- summarized by Alistair Des Moulins.
A topographic map is a two dimensional representation of part of the earth's surface. Contour lines, colours, and shading are used to indicate elevation.
There are four pieces of information you need to know about a map before it can be of much use to you. These are:
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1. Scale of the Map
The Scale of a map tells you how many units of length on the ground are represented by 1 unit of length on the map. Without this information you do not know whether one centimeter on the map represents 1 kilometer, 2 kilometers, or 10 kilometers on the ground.The topographic maps used for hiking in in the Rocky Mountains have a scale of 1:50,000, i.e. 1 cm on the map represents 50,000 cms ( or 1/2 km ) on the ground. There are also 1:250,000 maps which cover a larger area in less detail - these are useful for identifying mountains which are further away or for planning longer trips.
Some scales and their meanings:
| Scale | Metric meaning | Imperial meaning | Comments |
|---|---|---|---|
| 1:50,000 | 2 cms to 1 km | 1.25 inches to 1 mile | Canadian, British, and European maps |
| 1:250,000 | 1 cm to 2.5 kms | 1 inch to 4 miles | Canadian and other maps - used to get overall view of area |
| 1:63,360 | 1 cm to 2/3 km | 1 inch to 1 mile | Old British maps |
| 1:62,500 | 2 cms to 1.25 km | 1 inch to 1.04 miles | US topographic maps (15 minute series) |
| 1:25,000 | 4 cms to 1 km | 2.5 inches to 1 mile | More detailed maps (US, Canada, Britain, and Europe) |
| 1:40,680 | 2.5 cms to 1 km | 1 inch to 1 km! | Some newer US maps |
2. Orientation of the Map
There should be some indication of which direction North is on the map. Many maps have grid lines criss-crossing the map. Normally these run North-South and East West. However this is not always the case. There is a 1:250,000 scale map of Jasper National Park that does not follow this norm.Grid References
Grid lines are drawn on our 1:50,000 maps at 1 km intervals. A grid reference locates a position to the nearest 100 meters. However any 6 digit grid reference will also occur at points 100 km north, south, east and west of this point. So for a more precise reference always quote the map number or the full grid reference using the other numbers on the side of the map close to the corners.
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The first 3 numbers are the west-east reference and the last 3 are the south-north reference, e.g.:
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3. Magnetic Declination of the Map
There are 3 different Norths that we need to know about:- True North - This is the direction towards the North Pole. Lines of longitude meet at the North Pole.
- Grid North - This is the direction of the North-South Grid lines on the map. As grid lines are parallel lines not all of them can point to the North Pole. So there is a small difference between grid north and true north because the earth is a sphere and the map is flat.
- Magnetic North - This is the direction that the compass needle points. The position of the North Magnetic Pole varies a bit from year to year. Currently it is somewhere near Bathurst Island in the NWT (or may be Nunavut now).
Somewhere on the border of most maps there is a set of arrows to indicate the relationship between the 3 norths at a particular point on the map. Usually, there will also be a narrative like:
Magnetic North was 20 degrees east of grid north in 1976 and this angle is decreasing by 5' a year. (1' means 1 minute or 1/60th of a degree. There are 360 degrees in a circle.)
Some examples of magnetic declination:
Knowledge of the magnetic declination is needed in order to use the compass correctly to either find your way or identify distant mountains. Most maps and compasses have 360 degrees in the circle. However in Norway they use 400 degrees to the circle - so beware!
Taking Compass Bearings
From Map to Ground
This technique is used for finding out which way you want to go from a known point - used for navigation in treed areas and in bad visibility conditions above treeline.-
Using a compass with no built in adjustment for magnetic declination:
- Place a corner of the compass that is closest to the string hole on your known position on the map.
- Align the long side of the compass along the direction you wish to travel.
- Rotate the circle part of the compass so that the North mark on the compass circle and the lines within it align with the north-south grid lines on the map. N.B. The position of the compass needle and the orientation of the map are irrelevant!
- Read off the number on the circle part of the compass by the arrow or mark towards the direction you wish to travel. (This is the angle between Grid North and the direction you wish to travel)
- Convert this angle to a magnetic bearing by using the diagram of the 3 norths on the map: Place the compass on the norths diagram with the N of the compass circle pointing to grid north on the diagram. Rotate the compass circle the required number of degrees towards magnetic north on the diagram.
- Put the map away and align the compass so that the North end of the compass needle points to the North mark on the compass circle.
- Walk in the direction to the arrow or mark on the compass - keep watching that needle!
