Variation and deviation

this video is going to take a look at variation and deviation and we're going to look at the causes of both of them and how to calculate both of them as well both variation and deviation are effects on a magnetic steering compass and we're going to look at variation first from the point of view of navigating between a compass and a chart it's important to realize that there are two North's firstly there's the North Pole where Santa lives and there's a candy cane stuck in the ground and secondly there's a magnetic north where the magnets live our steering compasses and our handheld compasses are magnetic devices and they point towards the magnetic north not true north however our charts are all orientated to true north the resulting angle between magnetic north and true north is what we call variation and it will change according to where you are on the planet so why do we do this why don't we just orientate our charts towards magnetic north and be done with it charts are orientated towards the North Pole true north we'll call it because it's a fixed position on the globe and it doesn't move however magnetic north does move the magnetic pole or the center of the magnetic field is currently moving east around the top of the planet and because it moves consistently it's impossible to orientate a fixed chart towards a moving object therefore the problem we have as navigators is that we have to do all our planning relevant to true north using charts then we have to translate these true headings for our plan into magnetic headings that we can follow on a boat steering compass the further problem is if variation is different depending on where you are in the planet how do we make those changes when we're looking at a chart then we can come across this symbol this is called a compass rose it's a printed compass on the chart that's orientated to true north there are a couple of additional bits of information written within the compass rose this tells us that the variation in this area is 40 minutes west zero degrees 40 minutes west and that was in 2019 and this tells us that the variation is decreasing by 11 minutes every year remember there are 60 minutes in a whole degree so looking at that as an example then variation is zero degrees and 40 minutes to the West in 2019 that's decreasing by 11 minutes every year so in 2020 variation is 29 minutes 40 minutes - 11 minutes variation can move either east or west depending on where you are in the planet if you take it look at this example where we are here in Africa true north pointing dead upwards the variation would be moving to the west take a look at this image that was sourced from Wikipedia it shows the magnetic field across the planet variation changes wildly according to where you are for example here this green line represents zero degrees of variation so if you're on that green line your compass won't have any variation between true north and magnetic north however this blue line represents 40 degrees of variation so that's a 40 degree difference between your steering compass and what you can read off of your charts we need to know how to calculate the difference between true north and magnetic north otherwise we'll get into a problem if we do a plan for a vessel heading towards the Philippines in true north and then give them a heading to follow intrude on their compass they would be wrong by 40 degrees and end up heading towards the middle east so it's really important then that we use magnetic headings for the compass and true headings for the chart so how do we calculate the difference we could either be trying to go from a magnetic heading to a true heading or to a true heading to a magnetic heading we could have easterly variation or westerly variation and that could be changing wherever we are it all sounds very complicated but there's a simple tool I use to help me remember what to do I used this word cadet what this word reminds me is that if I'm working from a compass or a magnetic heading to a true heading I add easterly variation if I'm adding an easterly variation I would take away a westerly variation filling in the complete cadet picture it looks a bit like this so if I'm going from compass to true I would add east or take away West as it shows me across the top and if I'm working the other way from true to compass or true to magnetic I would take away in these silly variation and add a westerly variation let's pop that over here in the corner and look at some examples if I'm being given a magnetic heading of a hundred and ten degrees and I want to work out what that is to be able to put it on the chart I need to change it to true so in this example my heading is 110 degrees magnetic my variation is 5 degrees to the east looking at the cadet example in the corner I'm going from magnetic to true so I'm going to add East so a hundred and ten magnetic plus five degrees of variation to the east gives me a hundred and fifteen degrees true this is now a heading I can plot on the chart here's another example so the heading is ninety five degrees magnetic and the variation is nine degrees but this time to the west looking at the cadet picture I'm going from compass to true so I'm going to take away West so ninety five degrees minus nine degrees variation to the west gives me a true heading of 86 degrees look at this example going the other way this time I have a heading of 265 degrees true and I want to change it to a magnetic heading the variation is six degrees to the west looking at the cadet picture I'm going from true to compass so I'm going to add West two hundred and sixty five degrees plus of six degree Western variation gives me 271 degrees magnetic that is now a heading I can tell the helmsman to follow of the steering compass here's another example going from true to magnetic a heading of 65 degrees true with an eighth degree easterly variation looking at the cadet picture I'm going to go from true to compass so I'm going to take away east 65 degrees true - an eighth degree easterly variation gives me a heading of 57 degrees magnetic that I can follow on the compass so that's the cause of variation and how we can calculate the differences let's take a look at what deviation is as we've already mentioned the compass is a magnetic device and it gets interfered with by the magnetic pole like any magnet though it also gets interfered with by other magnetic four is on our boats this includes things like radios mobile phones speakers any kind of large metal objects the engines themselves all of these things create an electric or a magnetic field that will interfere with our compass in order to discover what the effect on our compass is we need to create something called a deviation card take a look at this example from one of our old boats in order to create a deviation card an engineer has to come on board with something called a gyro compass a gyro compass isn't a magnetic device is an electronic device what then happens is the boat goes on certain magnetic headings and the gyro compass records the difference between the true north and what the boat is steering we then try and install additional magnets to the compass Pole and bend the needle back to as close to true north as possible the result of effort gives us a card like this where we have minimal degrees of variation on different headings once we have this card set up we apply westerly deviation and easterly deviation in exactly the same way as we do for variation so the cadet bird still tells us the correct way to apply it the one thing we need to make sure of though is that we apply their changes in the correct order firstly we need to apply the compass correction so we need to use deviation which will then give us a magnetic heading once we've calculated the deviation we've dealt with the problem that's unique to our vessel we then have a magnetic heading that we can apply variation to to give us a true heading so this is the order of work then if you're working from a compass first to a true heading you applied deviation get the magnetic heading then apply variation to get the true heading conversely if you're going the other way from the true heading to a compass heading then you apply variation to get the magnetic heading then deviation to give yourself a compass heading say this have a rather forcing mnemonic to help remember this and it's cruisers don't meet virgins twice that's variation and deviation then try not to get bogged down in the detail and remember it's just simple arithmetic you just need to make sure you're going the right way on watch I often write the cadet were down on the chart and drawing the arrows and the plusses and minuses so I know that I don't make a mistake [Music] [Music] [Music] [Music] [Music] [Music]

Channel: Get Lost Sailing & Powerboat Training

Views: 22,797

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Id: OSsYYCCWRqU

Length: 12min 4sec (724 seconds)

Published: Fri Apr 10 2020