PortrayingTheEarth_lecture.docxCollege of Alameda
GEOG 1: Physical Geography Professor Bow
Ch. 2: Portraying the Earth
The goals and objectives of this lecture/chapter are to:
· Develop an understanding of the size and shape of the Earth.
· Become familiar with how geographers determine locations on Earth.
· Know the basics of the Geographic Grid.
· Explain what a map projection is.
· Explain how global positioning system works.
· List and give examples of remote sensing techniques.
· Determine how GIS is used to solve problems in physical geography.
The Size and Shape of the Earth
Our perception of Earth’s size, shape, and topography is often distorted. Three-dimensional wall maps and globes (such as the example of Europe shown to the right) exaggerate or emphasize landforms, such as mountain ranges and valleys. These are often exaggerated 8 to 20 times their actual proportional dimensions.
The diameter of our planet is only about 13,000 kilometers (7,900 miles). Diameter is defined as a straight line passing from side to side through the center of a body or figure. The figure to the left illustrates Earths diameter as measured from the North Pole to the South Pole and from the equator. Note that the two measurements are not equal (i.e. Earths diameter is slightly longer when measured at the equator, as opposed to the North Pole and South Pole. This will be addressed on page 3.
To put this in perspective, the Moon is 385,000 kilometers (239,000 miles) from Earth and the Sun is 150,000,000, kilometers (93,000,000 miles) away. The air travel distance from San Francisco to New York City is about 4,000 kilometers (2,500 miles)1.
Earth’s surface varies in terms of elevation. The highest point on Earth, as measured from sea level, is Mount Everest, which stands at 8.9 kilometers (5.5 miles). The lowest point on Earth, as measured from sea level, exists on the seafloor. This is known as the Mariana Trench and it exists in the Pacific Ocean (east of the Philippines and north of Guam). The trench is 11.03 kilometers (6.9 miles) below sea level. This means that the total relief (or the total distance between the highest and lowest points on Earth) is only 19.9 kilometers (12.4 miles)1. Think about that for a minute. The distance between the highest and lowest points on Earth is only 19.9 kilometers (12.4 miles). The distance from my house to the Oakland Airport is over 96.6 kilometers (60 miles) one way!
In terms of shape, Earth is almost, but not quite spherical. As mentioned previously, Earths diameter is slightly larger when measured from the center of the Earth along the equator, then measured from North Pole to South Pole. This means the surface of Earth flattens slightly at the North Pole and the South Pole and bulges out slightly around the equator. Why does this happen? Enter Physics & Astronomy – any rotating body has a tendency to bulge around its equator and flatten at the polar ends of its rotational axis1. Although the rock materials that make-up the Earth may seem quite rigid and immovable to us, they are pliable and flexible (you’ll see further examples of this when we examine Earths tectonic plates in a couple of weeks). What does all of this mean? Well, it means it is inaccurate to call our planet a perfect sphere. The correct term for its shape (slight bulging in the center and flatter at the poles) is an oblate spheroid.
The Geographic Grid
As mentioned in the previous chapter, geographers determine where and why. Where is usually the easier of the two questions to answer, so this is where we will begin our fundamental geographic work. In order to determine accurate locations on Earth, we have developed a grid system which consists of two sets of lines that intersect at right angles. This allows the location of any point on the surface to be described by the appropriate intersection. This grid system is known as The Geographic Grid or Latitude and Longitude.
Before we dive into latitude and longitude, it’s important to understand the difference between great circles and small circles on a globe. Any plane that is passed through the center of a sphere bisects that sphere (i.e. divides it into two equal halves) and creates what is called a great circle where it intersects the surface of the sphere. The equator is an example of a great circle, because it cuts the globe equally in half. Planes passing through any other part of the sphere produce what are called small circles where they intersect with the surface (i.e. they do not cut the sphere/globe into two equal halves). See the images on the following page for further details. Great circles have two properties of special interest for us:
1. A great circle is the largest circle that can be drawn on a sphere; it represents the circumference of that sphere and divides its surface into two equal halves or hemispheres. As well see later in this lecture/chapter, the dividing line between daytime and nighttime halves of Earth is a great circle.
2. A path between two points along the arc of a great circle is always the shortest route between those points. Such routes on Earth are known as great circle routes (which will be discussed more in the next chapter).
1. Latitude
Lines of latitude, also called parallels, are oriented in an east-west direction. Latitude lines always run parallel to each other, and hence they are always equal distance apart. Latitude lines never converge or cross. What are these lines measuring? When you see latitude values they are expressed as degrees (°). This is because latitude lines are a description of location expressed as an angle north or south of the equator. As shown in the image on the following page, we can project a line from any location on Earth’s surface to the center of the Earth. The angle between this line and the equator is the latitude of that location. The starting/beginning line of latitude is the equator or 0°. The equator is the starting line simply because it is the largest parallel (i.e. a great circle) that can be drawn on the globe. In other words, the equator cuts the globe into two equal hemispheres. All other parallels are small circles. The half of the globe north of the equator is the northern hemisphere and the half south of the equator is the southern hemisphere. Lines of latitude or parallels end at two specific points, the North and South Poles. These are represented at 90°N (North Pole) and 90°S (South Pole). This means that the values for latitude range from a minimum of 0° to a maximum of 90°. There is never a 91°N or a 200°S - 90 is the stopping point for latitude.
