Using:
gmt grdmath -R-156.1/-154.8/18.9/20.3 -I15s hawaii_contour_3000m_15s_C.txt ldist = ldist_to_contour_3000m_15s.grd
results in this error:
grdmath [ERROR]: gmt_M_err_trap: 2
grdmath (gmt_api.c:2370(gmt_get_header)): Not a supported grid format [hawaii_contour_3000m_15s_C.txt]
[Session gmt (0)]: Error returned from GMT API: GMT_GRID_READ_ERROR (18)
[Session gmt (0)]: Error returned from GMT API: GMT_GRID_READ_ERROR (18)