Finish draft CRS section. Close #53

02-spatial-data.Rmd

@@ -670,7 +670,21 @@ To discover what the 'magic number' `27700` means, we can retrieve the CRS again
 st_crs(sf_points)
 ```
 
-Note that the result shows that the `epsg` has been updated to this code.
+Note that the result shows that the `epsg` has been updated. But what does this mean?
+The answer can be found in another function, from the **rgdal** library:
+
+```{r}
+crs_codes = rgdal::make_EPSG()
+dplyr::filter(crs_codes, code == 27700)
+```
+
+This shows that the CRS we set was the British National Grid.
+Although this result could also have been found by searching online for "[CRS 27700](https://www.google.co.uk/search?q=CRS+27700)", or by plotting it with a command such as `mapview::mapview(sf_points)` and zooming out, the method showed above was useful for delving into the nuts and bolts of how EPSG codes work.
+The formula that converts a geographic point into a point on the surface of the Earth is provided by the `proj4string` element of the `crs` (see [proj4.org](http://proj4.org/) for further details):
+
+```{r}
+st_crs(27700)$proj4string
+```
 
 ### {-} Pro tip