title: Extract portion of netcdf file based on latitude and longitude in R
body: <p>I have a netcdf file of GRACE data containing change in groundwater storage  of all over the world from 2002 to 2016. That also contains lat/lon values
  I want to extract data of a particular study area from that world data based upon lat lon values of that area using R .
  The latitude variable runs from +90 to -90 and the longitude variable runs form 0 to 360.</p>
  
  <p>I wish to extract a subset of the overall grid using the following geographical corner boundaries.
   bottom left corner :(lat 21.5, loon 85.5)   Top left Corner :(lat 27.5, lon 85.5)     Top right corner :(lat 27.5 lon 89.5)    bottom right corner :(lat 21.5, lon 89.5) .</p>
  
  <p>Any advice would be greatly appreciated.</p>