Degrees, minutes and seconds are separated by colons, and each can
have a decimal point as well. First character must be a minus sign or
number, and last character must be W, E, S, N, or number.
References
Wessel, P. and W.H.F. Smith. 2004. The Generic Mapping Tools:
Technical reference and cookbook. Version 4. Available at
http://gmt.soest.hawaii.edu/gmt/doc/pdf/GMT$\_$Docs.pdf.