Ice fishing is the practice of catching fish with lines and fish hooks or spears through an opening in the ice on a frozen body of water. Ice anglers may sit on the stool in the open on a frozen lake, or in a heated cabin on the ice, some with bunks and amenities. Ice fishing methods have changed drastically over the past 20 years. The name of the game is Mobility for today’s modern ice anglers. The days of drilling one hole, waiting and hoping that a fish will swim by, are starting to fade. With light gear, battery-operated sonar units, and fast and powered augers, an angler can conceivably drill and check hundreds of holes in a single day. When the fish stop biting where they are, anglers can move to the next hole, checking it with their sonar first to look for activity, and if there are no fish they will keep moving until fish are found. In addition, schools of fish tend to move around; so a hole may be productive for 10 minutes and then slow down to nothing for an hour before a school returns to that location. This "fish where the fish are" technique and ease of mobility increases the catch rate of any angler, because it minimizes the wait between bites, similar to "trolling" in summer. Anglers can now use many available maps and surveys to help pinpoint lakes and areas within those lakes that make sense to try for specific fish, noting those locations in latitude and longitude coordinates. They are then able to use a handheld GPS receiver to aim them to those spots, usually with accuracy of less than 20 feet. Ice anglers then drill holes with whichever auger they have, checking the ice thickness for safety as they go. Using sonar, the angler can determine the depth of the water, bottom content, weed and structure cover, and even see if there are fish there. Also, by using sonar, they can place the bait according to where they think the fish are. If they are using "tip-ups" they can carpet the area at different depths and with different presentations (the number allowed being determined by local laws) and see what is the most productive. Modern ice anglers can also use modern reels mounted on shorter (18"-36"/45–90 cm long) fishing rods to actively fish by watching, by using their sonar, where their lure is relative to the fish, and jig accordingly to entice a bite. Ice fishing can be done at any time of day, and is typically most active around dusk and dawn. Different fish are active at different times of day, so anglers need to fish for them accordingly. There are fish houses large enough and comfortable enough to spend many days in a row out on the lake, fishing the entire time. One can even fish in one's sleep, by using audible alarms on one's lines to tell when a fish is biting. There are also many lightweight and highly mobile portable shelters that mount on plastic sleds and collapse for transportation. These can vary from small, one-person shelters (commonly called "Fish Traps") to large and complex shelters able to fit up to 6 people at once.