Finding the boundary points in a raster data file

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?

Solution

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.

# 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

 # 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)