Finding the boundary points in a raster data file

Question

So, I have with me the center point and the zoom level.
I have to plot some points on the map.

The map is stored in a raster data file, and is displayed on the R's widget.

The issue is that when a point is received, I need to check whether it falls inside the data of this raster file or not.
If the point is in the raster file, then I can safely plot it.
If not, then I need to load another raster file which contains that point, and then plot the point.

The raster package of R has a function named as.data.frame which loads the raster data into the data frame.

Is it possible, then, to figure out which points (lats and lons) are at the four corners? Am I on the right track?

Answer 1

If your data is of RasterLayer class then extent will give you the extent of the raster and xmin , ' min , ymax and xmax to access the various slots.

eg

# create a dummy raster
r1 <- raster(nrows=108, ncols=21, xmn=0, xmx=10)
r1[] <-1

extent(r1)
## class       : Extent 
## xmin        : 0 
## xmax        : 10 
## ymin        : -90 
## ymax        : 90 

You can access the various slots using

xmin(r1)
## [1] 0
xmax(r1)
##[1] 10
ymin(r1)
## [1] -90
ymax(r1)
## [1] 90

If your data is a SpatialGridDataFrame then bbox will return the bounding box

.grid <- as(r1,'SpatialGridDataFrame')

bbox(.grid)
##    min max
## s1   0  10
##  2 -90  90

Does my xy coordinate lie within the raster boundary

you can use cellFromXY to find the cell id , and it will return NA if it is outside the extext

eg

 # some data
 .points <- rbind(c(1,1),c(-4,1))
 # the first point lies within the raster, the second not

 # cell from XY will tell you this.
 cellFromXY(r1,.points)
 ## [1] 1116   NA

EDIT for ggmap

if you have a map acquired by get_map , it is a ggmap object, and will not work with the package raster without some help from you .

you can obtain the bounding box as the bb attribute.

  hdf <- get_map()
  attr(hdf,'bb')
##    ll.lat    ll.lon   ur.lat    ur.lon
## 1 29.38048 -95.80204 30.14344 -94.92313

A helper function that will create a RasterStack from a ggmap object

ggmap_rasterlayer <- function(map){
  map_bbox <- attr(map, 'bb') 
  .extent <- extent(as.numeric(map_bbox[c(2,4,1,3)]))
  my_map <- raster(.extent, nrow= nrow(map), ncol = ncol(map))
  rgb_cols <- setNames(as.data.frame(t(col2rgb(map))), c('red','green','blue'))
  red <- my_map
  values(red) <- rgb_cols[['red']]
  green <- my_map
  values(green) <- rgb_cols[['green']]
  blue <- my_map
  values(blue) <- rgb_cols[['blue']]
  stack(red,green,blue)

}

my_map <- ggmap_rasterlayer(hdf)