Passing Variables Into A Function From A Form

I have a vague idea of what might be going wrong but I have no idea what I should be doing instead. Here is what I'm trying to do: User inputs X and Y coordinates into form validate that they are numbers do a little bit of maths redirect to a php script with a single variable tacked onto the end of the url Here is the form code:

//part of a larger php script to make the form
echo "<form name='gotoForm' onsubmit='return coordCalc()'>
<fieldset>
<legend>Go to Square</legend>
X <input type='text' id='X' size='1' maxlength='3'/>
Y <input type='text' id='Y' size='1' maxlength='3'/>
<input type='submit'
[Code]...