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A map is a graphic representation of
a portion of the earth's surface as seen from above, drawn to scale. A
map provides information on the existence; location; and distance between
ground features such as towns and roads. While there are literally thousands
of different types of maps, they basically fall into one of these
Topographic maps not only depict a portion of the earth's surface to scale, but also incorporate other information to assist the map user. The following list details information that is currently standard to all USGS topographic maps.
Locations of features depicted on a topographic map can be determined by using latitude and longitude (also known as the geographic coordinate system); township and range land survey system; or the military grid reference system (MGRS).
The shortest distance between two points is a straight line, but on the surface of a globe the shortest distance is an arc known as a great circle. If you were to plot the route of a great circle on most maps, it would not appear to be a direct route. This is due to the distortion caused by projecting the curved surface of the globe onto a flat surface. Perhaps the most widely used map is the Mercator Cylindrical Projection. The Mercator map is very useful for navigation because a straight line on the map corresponds to a compass heading. If you look carefully at the map below, both parallels and meridians are straight lines and cross at right angles. The meridians are equally spaced, but parallels are not. This is because the Mercator projection is constructed by straightening the lines of longitude and by increasing the space between latitude equal to the space of longitudinal widening. This projection is most accurate within 15 degrees of the equator. Distortion is so severe near the pole that the northern and southern limits of the map are fixed at the 84th parallel. The Mercator projection has given many people a distorted perception of the size of the continents. Greenland, for example, appears larger than South America when actually Greenland is only one eighth of the size of South America.
Because lines of longitude merge at the poles, the distance represented by one degree of longitude is greatest at the equator and decreases as latitude increases. For example, at the equator a degree of longitude is about 111 kilometers (or 69 miles), while at 60 degrees latitude, a degree of longitude is only about 56 kilometers (or 35 miles.) At the poles, the meridians intersect, so a degree of longitude is zero kilometers (that's because there are no degrees of longitude at the poles.) Nevertheless, we can easily determine the distance between two places on the globe even if their latitude is not the same.
Global Positioning Systems (GPS)
GPS was developed by the U.S. Department of Defense as a system that would provide global, all-weather, 24 hour positioning capability. GPS is now also used in civil applications mostly for navigation and mapping.
Presently, 24 GPS satellites orbit the earth at a distance of about 20,000 km. Four satellites are in each of six different orbital planes. This arrangement insures that at least four and as many as eight satellites are visible above the horizon from any spot on Earth.
Onboard each satellite are atomic clocks that keep precise time. Each satellite broadcasts its location and time information as a code on two microwave carrier signals (L1 frequency of 1575.42 MHz and L2 frequency of 1227.60 MHz).
The basis of GPS positioning is that your location can be determined if you know the distance to four (4) different satellites. This technique is called triangulating or ranging. The GPS receiver measures the distance to the satellites using the travel time of a coded radio signal and the speed of light. Because your handheld GPS unit does not contain an atomic clock to precisely measure time differences, it contains a directory (called an almanac) of the projected position of each of the satellites in the orbital planes. The GPS receiver uses this information to calculate the time differences and thus the distance to each satellite.
In order for GPS to work, the antenna at the end of the GPS unit needs to receive the signals from the orbiting satellites. Therefore, you need to be outside with a view of the sky. Mountains, trees, buildings and other obstacles can block the satellite signals or cause the signals to bounce around creating positioning inaccuracies. When you turn on the GPS unit, it will take some time for enough information from the satellite to be processed before your position is "fixed" or known. The military uses an encrypted code on the L2 frequency to obtain ±5m accuracy on handheld GPS receivers. The signal code carried by the L1 frequency used by civil GPS units is randomly degraded and scrabbled (called selective availability, or SA.) Therefore, a position determined by the handheld civil GPS units is approximately ±100 m.
Map Projections & Types
A map projection is the systematic arrangement of a planet's parallels and meridians onto a plane surface. These meridians and parallels become the projection graticule. The graticule takes on different forms depending on the type of projection plane surface; the point or line of tangency; the aspect; and direction of an imaginary projection light source. The projection process also involves the transformation of land features such as coastlines and land boundaries.
All map projections have some type of distortion or deformation. Depending on the projection properties, the distortion may be of area; shape; size; distance; direction; or scale. No projection is free from all distortions, but each contains only some distortion. The cartographer or mapmaker must select a projection which will result in a minimum of distortion in relation to the map theme or purpose; the amount of land area shown; and the portion of the planet's surface being represented on the map. As previously noted, all map projections contain some types of distortion. Some projections preserve shape and direction while distorting area. Others maintain area but distort shape and scale. In many projections scale may vary from place to place and in all projections distortion will increase away from the places of tangency. The types of distortions are a function of the way the projection is constructed. As most projections have been derived mathematically, the type of distortion is often a function of certain mathematical relationships specific to a given projection.
Map projections are grouped into three families: Cylindrical, Conic and Azimuthal, with Pseudocylindrical projections forming a variation on the Cylindrical Family. These families are based on the configuration of the plane onto which the globe (sphere) is projected.
In addition to map projections, there are an endless variety of types of maps. Below are just a few of the types available.
The modern system of map symbols owes much to the reign of Napoleon Bonaparte. During the first part of the 19th century France became the leader in cartography with the introduction of such cartographic innovations as a comprehensive system of incorporating symbols onto topographic maps.
Table of Map Symbols
(Be sure to expand picture in order to read fine details)
a standard topographic map, the colors used and the features they represent
Black: Indicates man-made features, such as buildings.
Gray: Indicates built-up areas; relief features; and elevation. This color enables the map to be red-light readable.
Red: Depicts major roads and highways.
Brown: Depicts contour lines.
Green: Identifies vegetation such as woods; orchards; and vineyards.
Blue: Identifies water features such as lakes; swamps; rivers; and streams. Intermittent water features are depicted with a dashed line.
The main advantage of a topographic map is that it portrays the elevations and relief of the portion of the earth's surface it depicts.
The representation of the shapes of natural terrain features (the shape of the ground) is called the relief.
Contour lines are the most common method used to depict relief; elevation; and depression on a topographic map. A contour line is an imaginary line on the ground above or below a datum plane (sea level - which is measured as either "0 Feet" or "O Meters" depending on map scale). There are three (3) types of contour lines:
In addition to contour lines, bench marks are surveyed elevations at specific points and are usually indicated by the letter "X." Finally, spot elevations are points that may be surveyed and are used to depict elevations at prominent terrain features, such as hill tops and road intersections.
Although the use of contour lines provides a more scientific and accurate portrayal of elevations, the relief that is depicted by contour lines can be difficult to visualize in 3-D. The following methods are used in depicting certain terrain features:
Summary Contour Map labeling all the above terrain features.
As we have stated, relief is basically the shape of the land. As such, the spacing of the contour lines also tells much about the relief. For example, you already understand how contour lines can represent a hill, but how steep is the hill and what type of slopes can be found on the sides of the hill?
Uniform Gentle Slope
This type of slope is depicted by contour lines that are evenly spaced and wide apart.
Uniform Steep Slope
This type of slope is depicted by contour lines that are evenly spaced and close together.
This type of slope is depicted by contour lines that are close together towards the top of the terrain feature and are widely spaced towards the bottom of the slope.
This type of slope is depicted by contour lines that are widely spaced towards the top of the terrain feature and are close together towards the bottom of the slope.
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