The following series of eight introductory articles on navigation appeared in the Newsletter of the Melbourne Bushwalkers. There are questions relating to the articles for both expert and novice, with answers. Possession of a map, preferably of scale 1:25,000 or 1:50,000, will be helpful.
All maps have a scale, usually represented as 1:25,000, 1:100,000 or similar. 1:25,000 means that the map is 25,000 times smaller than the ground - that is, that 1 cm on this map is equivalent to 25,000 cm on the ground (or 250 metres) - so 4cm is equivalent to 1000 metres (or 1 km) and vice versa.
Also almost all the maps we use have a grid which can be used together with the scale to judge distances and can give a feel for the size of things. Have a look at the map below, which has a 1km grid on it – that is the grid lines are 1km apart.
(Notice that you were not told the scale of the map yet you were able to answer the questions using the grid – Conclusion: Grids are very useful for estimating distances on a map.)
You draw a line on a map from one point to another which is approximately to the north of you, where you want to go. You notice that the line is exactly parallel to the Magnetic North arrow on the declination diagram in the margin. Can you set the bearing without putting the ("Silva" style) compass on the map? What bearing would you set?
The obvious answer is that it is possible to set the compass to zero, or zero plus or minus the annual variation, however this is incorrect. On most maps the declination diagram it just that – a diagram which is not necessarily drawn to scale. For instance on the Dargo Plains – Cobungra 1:50,000 map the grid north-magnetic angle is quoted as 12 degrees, but it actually measures 20 degrees!
So the number of degrees quoted should be used, not the graphical diagram.
By map legend I mean most of the things in the margin of the map. Let us look at this marginal information in order of importance.
There will be a bar scale which you can use to measure things on the map. The actual “representative fraction” such as 1:50,000 will also be shown. The user should always keep in mind the actual scale of the map eg 1:50,000 , even though as we saw in the previous lesson it is possible to do some useful measuring without actually knowing the scale. The reason for knowing the scale is that it determines the level of detail shown. For instance, a 1:25,000 map shows farm fences, whereas a 1:100,000 map generally does not (except in sparse areas where the fence is considered as a navigational feature).
It is essential that you know this for many purposes. We will discuss why in a future article.
Date on the map
The date on the map always appears in the “credit note” – a little block of text that tells how the map was made, when, by whom, and any accuracy limitations.
The actual symbols used to portray roads, streams, timber cover, etc. Note that just because something appears in the legend doesn’t mean that it will be shown on the map, even if it exists on the ground. A good example is fences. The legend will show them for most scales, but they only appear on the map if they have some significance. If there are lots of them, they are all left off because they don’t serve much purpose, whereas one isolated fence might be shown as an aid to navigation.
Also each map series may have a different set of symbols to portray the same thing. For instance, on some maps roads are just plain red lines, and on others they have “casings” that is, a pair of parallel black lines filled in by a red infill. Lesser standard roads might be shown by a narrower red line in the first case, or in the second case, by leaving out the red infill or breaking the red infill into very small dots so that the line appears to be pinkish.
The declination diagram
This is a little diagram showing the relationship between various north points – more on this in a later lesson.
Answer the following questions about any map that you happen to have. The maps most suitable would be any 1:25,000 or 1:50,000 metric scaled map for Victoria, and a good example is the Reedy Creek 7923-3-1 map which contains the area of a permanent rogaining course occasionally used for our club navigation training – this map or any map (perhaps of the area of your next walk, so you could test your skills), may be purchased from map shops (e.g. Melbourne Map Centre, Chadstone), or you could peruse one of the clubroom maps.
On maps of 1:25,000, 1:50,000, and 1:100,000 the grid lines always have two digits. On a 1:250,000 map they only have one digit (except on some very old maps). Why is this so?
All the blue lines on a map relate to water. The reason we are studying them at this stage is because they help map users to get a picture of the country without using contours.
The vast majority of the thin blue lines represent gullies which would only have water in them when it is raining. At the end of summer, almost all of the single line streams would be dry, and perhaps many of the double line streams as well (those with a blue infill between the blue lines).
You may ask, “If most of the blue lines are dry most of the time, why are they shown blue, suggesting water?” There is a very good reason, and that is to help the map user to decide which are gullies and which are spurs, and to show the general shape of the country. Without the blue lines it would be harder to tell. Also the blue lines help us to define a watershed which is all the land upstream of a given point on a stream, and thus all the rain falling in the watershed will flow past that point.
You need to be able to identify the main watersheds in an area, because the most desperate form of being lost is when you think that you are in one watershed when you are actually in another. This can be avoided by identifying the watershed boundaries so that you don’t inadvertently cross from one into another. Note that a watershed boundary never crosses a stream, no matter how small.
Another use for identifying watershed boundaries is for determining the likelihood of finding water. Ignoring for the moment any considerations of geology or the presence of moss beds or springs, the bigger the catchment, the more likely it is that a given stream will have water in it.
It is often said that if you are lost, follow water downstream and you will eventually come to civilization. Whilst this is true, streams often have the thickest scrub so the advice needs to be modified to suit the area.
Now, to test what you have learnt, try these questions referring to the map:
A friend has given you a compass from the Northern Hemisphere. Will it work properly here?
A grid reference is a number which gives a position on a map. Most maps have a little instruction box on them to show how to give a grid reference, but I will repeat the important parts here.
