NAVIGATION

NAVTREK

MAP AND COMPASS

This navigation information is the beginning of a series helping you to become more proficient at using the map and magnetic compass.

This first lesson is a brief overview of the things about mapping you would need to take into account if you were preparing to undertake an expedition suitable for example in the Duke of Edinburgh award scheme.

Step 1. Identify and record the following.

The Map name. for the purpose of this explanation i have used the Camden map. NSW  Australia.

Step 2. Locate the scale, usually at the bottom of the map.

in this case it is 1:25000. This means that for 1mm or inch on the map this represents 25000 mm or inches on the ground.

You will also find the date of the field revision, very important when calculation the magnetic deviation. This map was revised in 1975

Step 3. Locate the magnetic deviation arrows. at the bottom of this map, sometimes on other maps it is found halfway down the right side.

Now is the time to calculate the current magnetic deviation.

The information on the map indicates that in 1975 the deviation was 10.5 degrees. So this year is 2010 minus 1975 equals 35 years. The deviation changes by 0.1 degrees every three years, therefore divide 35 by three = 11.6 Now 0.1 times 11.6 lets say 1.16  Add this to the 10.5 and you get 11.6 degrees.

You may have heard the rule East is least and West is best relating to this deviation. This simply meas that if the magnetic arrow is pointing East of the true north arrow then the declination is positive and you add the calculation to your grid bearing. If the magnetic arrow is pointing west of the grid north arrow then the declination is negative and therefore subtract the calculated deviation from your grid bearing. Check out step six for this application.

Question 1. Why does the magnetic deviation change?

Question 2. Why is this magnetic deviation calculation so important?

Step 4. The legend on this map is at the bottom of the map and is divided into two. Some maps have the legend in other places.

The legend helps to identify ground features. They are usually standard for all topgraphic maps world wide.

Step 5. Locate the Eastings and Northings on the map. These are the words that describe the gtrid references that help locate an item on the map. usually we use a six figure reference the first three numbers are the Easting and the last three numbers are the Northing. Check my map for example.

Right down the bottom right corner of this map is a wier on the Nepean river, just next to the railway bridge.

The grid reference for this wier would be 917222

Read the first two numbers directly from the bottom edge 91 and the estimate the 1/10 divisions to the next number in this case the wier is 7 divisions along the easterly direction. Now read the northerly numbers of 22 and find that the weir is 2/10 along the northerly edge in the northerly direction thus the number 917222 is obtained.

Step 6. Taking a grid bearing is now the main reason for this mapping stuff, starting point and finishing point what bearing should I set to get to my desired end location.

Lets assume for this exercise your starting point is on the Hume highway at the corner of Cawdor rd This grid reference is 875225 and your finishing point will the Wier on the nepean river 917222

To find the grid bearing there are three ways this can be done. Each with varying ease and accuracy.

First method, use your compass and orient the map. then lay the compass along the line of travel and rotate the compass bezel to determine the bearing ( take int account the deviation ). Plenty of other people have very good UTUBE EXPLANATIONS.

Second method. use a circular or at least half circular protractor and lay the protractor on the map lining up the base of the protractor with the grid lines on the map. Place the centre of the protractor on the start point. Use a line from the centre of the protractor to the destination point and read the Grid bearing bearing from the protractor. Now add or subtract the magnetic deviation to get your magnetic bearing. Full and detailed explanation next time.

Third method. This is the most accurate but also involves a bit of mathematical calculation involving X and Y references and then using Trigonometric tables to convert the fraction to degress. What the !!!!!!!!!!!. Full explanation next time.