fishnet function creates a regular grid of locations covering
the study area at various grain sizes.
fishnet(mask, res = 0.5)
a vector polygon covering the boundary of the survey region.
the grain size of the grid cells in decimal degrees (default).
A spatial vector polygon object of equal area grid cells covering the defined area.
Phillips, S.J., Anderson, R.P. & Schapire, R.E. (2006) Maximum entropy modeling of species geographic distributions. Ecological Modelling 190: 231-259.