Maps and Navigation
Maps and Navigation - Part 1
Before you use maps for navigation, you must understand something about them. Understanding maps and their symbols, accuracy, and scale will make navigation much easier. This lesson will just focus on understanding the map.
A map is a line drawing, to some scale, of an area of the earth's surface. It shows objects and features by conventional signs. A map is also a graphic representation of a portion of the earth’s surface. Maps, although drawn to a scale, are not absolutely accurate because they represent a curved and uneven surface on a flat piece of paper.
There are three types of maps and each has its uses and purposes. Here is an explanation of them.
1. Planimetric Maps
Show the positions of features without showing their relationship to the hills and valleys of the land. They can include rivers, lakes, roads, trails, boundaries, or other human-made, symbolic features. For example:
a. Common road maps: Road atlas and street maps.
2. Topographic Maps
Shows both the horizontal and vertical (relief) positions of features. Topographic maps are often referred to as quadrangles or quads. For example:
a. Contour maps are the most common method of representing the shape and elevation of the land. A contour is a line of equal elevation on the ground that delineates the same elevation above or below a specific reference elevation, usually sea level.
b. Shaded-relief maps are pictorial. They are shaded to simulate sunlight on the terrain. This shadow effect accentuates the shape of the physical features.
3. Orthophoto Maps
An orthophoto map is a map depicting terrain and other features by color-enhanced photographic images. It is an aerial photograph geometrically corrected ("orthorectified") such that the scale is uniform of the land. Unlike an uncorrected aerial photograph, an orthophotograph can be used to measure true distances, because it is an accurate representation of the Earth's surface Some orthophoto maps are overlain with contour lines and other features commonly associated with topographic maps. These maps are corrected for scale and are the same size as U.S.G.S. quads.
A legend shows information needed to interpret a map. Each type of map has information represented in a different way relating to its subject matter. The legend can explain map scales, symbols and color.
The map scale indicates the ratio or proportion of the horizontal distance on the map to the corresponding horizontal distance on the ground. A representative fraction (R.F.) scale expresses the ratio of the map distance to the ground distance in like units of measurements. It is usually written as a fraction or ratio.
A representative fraction is always written with the map distance as 1 (one). A representative fraction of 1/24,000 (1:24,000) means that one UNIT of measurement (inches, millimeters, feet, etc.) on the map is equal to 24,000 of the SAME UNITS on the ground. You CANNOT mix units in a representative fraction.
- If it is one INCH on the map, it is 24,000 INCHES on the ground.
- EXAMPLE: R.F. is map distance = 1" OR 1:24,000 (ground distance 24,000")
The larger the representative fraction (the fraction 1 divided by 24,000 is obviously larger than 1 divided by 250,000), the larger and clearer the details shown on it. But on the other hand, the larger the fraction, the smaller the area covered by the same size map sheet.
A graphic scale (G.S.) or comparison scale is entirely different. It usually COMPARES map distances to the ground distance in DIFFERENT units of measurements. Usually a graphic scale is a line marked off on a map indicating so many inches or millimeters equal so many feet, kilometers, chains, or miles on the ground. A comparison scale of 1" to 2000 feet means that 1 (one) inch on the map is proportioned to 2000 feet on the ground.
EXAMPLE: G.S. is map distance to ground distance: 1" to 2000 feet
It is common practice for maps to be oriented with north at the top. Most maps have a symbol of an arrow pointing to the north. It is then understood east is right, west is left and south is bottom of the map. These are also called cardinal points and there are four of them: N, S, E, and W. A compass points to the magnetic north pole. This is different from the true north pole as represented on maps. If you are using a map to navigate with, you must correct the difference by adding or subtracting the difference. More on this in Part 2 of this lesson maps.
Some maps have a revision date, which is when the map was last updated to reflect changes. If the map is very old, it may not be very accurate.
In order to make the identification of features on a map easier to interpret and to provide more natural appearance and contrast, map symbols are usually printed in colors with each color representing a class of features. The colors are as follows:
1. BLACK - Most cultural or human-made features, boundaries.
2. BLUE - Water features such as lakes, rivers, or swamps.
3. GREEN - Vegetation such as woods, orchards, or vineyards.
4. BROWN - All relief features, contours, cuts and fills (Topographic maps).
5. RED - Main roads, built-up areas, boundaries, special features.
6. Other colors may be used for special purposes. Their key will be found in the margin/legend of the map. In the map below, brown indicates built up areas (towns) containing people. Yellow indicates a snowmobile trail.
1. In general, what does a legend show ?
2. Referring to a map scale, describe a representative fraction and give an example:
3. What is a graphic scale on a map ? Give an example:
4. What are the five most common colors found on a U.S.G.S topographic map and what features do those colors represent ?
A. __________ ________________________________________
B. __________ ________________________________________
C. __________ ________________________________________
D. __________ ________________________________________
E . __________ ________________________________________
5. If you had three different types of maps, could you use the same symbol key for all of them ? Why or why not ?
6. What cardinal direction is usually indicated at the top of map? ________________
Bottom of a map? ___________________
Right of map? ______________________
Left of map? _______________________
7. List the three categories of maps.
8. List three examples of planimetric maps.
9. List the two types of topographic maps.
New week: Learn how to use maps and a compass to navigate from one place to another.