CTC Bootcamp 2015 - Tutorial on reading bearings

Tuesday, March 17, 2015
A few teams requested help to calculate a bearing between two points on a map. Below I put a small tutorial together. This should make it easy to execute assignment 7 at home: calculating bearings on map. 

Say for example we are standing on top of our 200m hill near Chengalpattu and we wish to take a bearing to the pond Southeast (both marked with red circles):

Step 1: draw a vertical (0 deg bearing, North) and horizontal (90 deg bearing, East) lines through your current location:

Step 2: Copy any compass image onto your map and align it with your current position: e.g. match the 0 bearing and 90 deg bearing as shown below
This will ensure that the center of the compass corresponds to your current position

Step 3: Draw a line from the center of the compass (=your current position) to your point or reference (e.g. lake pond)
Where the line cuts the compass you can easily read the bearing: in this example: 125 degrees:
So from your hill top the bearing to the pond is 125 degrees (useful to understand the direction of the pond from your current position)
And from the pond to the hill top the bearing is the diagonal opposite: 125 + 180 = 305 degrees (useful to calculate your current position from the pond location on the map)

(Taking a bearing to a reference point in the field is normally done using a compass: 
Standing at your current location you ll point the compass straight to the reference point and read the approx. bearing from the compass
For this home assignment we are doing it using Steps 1+2+3 so we don't have to go out into the field (yet)

Step 4: After you have taking a bearing in the field with compass you can then use this to identify your current position on the map:

Put the compass center at the pond location (see step 1+2 above)
Draw a line from the center of the compass through 305 degrees (=pond bearing + 180)
You can see below that this line (=bearing) cuts through the hill top which is your current position

Hope this clarifies the 7th assignment a little bit more!

Types of Compass

The above tutorial shows you how to take bearings on a map with a simple compass image
A more practical way to take bearings is using a compass: (you ll of course need to buy one first :-)

1. Hand bearing Compass

For taking accurate bearings in the field this type of compass is best suited:

First you point the compass to the reference point in the field
Look through the narrow vertical opening of the top piece and ensure that the vertical wire cuts the reference point (this will ensure the compass is perfectly aligned with the point)
Next you look through the magnifying glass to read the accurate bearing from the main compass

2. Map bearing Compass

A map bearing compass is useful to take bearings on maps in two scenarios:

A. Take bearing between two known points: (Step 1-2-3 above)

Align the side of the outer rectangular plate with your two points (current location at bottom, reference point at top)
Turn the North arrow of inner compass wheel towards top of map (=North)
The direction (arrow) line on the plate now shows the bearing on the inner compass wheel: 320 degrees in the example below

B. Find your current position using a known bearing from a reference point (typically taken using a hand bearing compass in the field) (this is another method for Step 4 above)

Align the side of the map bearing compass plate with your reference point
Now rotate the plate (ensure the side remains aligned next to your reference point) until the direction (arrow) line points to the given bearing + 180 degrees 
(while rotating the plate also rotate the inner compass to ensure its North continues to point to the top of the map)
When both the plate angle matches the bearing + 180 degrees and the North of the inner compass wheel aligns with top of the map then you can draw a line from the reference point along the plate of the compass
This line will intersect with your current location on the map



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