Take for example the grid reference number 288769, it is actually composed of two parts, the Easting 288 and the Northing 769. Remember that the parts must be in alphabetical order, ie E,N. The Eastings are the grid numbers at the top and bottom of the map which increase as you move to the east, and the Northings are the numbers on the sides of the map which increase as you move to the north.
With a six digit grid reference you need to estimate the tenth parts of the grid square (assuming that you are using a map of 1:100,000, 1:50,000 or 1:25,000 scale). Only use the large numbers on the grid lines, not the small numbers which are ignored. Make sure you understand how they are derived because from now on we are going to use grid references a lot.
Note that a six digit grid reference is accurate to 100m on the ground. It is also possible to use only four digits to indicate a general area of 1km square.
We are learning about grid references at this stage so that we can use them to locate specific features on maps. For instance they may be used in trip reports to indicate where you went eg – we started at Mount Warby GR (short for grid reference) 288769, had lunch at the creek junction at GR 278752 etc….
Now for some questions for you to answer:
You are following a straight high tension line but have lost track of how for you have gone along it. You look at about right angles to the line and see a prominent peak about 3 km away. You read a bearing to it (standing under the line but well away from a tower) plotting the bearing backwards to intersect the power line and thus determining your position. Later you discover that the position was in error by about 600m. What has gone wrong? What if you were 1200m out of position?
Contours are used on topographical maps to show the shape of the land. To the uninitiated they look like a mess, cluttering up the map, however to the expert they describe in great detail the shape of the country; its steepness, and many topographical features, such as knolls and saddles which aid in navigation.
Each contour is an imaginary line at the same height above sea level. If the contour level is say 10m, then the next contour will be 10m higher than the previous one etc.
Refer to the diagram below which shows some typical topographical features and the contour lines which define them.
Note the following points:
From examining the map above:
You are in an area where there is a known magnetic anomaly such as the one near Falls Creek at Mt. Jim. Is a compass of any use in such a situation?
Possibly. For ordinary navigation no, however at a given location all bearings are affected equally by a magnetic anomaly. You can stand in one location, read bearings to several points, and if one of the points is a known one such as a trig, you can determine the compass error and then apply this error to all other bearings read from the same position. Thus, the angles at a point can be correctly deduced, however be sure to read all the bearings without moving, other than rotating.
Usually about every fifth contour is shown by a heavier line. These are known as index contours. When looking at the general shape of the country you need only look at the index contours. The thinner non-index contours are only of use when looking at the detailed shape of the land at a particular location. When determining how far up or down, count the index contours, not every contour.
How steep is it?
You can work out exactly how steep a slope is by measuring how far apart the contours are. Say the index contours are 100 vertical metres apart. Using the scale at the bottom of the map you determine that a pair of index contours are 400m apart horizontally. The grade is then 100/400 = 0.25 which is more often quoted as 25%, or 1 in 4, which means that for every metre you travel horizontally, you will rise 25% of that, ie 0.25m
To help answer the Q’s below, revision of Lesson 3 on drainage, Lesson 4 on grid references and Lesson 5 on contours may be useful.
On the Warby Range map first look at the drainage pattern (the heavier lines). You will see that the water flows in the opposite directions on the two sides of the map. Therefore there must be a ridge (a watershed boundary) in the centre of the map. This fact alone should help you to visualize the general shape of the land shown. The land in the grid square bounded on the south west corner by GR 290750 (or 2975) is gentle, the next grid square to the north has some medium slopes in it, and the next one to the north of that contains some steep slopes (ie close contours). Just how steep you can work out if you are given that the contour interval is 10m and the grid square interval is 1000m.
You are navigating through the Otways rain forest and have cut off the marginal information from the map to save weight but you remember that the grid magnetic angle on the recent 1:25,000 map adjacent to the east was 12 degrees east. Would it be safe to assume that the angle was essentially the same where you are?
Generally it is more important to be able to read a map effectively than use a compass, however a compass is essential in featureless country, fog, dense scrub or the like.
The best type of compass for bushwalking is the orienteering type with the compass mounted on a flat rectangular piece of transparent plastic (the baseplate) and to have the needle in oil (as it allows the needle to stabilise quickly) Also the string attachment should be long enough to allow the compass to be held at waist level with the string around your neck.
Other features are the line in the centre of the baseplate of the compass with an arrow on one end is called the “direction of line travel” or the index line, and the compass housing can be rotated in relation to its base. The base of the compass also has a thick centre arrow with parallel lines (Orienting lines) on either side of it.
The red coloured end of the compass needle aligns to magnetic north (the direction of the magnetic north pole), which is usually different to grid north, which is the direction of the grid lines on the map. Don’t worry about true north (the direction of the geographic north pole) – it is of no interest to us for navigation purposes.
Obtain a map (we will assume you have a compass as described above and illustrated below) – any map will do, just so long as it has grid lines and a declination diagram to the side (which shows the relationship between the various north points). Mark 2 points – say A and B. Imagine we are at A and will now attempt to get to B using the compass.
* Anywhere in Victoria you will have to subtract, but in other parts of Australia you may have to add the grid-magnetic angle. An aid to help when converting the grid angle on the map to the compass bearing you would follow on the ground is to use the mnemonic “GrandMa’s Socks” ie Grid to Magnetic Subtract, and the reverse also holds when converting a magnetic angle to a grid angle – the grid-magnetic angle is added to the magnetic angle you have measured on the compass – so the mnemonic MGA (for the car) can be used – ie Magnetic to Grid Add.