Map and Compass Guide
Home Made Map and Silva Compass
What is a Map?
A map is a smaller scaled down version of the ground below your feet. Normally I would purchase 1;25,000 and 1;50,000 scaled maps such as Ordnance Survey. So this means (taking the 1;25,000 as an example) that 1 square on the map can be multiplied by 25,000 to achieve the actual real area of the ground.
The map in the picture with the use of Google Earth and Garmin Connect is scaled with 1cm representing 200 metres.
A good map should contain detailed information such as landmarks, contour lines, altitude, a scale and a legend (explaining what that line on the map means).
Map and scale is a birds eye view of the ground, drawn to scale capable of showing physical features, man made features and relief information. When it shows all of these it is known as a Topographical Map.
By the time I post maps on this blog I will aim to provide as much useful information as possible to make navigation as easy as possible.
Ideally maps should be orientated North such as the one pictured above. This means that the top of the page is North and the bottom is South. This makes it easier to navigate compared to a map that isn't orientated North. This will become clearer shortly.
Types of North
Just to confuse you there are three types of North. It is worth noting that this is provided for information purposes as this is more relevant to those who wish to travel over much longer distances. However best practice is to transfer the grid bearing into a magnetic bearing and should always be done when possible.
- Grid North: The North that is on the paper map is straight up towards the North Pole.
- Magnetic North: The North that your compass points to is in the direction of Canada.
- True North: In-between Grid and Magnetic
In other words, if you take a compass reading from the paper map and use it on the ground, over time and distance you will actually be walking off course. Over 100 metres this isn't really an issue, however over 1000 metres or more you can notice a big difference.
To adjust your compass to a magnetic bearing you have to work out the current GMA or Grid Magnetic Angle and add this figure to the compass bearing. If you go the opposite way and want to achieve a grid bearing from a magnetic bearing you have to subtract the GMA.
On an Ordnance Survey map it will tell you the year it was made and you should subtract this year from the current year and write this figure down. For example; a map made in 2008 used in year 2016. 2016-2008=8
Now you need to look on the map for the annual change rate in mils. This means that every year the ground changes as the Earth is moving so your map needs to be adjusted also.
The annual change rate on the map will say something like 2.5 mils = Annual Change.
So now you should multiply 8 (the difference in years) by the annual change rate (2.5 mils)
8 x 2.5 = 20 mils
Now look at the current map's GMA and subtract 20 mils
Example 39 mils (current GMA) subtract 20 mils = 19 mils
If you purchase a map made in 2016 and use it in 2016 obviously there is no need for adjustment and you would use the GMA on the map.
Once you have obtained the GMA it can be applied in two ways. If you subtract GMA from a magnetic bearing you will have a grid bearing. If you have a grid bearing you must add the GMA to obtain a magnetic bearing.
Grid to Mag = Add
Mag to Grid = Get Rid
Bearing taken from map = 0800
GMA is 27
0800 + 27 = 0827
Now you would set your compass to 0827. If you were to set your compass to 0800 without adjustment, as said before, over time and distance you would be dramatically off course.
The GMA formula: Current year - Year of map x annual change rate - from current GMA = Adjusted GMA.
Note: The formula above is based on mills used on a military compass. By military I don't mean the round green one with a sight, I mean the Silva compass which has the mills displayed around the edges (See link below for info). If you are willing to spend the extra money I would recommend you buy the military version. Although for use in the UAE with Google images as a map, the one used in this blog is more than enough.
Silva Military Compass
Understanding the Compass
The compass (pictured above) can be used either by pointing at an object and taking a magnetic bearing or placing onto a map to take a grid bearing.
The compass starts at 000 degrees and finishes at 360 degrees (see picture above). On a military compass you will also have mils displayed around the bezel (0000-6400) If you read 46 mils on the military compass, in reality the number is 4600.
The scale on the left hand side is for use with maps scaled to 1;25,000 and the scale on the right is for use with maps scaled to 1;50,000.
The arrow on the top of the compass (in black) is called the direction arrow. In other words, when the bearing is set, the North end off the needle (red) is in line with the declination arrow and you hold the compass in front of you, this is the direction you should follow.
The round piece is called the bezel and is able to turn 360 degrees. Inside is filled with water and there should be no bubbles visible as this is a sign of damage and must not be used.
The red arrow inside the bezel is known as the declination arrow.
The red and white needle inside the bezel faces to North (red) and South (white)
The lines inside the bezel are useful when taking a bearing to ensure they are facing directly up the page. This is more useful with a scaled map.
Taking a Grid Bearing (Step 1)
Taking a Grid Bearing (Step 1)
Looking at the map pictured above we are going to take the first grid bearing. The starting point is highlighted in green and the first distance is 600 metres which was measured using the scale of 1cm = 200 metres.
As you can see the direction on the map is from left to right.
Ideally the line should be straight unless it's over a short distance where you account for the slight bend.
In reality I should split that first point into two but we can imagine it's straight.
Taking a Grid Bearing (Step 2)
Taking a Grid Bearing (Step 2)
Place the edge of the compass along the desired line of travel (red line) with the direction arrow pointing in the direction of travel (black arrow)
Leaving the compass in this position, move onto step 3.
