Maps
From WildernessWiki.org
Note: This article refers to the National Topographic System (NTS) maps published by Natural Resources Canada in 1:50 000 scale. Although the maps you use may appear different, the same principles apply, although some information, including colours and units of measure, may not be the same.
A map is simply a visual representation of an area of land, designed to depict in two dimensions the features of the three-dimensional terrain. Because a map with all known information about a particular area would be cluttered and overloaded with too much data to be particularly useful, maps are identified according to their purpose.
Looking through an atlas will show many different maps for the same area, all dealing with a particular type of information: political boundaries, mineral distribution, and climate data, to name a few. Looking in the glove box of most vehicles will reveal a street map. Here, the relationship between objects is much more important than distances and elevation. On a street map, the trip from A to B can be figured out easily in terms of driving time and left or right turns. Once we're traveling from A to B on foot, details such as elevation gain and obstacles become important to know in order to be able to navigate effectively.
The type of map we use for this is a topographic map. These maps have specific feature to provide a reasonably accurate way of showing the three dimensional terrain on a two dimensional map. Because they do not contain this information, it is important not to rely on street maps for navigation outside of urban centers.
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Projection
A map’s visual representation of the terrain is complicated not only by representing varied elevation on a flat surface, but also by the fact that the earth is a sphere. It is easy to see the difficulty cartographers face if you think of peeling an orange and trying to get the peel to lie flat on a table in a single piece. While maps drawn on curved surfaces would be more accurate depictions of sections of our planet, they would be difficult to transport and to use.
The curved surface of the earth is transferred to flat maps through a method called projection. In essence, this takes a spherical shape and projects it onto a hypothetical sheet of paper wrapped around the sphere, which is then 'unrolled' to give us a map sheet. The hypothetical sheet of paper can be ‘wrapped’ around the globe in different ways, providing projections with varying shapes and views. The most commonly used projection—and the one most relevant to this field of discussion—is the cylindrical Mercator projection. See here for details on this and other projection methods.
An unavoidable side effect of the projection process is that maps made this way only show truly accurate distances and relationships between points close to the center of the map. Fortunately, the Earth’s radius of curvature is so large that major distortion only occurs when the map sheet covers a very large area.
Scale
This leads into a discussion of scale. A scale of 1:50,000 indicates that one unit of measure corresponds to fifty thousand of the same units on the ground (inches, centimeters, etc). By adjusting this ratio, we can vary the size of the area of terrain represented on a map. 1:50,000 allows the user to measure distances accurately between points that are walking distance apart and shows the detail necessary to navigate on foot.
Marginal Information
The space that forms the border around the outside of the map is the margin. This space is packed with helpful pieces of information such as the name and date of the map, the date the map's information was collected, projection, scale,the key or legend of symbols used on the map, and declination.
Color
An important facet of any topographical map is the coloration of various types of terrain. On CGS map sheets, green represents tree cover with the intensity of color often showing the density of tree cover. Blue shows water and permanent ice, and white indicates open ground: clearings, power and rail rights of way, snowfields, etc. Pink or red are used to indicate urban areas and black indicates man-made structures (and text). Elevation is indicated with brown contour lines.
Contour Lines
The common method of indicating elevation is to imagine that the terrain is made of many layers of uniform thickness, much like an architects model. Rather than gradual slopes, the ground is 'terraced'. If we now take each of these layers and draw their outlines, then superimpose them on top of one another, we generate an effective representation of what the terrain looks like. Areas where the lines are close together indicate land formations where the elevation change is great in comparison to the distance over the ground. For the maps I've mentioned, the contour interval (distance between the lines) is 50 feet. This is given in the marginal information and every fifth line has its elevation given (in brown) on the map. If the contour interval and the scale are changed disproportionately, the look of the map will change. This can lead to errors if one is using multiple maps simultaneously. After a bit of practice, you can actually 'see' a contoured map in three dimensions. The use of contour lines will be covered in an article on navigation.
Latitude and Longitude
Between the margins and the map's body (the 'meat' of the map - the reason you're reading this article!) is where the lateral and longitudinal information is placed. This becomes important once we navigate off of the map sheet, as it firmly fixes the map body's location on the surface of the earth. Latitude is a measurement of distance north and south of the equator and is represented by horizontal lines often called parallels. Imagine horizontal rings around the earth about 69 miles (111km) apart. That distance does vary slightly because the earth is shaped like a squashed beach ball, but the difference is negligible. Longitude is a little more difficult to calculate, so the method here is to picture lines running vertically between the north and south poles. The distance between any two adjacent lines (also called meridians) varies as we go north and south due to the convergence of these lines at the poles. While a degree of longitude is about 69 miles (111km) at the equator, it decreases to zero at the poles. This causes some of the most difficulty in navigation. These distances are far too great to divide accurately for navigation on foot, so we divide them further on the map sheet into grid lines.
Grid Lines
To make land navigation practical, a map sheet has its degrees of latitude (lat) and longitude (long) divided into smaller 'squares' by the use of faint lines called grid lines. A ruler measuring only inches isn't very useful, so we break down the inches into eighths, sixteenths, etc. Grid lines serve to break down degrees of lat and long the same way. On the CGS maps, the 'squares' correspond to 1 square kilometer - a very convenient unit for navigation. This does present us with another problem though. While the parallels of latitude are...well...parallel, the meridians of longitude are closer together at the top of the map than at the bottom. The opposite is true south of the equator. Fortunately, because we are dealing with a very small portion of the earth's surface in this context, the error is small. It does, however, mean that the north indicated by the top of the map (grid north) is not an accurate indication of the direction to the north pole (true north).
Declination
Now that we've covered the basics of how a map is 'built', we need to look at how we can use it. The obvious tool that comes to mind is a magnetic compass. Because this is covered in another article, we need only concern ourselves with using the two together. The biggest problem with a magnetic compass is that the compass needle doesn't point to the north pole. The earth's point of maximum magnetic attraction (mag north) is actually south of true north. If a compass were placed at the north pole, the needle would actually point towards Hudson's Bay. If you were to point with one arm toward true north and one arm toward magnetic north, the angle between your arms is the magnetic declination and it will change as you move over the ground.
Because of this, the declination given on the map is calculated for the center of each map sheet and incorporates any local magnetic anomalies (deviation). It is also given the designation of east or west depending on the location of the map with respect to mag north. Declination is given for the year that the map is printed, so we need to update it for this year. The annual change is also given, and whether the change is increasing or decreasing. If we take the number of years since the map was printed and multiply it by the annual change, we have the net change. We add or subtract it from the declination given, depending on whether it's increasing or decreasing. Great, we now know what the difference is between the top of the map and the direction the compass is pointing. So how do we use it ?
The declination is given as east or west, depending on which 'side' of mag north the map represents. We must add or subtract the magnetic declination from each bearing we take, otherwise the course we plot on the map won't relate to what's actually on the ground. To do this, remember the saying "East is least, west is best" If the declination is given as east, it is subtracted from the bearing and west declination is added to the bearing. Many compasses on the market have a declination scale scale built in which are manually adjusted to allow for this. Once set, they don't need to be adjusted until the following year or until a different map sheet is used.
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