# How to use a compass: bearings, maps and declination Canonical: https://romeoapps.app/en/surv/how-to-use-a-compass/ Author: Roméo Gambino · Published: 2026-09-10 · Updated: 2026-09-10 · App: Surv (https://romeoapps.app/en/surv/) The parts of a baseplate compass, how to take and follow a bearing, orient a map, correct for magnetic declination, and avoid readings disturbed by metal. ## Why learn it when your phone has one A phone compass is convenient until the battery, the cold or the water gets to it. A baseplate compass has no battery and no screen, and it works the same way every time. It also has a limit worth knowing from the start: as the US Geological Survey puts it, at most places on the Earth's surface the compass does not point exactly toward geographic north. Using a compass well is mostly about handling that difference calmly. ## The parts of a baseplate compass A baseplate compass is a clear rectangular plate with straight edges, often with a ruler printed along them. Along its length runs the direction-of-travel arrow. On the plate sits a rotating dial marked in degrees from 0 to 360, with an index line where you read the number. Inside the dial are the orienting arrow and a set of parallel orienting lines, which turn with the dial. Floating above them is the magnetic needle; its red end points to magnetic north. ## Hold it properly Hold the compass flat and level in front of you, at about chest or waist height, with the direction-of-travel arrow pointing straight away from your body. A tilted compass lets the needle drag or stick. Turn your whole body rather than the compass alone, so the arrow always points where you are facing. Give the needle a moment to settle before you read anything. ## Keep metal and magnets away The needle responds to any magnetic field near it, not only the Earth's. Steel and magnets pull it off: a car, a metal fence, a bridge rail, keys, a knife, and a phone or a phone case with magnets held next to the compass. Power lines can disturb it too. If a reading looks wrong, step a few paces away from the object and read again. A compass is only as good as the space around it at the moment you read it. ## Find a direction without a map Suppose you know you need to walk at 90 degrees, due east by the compass. Turn the dial until 90 sits at the index line. Hold the compass level and turn your body until the red end of the needle sits exactly inside the orienting arrow. The direction-of-travel arrow now points along your bearing. This is the basic move, and every other technique in this guide uses it. ## Take a bearing on something you can see To measure the direction of a landmark, point the direction-of-travel arrow at it, holding the compass level. Then, without moving the plate, turn the dial until the orienting arrow lies under the red end of the needle. Read the number at the index line: that is the bearing to the landmark. Writing it down lets you return along the reverse direction later, which is the bearing plus or minus 180 degrees. ## Follow a bearing across ground Walking with your eyes on the compass is a good way to trip. Instead, set your bearing, sight along the direction-of-travel arrow and pick a distinct feature on that line: a tree, a rock, a post. Walk to it, then repeat. In poor visibility, choose closer features. If an obstacle blocks the line, you can walk around it and use a feature you picked on the far side to rejoin the same bearing. ## Read the map before the compass A compass tells you direction; a map tells you where things are. The USGS explains that topographic maps show the shape of the land with contours, imaginary lines that follow the ground surface at a constant elevation, usually printed in brown, with heavier index contours at intervals. Natural and man-made features use standard symbols and colours: woodland in green, water in blue. Spend time with your local map before you need to navigate with it. ## Take a bearing from the map Place one long edge of the baseplate on the map so it joins where you are to where you want to go, with the direction-of-travel arrow pointing toward your destination. Hold the plate still and turn the dial until the orienting lines are parallel to the map's north-south lines, with the orienting arrow pointing to the top of the map. Read the number at the index line. This is a map bearing, measured from the map's north, not yet from magnetic north. ## Declination: why the needle is not map north NOAA's National Centers for Environmental Information define magnetic declination, sometimes called magnetic variation, as the angle between magnetic north and true north. Declination is positive east of true north and negative when west, and it changes over time and with location. The USGS adds that at very high latitudes the compass can even point south. Where declination is small it barely matters over a short walk; where it is large, ignoring it sends you steadily off course. ## Where to find your declination The USGS notes that the margin of its topographic maps shows the magnetic declination at the centre of the map for the year the map was made, and that most of these maps carry a diagram with three north arrows: true north, grid north and magnetic north. Because declination changes over time, the figure on an old map may be out of date. NOAA provides an online calculator that estimates declination for any latitude and longitude on a specific date. Look yours up and write it on the map. ## Apply the correction With declination defined as positive east, the compass bearing equals the map bearing minus the declination. So with an east declination you subtract it from your map bearing before walking; with a west declination you add it. Going the other way, from a compass bearing to the map, you do the reverse. Some compasses have an adjustable declination setting that does this for you once set; check whether yours has one and whether it is set for where you are. ## Orient the map to the land An oriented map is turned so that its north matches the real north around you, and features on the map line up with what you see. Lay the compass on the map with its edge along the map's north-south lines. Set the dial so the declination correction is included, then rotate the map and compass together until the red needle sits in the orienting arrow. Now a road that runs left on the map runs left in front of you. ## Phone compasses and their limits A phone measures the same magnetic field with a small sensor, so the same interference applies: cases with magnets, car mounts and nearby metal all shift its reading. Phones may ask you to calibrate the sensor by moving the device, and many compass apps let you choose between true north and magnetic north, so check which one yours is showing. NOAA's declination page links a compass web app that uses the phone's sensor. It is a useful tool, and it still depends on a charged battery. ## Where Surv fits Surv includes compass and coordinates tools that work on the iPhone without a network, alongside its offline guides. They share the phone's limits: battery, sensor calibration and magnetic interference. Carry a baseplate compass and a paper map of the area you are in, and practise the moves in this guide somewhere familiar, where a mistake costs a short walk back rather than a real problem. ## Questions ### What is magnetic declination? NOAA defines it as the angle between magnetic north and true north. It is positive east of true north and negative west, and it changes over time and with location. ### Where do I find the declination for my area? In the margin of a topographic map, where USGS maps show it for the year the map was made, or with NOAA's online calculator for any location and date. Old maps may be out of date. ### Why does my compass give a strange reading near my car? Steel and magnets near the needle pull it off magnetic north. Step away from vehicles, fences and railings, and keep phones, keys and magnetic cases away from the compass while you read it. ### Is a phone compass as good as a real compass? It measures the same magnetic field and suffers from the same interference, but it depends on a battery and may need calibrating. Carry a baseplate compass as a backup. ## Further reading - NOAA NCEI: Magnetic declination — https://www.ngdc.noaa.gov/geomag/declination.shtml - NOAA NCEI: Magnetic field calculators — https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml - USGS: What is declination? — https://www.usgs.gov/faqs/what-declination - USGS: Finding your way with map and compass — https://www.usgs.gov/publications/finding-your-way-map-and-compass