Drawing variation lines

As I introduced in the previous post, on-the-go navigation requires a continuous updating of position, followed by motion in a continuous loop until you reach your destination.

The concept of On-the-Go-Navigation is finding ways of navigating that are fast and robust.   This can be particularly important when bouncing up and down in waves, or with an impatient partner who doesn't like the fact that you stop and read maps.

Here is a list of topics

1.) Drawing variation lines on a chart (this post)
2.) Measuring distances on a chart
3.) Finding bearings on a chart
4.) Finding bearings to objects

I'm assuming that you carry a compass with you - although this isn't always a given.   One of the more befuddling aspects of using a compass is adjusting for magnetic variation (or declination for you land-lubbers).    Variation (or declination) is the difference between true north and magnetic north.   If you don't get it right, it can make a world of difference!

Steve Whittaker is a DC Lobbyist who wanted to recreate the journeys that his dad took and set out to cross Nova Scotia by canoe.    He brought his compass along and got east and west declination confused.   To be clear: the compass needle points about 18 degrees west of true north in Nova Scotia, but Steve for some reason, thought it pointed 18 degrees east, which meant that he was traveling 36 degrees in the wrong direction.   He got lost and it only went downhill from there.   You can read about it here.

There are many ways of figuring magnetic variation.   You can find it on websites.   Any decent topographic map or nautical chart will have it.

Compass rose seen on a modern nautical chart.

The above figure shows a typical compass rose found on a modern nautical chart.   The outer circle contains angles from due north, called azimuth.   The inner circle contains angles from magnetic north - the direction of the local magnetic field lines.   In the inside of the inner circle, you can see the writing:  "Var 15^o 15’ W (2011)" - meaning that the magnetic variation is fifteen degrees and fifteen minutes west - the compass points this distance west of true north.   Below that is the phrase "ANNUAL DECREASE 4'".   This means that the value of the variation decreases by 4 arc-minutes every year.   The date of the observation is given as 2011.    As I write this, it's 2014, which is 3 years since the reading, the actual variation is more like 15^o 3’ W.   Since most normal people don't worry about a few measly arc-minutes, you can probably trust this compass rose. 

Now, when people are taught navigation, there's a lot of dinking around with figuring out what is true north from a magnetic compass reading - whether to add or subtract variation.   There are phrases like "True Virgins Make Dull Companions", and CADET  as mnemonics about whether to add or subtract variation whether going from map to real world or back for east or west variation.   I'll talk about one of these mnemonics later in a future post, but for now, I'm going to cut out the middleman and use a trick that people who do the sport of orienteering use.   

Orienteering is a race from controls that are laid out on a course.   The orienteer gets a map at the start of their turn through the course, and uses a compass to navigate the course.   The lines on the orienteering map are based on magnetic north.    So, borrowing from the sport, I draw my lines of magnetic north ahead of time on the map and base all my navigation work off the magnetic lines.   Furthermore, I space the lines some set distance apart to make the navigational tasks easy. 

You can do this with a protractor and ruler, or just a magnetic compass, although it takes a bit of time.   The easiest is to purchase a parallel:

Set of parallels - can be purchased as Staples or equivalent stationery supply store.

And a drafting compass:

Drafting compass - also available at a stationery supply store. 

So, now, I assume you have a drafting compass and a set of parallels.   The first step in putting on variation lines is to use the compass rose on the chart and extend a line of the perpendicular to the magnetic north on the chart.  This is shown in the figure below. 

The first step is to draw a line along the perpendicular to the magnetic north line using the compass rose on the chart and a set of parallels.

If you're using a topographic map, you'll probably find a small symbol at the bottom of the map that indicates the magnetic declination (topos call it "declination", charts call it "variation).   You can do the same thing with a protractor. 

The next two steps are illustrated below.   Us the drafting compass to get the scale from the bottom of the map.   In this case, I've chosen a scale of a nautical mile (one nautical mile is 1.15 statute miles).  Expand the drafting compass until it has a width of one nautical mile.   For different scale maps, you might choose a different distance.  The next step is to draw the actual magnetic north line extended from the compass rose on the chart. 

Next steps - open the drafting compass to one nautical mile (or other distance you choose), and extend the line of magnetic north from the compass rose on the chart. 

The next step is to use the perpendicular line to mark off ticks using the drafting compass at 1 nautical mile intervals.

Use the drafting compass with width set to one nautical mile to make a set of tick marks at intervals along the perpendicular to magnetic north. 

The next, final, step is to use the parallels to draw lines of magnetic north at widths of one nautical mile.  Start with the parallel lined up with the line you drew from the compass rose, and then walk the parallels over with the ticks you marked off the drafting compass.   

Use the parallels to mark off lines of magnetic variation separated by one nautical mile using the tick marks you put on the perpendicular on the previous step. 

Now you have a fully prepared chart with magnetic lines spaced one nautical mile (or other) apart.   This makes it easy to do chart-work on the water without worrying about adjusting for magnetic variation. 

NOW... some might claim that the drawing of variation lines on a chart can obscure important information, such as rocks or buoys.  Unless you use a very thick sharpy, this simply isn't going to happen.   Choose a pencil that's relatively sharp and thin and you can see everything you need on a chart.   

To this end, I have here two scans of a chart I prepared and even when the lines go directly over a rock or a buoy, they're easy to see.

Prepared chart off Stonington.   All rocks and aids to navigation are visible. 

Blow-up of chart.  The buoy and associated information are plainly visible, and even the rock to the NW of Wreck Is. is plainly visible. 

In the next post, we use the spacing of the variation lines to figure distances.