Taking a Grid Bearing (Step 3)
Taking a Grid Bearing (Step 3)
Leaving the compass in the exact position, slowly turn the bezel until the red declination arrow points to the top of the map (North, in other words).
This is why (as mentioned earlier) it is important to use maps that are orientated North such as the one in the picture.
Double check that the compass is still facing the direction of travel and the declination arrow is facing to the top of the page.
You can now pick up the compass and record the reading. In this case 092 degrees. This is the first grid bearing.
If you look at the picture of the compass between 080 and 100 there is a black line which indicates the bearing.
Setting Your First Bearing
Using the Compass on the Trail
Now we have our first bearing of 092 degrees and we have reached the starting point of our trek we can now begin to walk the first distance.
Take out the compass and turn the bezel until 092 degrees is in line with the black line we spoke about earlier.
Hold the compass out in front of you (away from any metallic objects such as a watch) and move your body (compass in hand) until the red needle (North) falls in line with the declination arrow (see picture above).
Hold the compass out in front of you and look at where the direction arrow (the black one) is pointing. This is the direction you want to follow. Remember that this is a grid bearing and not a magnetic bearing. In reality you could easily walk 600m on a grid bearing as long as you understand that there will be a slight error.
For ultimate accuracy follow the steps mentioned earlier to achieve a magnetic bearing.
Before you hit the trail you may want to understand how many steps you should take to achieve 100 metres.
To work out your pace you need to find a marked out race track such as the ones you find near the beach. Stand on the start line and start walking (as natural as possible). Try not to over/ under step.
Pick a foot, either left or right that you will use to count the steps. Lets say you choose the left foot.
When you begin walking step off on your left foot followed (naturally) by your right foot, when your left foot hits the ground this is your first step.
Repeat this process until you reach the 100 meter finish line. Record how many steps it took you to reach 100 metres and this is your pace.
In this example: Left, right, left, right, left you should have counted two paces (based on the left foot)
It will probably be around the 65 mark but everyone is different.
Always Know Where You Are
So you have just walked the first 600 metres on a bearing of 092 degrees unless you used the magnetic bearing which would be a slightly higher bearing (remember grid to mag = add) and you want to understand if you are on track.
Firstly, when you started walking you should have orientated the map to the ground. This means that when you hold the map out in front of you it should accurately match your surroundings. For example, the first 600 metres you should hold the map out in front of you with the number 5 to your left and the number 1 directly in front of you. (Numbers in white bubbles)
If you had an obvious land mark to your left such as a village and one to your right, such as a flowing river these should also be on the map. That way when you look at the map and it says there should be a village to your left, there literally should be one or you are not in the right location.
Orientating the map with a compass
Place the compass on the map and turn the bezel until the declination arrow (red) points up the map to grid North.
Rotate the map and compass together (not your body) until the red needle (North) falls in line with the declination arrow (also red).
Confirm that the features/ landmarks match the reality.
Without a Compass
Simply hold map out in front of you and turn until the ground features/ landmarks match
When You Reach the 600 Metre Point
When you reach 600 metres and require to change the bearing again you should confirm that you are actually in that location. Look at any features or landmarks that are highlighted on the map. This could be anything from a large boulder to a house or a tree. Once you can 100% guarantee your exact location you can re-set the compass and keep moving.
This process should be repeated so that you always know your location on the map.
Taking a Magnetic Bearing
Lets say you are lost and you can see your target in the distance such as a telegraph pole. You want to reach the pole but you can see that it's beginning to get dark or the weather is changing.
Hold the compass out in front of you and point the direction arrow (black) at the telegraph pole. Keeping the compass pointing at the pole, start to turn the bezel until the red needle falls within the red declination arrow. The compass is now set to a magnetic bearing.
Rather than walking with the compass in front of you constantly, do this instead;
Hold the compass out in front of you set to the required bearing, move the compass around until the red needle falls within the red declination arrow, look up and see where the direction arrow is pointing, look for something in the distance that is in the same line as travel (a large tree), put the compass away and walk to the tree. When you get there you can repeat the process.
Update on Compass Adjustment
This is an update on adjusting your compass to a magnetic bearing to be used with a bearing from Google Earth maps.
It seems that the North on Google Earth images isn't Grid North. The theory, as described earlier (grid to mag, add etc.) isn't relevant with screen shot maps from Google Earth.
I took two separate "grid" bearings on an image of Dubai (exactly the same way as I described earlier) and then took two magnetic bearings with the Silva Compass at the location to see the difference in degrees. The result was a difference in 5 degrees on both occasions. In this case to achieve a magnetic bearing from the grid bearing (Google Earth Grid Bearing) I subtracted 5 degrees.
Proved Example: The grid bearing (from the Google Earth image) was 308 degrees. To get the magnetic bearing (the compass) I subtracted 5 degrees to achieve 303 degrees. I confirmed this by pointing the compass at an obvious landmark that was also printed on the map.
What does this mean?
Not much really. Because it's such a small adjustment you can easily get by using the grid bearing taken from the map. To make it more accurate, just subtract 5 degrees when you set your compass bearing. When I post the map and directions I will supply details of the grid bearing (at least Google Earths version) and the magnetic bearing adjustment.
I will keep testing this theory and update if necessary.
- Amazon.com: silva compass: Sports & Outdoors
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