Update gps vignette

vignettes/gps-tracks.Rmd

@@ -48,9 +48,28 @@ st_layers("trace.gpx")
 ```
 
 The previous code code chunk downloads the trace from OSM and queries the 'layers' that can be imported (note there are 5, but only 'tracks' and 'track_points' are available).
+The following code chunk imports and plots the 'tracks' layer, which is a MULTILINESTRING geometry in sf, and only 1 row of data:
 
 ```{r}
 r = st_read("trace.gpx", layer = "tracks")
+class(r)
+st_geometry_type(r)
+nrow(r)
 plot(r$geometry)
 ```
 
+Other than the geometry, this object has limited information about the route in the fields. 
+The richest field is 'Description', which contains the following summary data:
+
+```{r, results='asis'}
+class(r$desc)
+as.character(r$desc)
+```
+
+The data can also be imported as points, as follows:
+
+```{r}
+p = read_sf("trace.gpx", layer = "track_points")
+plot(p)
+```
+