title: R code that evaluates line-of-sight (LOS) between two (lat, lon) points
body: <p>I'm having trouble figuring out how to calculate line-of-sight (LOS) between two (lat, lon) points, within R code. Any advice on how to approach this problem would be appreciated. I would like to use the R package - raster -  for reading in the terrain elevation data. It seems the spgrass package could be leveraged (based on <a href="http://grass.osgeo.org/grass70/manuals/r.viewshed.html" rel="noreferrer">http://grass.osgeo.org/grass70/manuals/r.viewshed.html</a>) but I wanted to avoid loading up a GIS. Thanks. </p>