While it is not expressly against any rule that I'm aware of, it is a very bad idea because of the chance to burn one of the rocks. If that were to happen, you will automatically lose the measurement before it happens. What happens if two rocks are so close to the center pin on the tee line that a measuring device cannot be used? Rule 14(5) states, "If two or more stones are so close to the tee that a measuring device cannot be used, and if a visual comparison cannot determine which stone is closet to the tee, the stones shall be considered tied. If these stones were to determine who counted in the end, the end shall be considered blank." When can you use a measuring device before an end is complete? Rule 14(2) says "no physical device to aid visual observation shall be used in measuring prior to the last stone delivered in the end coming to rest except as provided for in Rule 12(7) and 14(4)." So what are those rules? Rule 12 concerns the free guard zone. Rule 12(6) says in part "they shall make the determination by using the six foot measuring stick. If the position of another stone(s) hinders the use of the six foot measure, they may reposition the stone(s), complete the measurement and replace the stone(s) to its original position." Rule 14 (4) says, in part, you can also measure a stone at any time during the end to determine if it is still in play or not "when a stone is located on the back line in proximity to the centre line." It further explains that if one or more stones are in the way of the measure, you are not allowed to move them like you can in the FGZ rule referred to above. In that case, "they may request a non-partisan third party to render a decision." There you have it - the two occasions when you are allowed to measure a rock before an end is complete.