Validating groups of inputs

[eluser]vertmonkee[/eluser]
I have a form where a user is entering a location. The name and description are required so I have set my rules like this

Code:

$rules['locationName']        = "required";
$rules['locationDescription'] = "required";

They can then choose a coordinate system from three choices.

1. Decimal latitude / longitude
2. Grid reference
3. Latitude / longitude

How would I go about setting a rule for this?

Thanks for any help