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Using a compass with a built in adjustment for magnetic declination:
- Make sure the declination marker arrow is set correctly for the map you are using.
- Place a corner of the compass that is closest to the string hole on your known position on the map.
- Align the long side of the compass along the direction you wish to travel.
- Rotate the circle part of the compass so that the North mark on the compass circle and the lines within it align with the north-south grid lines on the map. N.B. The position of the compass needle and the orientation of the map are irrelevant!
- Put the map away and align the compass so that the North end of the compass needle points to the declination marker arrow.
- Walk in the direction to the arrow or mark on the compass - keep watching that needle!
From Ground to Map
This is used for:- Identifying unknown objects or mountains from a known point or
- Finding your unknown position on the map from 2 or more known visible objects.
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Using a compass with no built in adjustment for magnetic declination:
- Point arrow in body of compass at unknown object.
- Rotate the circle part of the compass so that the north end of the compass needle points to the north mark on the compass circle. (Keep compass body pointing at the object)
- Read off the number on the circle part of the compass by the arrow or mark on the compass that was pointing towards the object. (This is the angle between Magnetic North and the direction of the unknown object)
- Convert this angle to a Grid bearing by using the norths diagram on the map. Place the compass on the norths diagram with the N of the compass circle pointing to magnetic north on the diagram. Rotate the compass circle the required number of degrees towards grid north on the diagram.
- Get the map out. Place a corner of the compass that was nearest you when you looked at the object, on your known position on the map. N.B. The position of the compass needle and the orientation of the map are irrelevant!
- Keeping that corner of the compass in place align the long side of the compass so that the North on the compass circle and the grid lines in the circle align with the North-South grid lines on the map.
- The unknown object is somewhere on the line of the long side of the compass in the direction of the arrow on the compass body. You will need to extend the line beyond the compass. The object may not be on your map!
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Using a compass with a built in adjustment for magnetic declination:
- Make sure the declination marker arrow is set correctly for the map you are using.
- Point arrow in body of compass at unknown object.
- Rotate the circle part of the compass so that the north end of the compass needle points to the north end of the declination arrow of the compass. (Keep compass body pointing at the object)
- Get the map out. Place a corner of the compass that was nearest you when you looked at the object, on your known position on the map. N.B. The position of the compass needle and the orientation of the map are irrelevant!
- Keeping that corner of the compass in place align the long side of the compass so that the North on the compass circle and the grid lines in the circle align with the North-South grid lines on the map.
- The unknown object is somewhere on the line of the long side of the compass in the direction of the arrow on the compass body. You will need to extend the line beyond the compass. The object may not be on your map!
4. Contour Interval of the Map
A contour line is a (usually brown) line on a map joining points of equal altitude (height above mean sea level).The contour interval is the distance vertically between the altitude each individual contour line represents. When reading a map, contour lines give you information on what the topography of the area is likely to be. The closer the contours the steeper the ground. By interpreting contour lines it is possible to imagine a 3 dimensional picture of what the ground might look like.
Some 1:50,000 scale maps of the Rocky Mountains have a contour interval of 100 feet; i.e. separate contour lines are drawn joining points of altitude 4000, 4100, 4200, 4300 etc feet above mean sea level. 1:250,000 maps of the area have a contour interval of 500 feet. Using a 100 foot interval on these maps of the Rockies would make them too cluttered. There are some 1:50,000 scale maps of the Rockies that have a 20 meter contour interval below 2000 meters and a 40 meter interval above 2000 meters - these can be very confusing. Others have a 50 meter contour interval. So it is important to check the contour interval on every map! However in Saskatchewan some 1:50,000 scale maps have a contour interval of 10 feet and 1:250,000 scale maps have a contour interval of 100 feet. This is necessary to show any variation of altitude there. In the mountainous areas of the US, contour intervals seem to be 80 feet on the 1:62500 scale maps and 40 feet on the 1:40680 scale and 200 feet on the 1:250,000 maps.
Understanding Contour Lines
At first glance all the wiggly brown lines on a map may seem meaningless but detailed study of them can reveal a lot about the landscape. With practice, from studying contour lines you will be able to form a 3-dimensional picture in your mind of what an area may look like.- The closer the contour lines the steeper the ground.
- Closed contour lines represent mountains or hills.
- Closed contour lines with dashes represent depressions or hollows.
One must realize that this is an approximation of the terrain. The following illustrates two equally valid interpretations of contours:
With continued study of topographic maps and contour lines you will be able to picture your hike from the map while sitting in your armchair. However that must not stop you getting out there and doing it!!