
+The above list of examples and resources is far from comprehensive.
+Feel free to [contribute or suggest](https://github.com/ropensci/ruODK/issues) 
+other working examples for turning data from `ruODK` into spatial formats.