A small variation on this theme is to place the labels parallel to the line, use spherical degrees for placement, append the degree symbol as a unit for the labels, choose a rounded rectangular textbox, and inverse-video the label:
pscoast -R50/160/-15/15 -JM5.5i -Glightgray -A500 -K -P > GMT_App_O_7.ps
grdcontour geoid.grd -J -O -K -B20f10WSne -C10 -A20+d+um+s8 -Gl50/10S/160/10S -S10 \
-T:-+ >> GMT_App_O_7.ps
psxy -R -J -O -SqD15d:+gblack+kwhite+Ld+o+u-\\260 -W1p transect.d >> GMT_App_O_7.ps
The output is presented as Figure O.7.
