if statement from drop down selection and then save data

I have a box for user to enter altitude number and a drop down box right next to it so the user has option to choose "feet" or "meters". I don't know how to make an if statement that will recognize if the drop down box option was chosen in feet, it will then convert to Meters (I want to have meters as default unit in the system but still give user option to enter in feet if needed to)and then save the database.

Altitude : <input type = "text" name = "Altitude" value = "0" select id = "altUnits" name = "AltUnits"> <option value = "FEET" > FEET options  <option value = "METERS" > METERS options select

input type = <"submit" tabindex = 2 name = "jdbc_query_add_Newpoint" value = "ADD"/>