Trying to convert Sandwell's topex topo data

gmt img2grd -V -R-180/180/-80/80 topo_18.1.img  -Gtest2.grd -T1 -D
img2grd: Expects topo_18.1.img to be 21600 by 17280 pixels spanning 0/360.0/-80.738009/80.738009.
img2grd: To fit [averaged] input, your topo_18.1.img is adjusted to -R-180/180/-80.0023237126/80.0023237126.
img2grd: The output grid size will be 21600 by 16752 pixels.
img2grd: Created 21600 by 16752 Mercatorized grid file.  Min, Max values are -10914  8550
img2grd: Undo the implicit spherical Mercator -Jm1i projection.
grdproject: Processing input grid
grdproject: Transform (-180/180/-80.0023/80.0023) <-- (0/360/0/279.199726827) [inch]
grdproject: gmt_grd_project: Output grid extrema [-18704/16090] exceed extrema of input grid [-10914/8550] due to resampling
grdproject: gmt_grd_project: See option -n+c to clip resampled output range to given input range
grdproject: Proj4 string to be converted to WKT:
	+proj=longlat +no_defs