Error trapping

I'vo got a little form that I use on my site here

http://www.windsurf.me.uk/Links/links2.html

with the forms being accessed by the buttons under the map.

The form was generated with phpformgen here

http://phpformgen.sourceforge.net/

I now need to add some error trapping to the longitude and latitude
fields. People have been entering the data in a format other than the
one required and this kills the map until I edit the database and
correct it.

Any suggestions how to do this greatly appreciated.