When determining the latitude of a location you must designate which hemisphere you are located in. As you can see by the images below, the northern and southern hemisphere are mirror images of each other. If I said I buried one hundred million dollars’ cash at 35° latitude, you have two choices – 35°N or 35°S. This is not very accurate. If I said I buried one hundred million dollars’ cash at 35°N, then it gives you a narrower focus and you know to look in the northern hemisphere along the 35°N parallel. The only time you do not have to designate which hemisphere you’re in is when you are referring to the equator. The degrees for the equator is always simply 0° latitude because you are at the line in-between both hemispheres.
The image to the right is illustrating how lines of latitude are determined. For example, the 30°N parallel is located where it is because the angle that is created between the equator, the position on the surface, and the center of the Earth is 30°, so the latitude is 30°N. Same for the North Pole – the angle that is creased between the equator, the North Pole, and the center of the Earth is 90°, so its latitude is 90°N
Remember, lines of latitude:
· Are known as parallels
· Run in an east-west direction
· Measure distance north or south from the equator
· Are parallel to one another and never meet
· Get shorter toward the poles; the equator is the only great circle
There are seven significant parallels or lines of latitude you should familiarize yourself with:
1. North Pole = 90°N
2. Arctic Circle = 66.5°N
3. Tropic of Cancer = 23.5°N
4. Equator = 0°
5. Tropic of Capricorn = 23.5°S
6. Antarctic Circle = 66.5°S
7. South Pole = 90°S
Note: The North Pole and South pole are points, rather than lines (or you can think of them as indefinitely small parallels. We will discuss the significance of these shortly.
2. Longitude
Lines of longitude, also called meridians, are oriented in a north-south direction. Unlike lines of latitude, meridians are not parallel. They do not cross, but they do converge at the North Pole and South Pole. Lines of longitude extend from pole to pole and cross all parallels at right angles. Any pair of meridians is farthest apart at the equator, becoming increasingly close together northward and southward and finally converging at the poles. What are these lines measuring? When you see longitude values they are expressed as degrees (°), same as parallels. This is because longitude lines are a description of an east-west location as measured from the Prime Meridian.
The Prime Meridian has an interesting history. The equator is the natural baseline from which to measure latitude, but no such natural reference lines exists for longitude. Consequently, for most of recorded history, there was no accepted longitudinal baseline; each country would select its own “prime meridian” as the reference line for east-west measurement. Thus, the French measured from the meridian of Paris, the Italians from the meridian of Rome, and so forth. At least 13 prime meridians were in use in the 1880s. Not until the late 1800s was standardization finally achieved1. The catalyst for this standardization was the Unites States and Canadian railway. Railway executives needed to adopt a standard time system (the Prime Meridian is also the reference for standard time). Prior to standardization, different countries and even cities were on different times due to the use of different prime meridians. This made railway commutes and scheduling difficult. In 1883 all North American railroads adopted a standard time system and the following year, an international conference was held in Washington, D.C., to achieve the same goal on a global scale and to agree upon a single Prime Meridian. After weeks of debate, the delegates settled on the meridian passing through the Royal Observatory in Greenwich, England as the Prime Meridian for all longitudinal measurement. The principal argument for adopting the Greenwich meridian was that more than two-thirds of the world’s shipping lanes already used the Greenwich meridian as a navigational base.
Longitude is measured both east and west of the Prime Meridian to a maximum of 180° in each direction. The total number of degrees in a globe/circle is 360. If you divide 360 by two (i.e. split the eastern and western hemispheres), you get 180. Exactly halfway around the globe from the Prime Meridian, in the middle of the Pacific Ocean, is the 180° meridian – this is also known as the International Date Line.
When determining the longitude of a location you must designate which hemisphere you are located in. As you can see by the images below, the eastern and western hemisphere are mirror images of each other. If I said I buried one hundred million dollars’ cash at 120° longitude, you have two choices – 120°W or 120°E. This is not very accurate. If I said I buried one hundred million dollars’ cash at 120°E, then it gives you a narrower focus and you know to look in the northern hemisphere along the 120°E meridian. The only time you do not have to designate which hemisphere you’re in is when you are referring to the Prime Meridian or the International Date Line. The degrees for the Prime Meridian are always simply 0° longitude. The degrees for the International Date Line are always 180°. This is because you are at the line in-between both hemispheres at both of those meridians.
Remember, lines of longitude:
· Are known as meridians
· Run in a north-south direction
· Measure distance east or west from the Prime Meridian
· Are furthest apart at the equator and meet at the poles
· Cross the equator at right angles
· Are equal in length
· Are halves of great circles
3. Locating Points
Where a line of latitude and a line of longitude intersect, is a point (otherwise known as coordinate). You’ll often see coordinates expressed as either degrees, minutes, and seconds or decimal degrees. The reason why you don’t see coordinate expressed as whole degrees is because of the amount of space in between each degree. In other words, there is a LOT of space between each parallel (just under 70 miles to be exact; see the image to the right on the previous page). In order to account for this space, there needs to be additional lines in between each whole degree. So, in between each line of latitude and longitude are 60 lines (or minutes) and in between those lines are 60 more (which are referred to as seconds). Having these extra lines in between each whole degree means almost every inch of space/surface of Earth is accounted for. Here is an example of the degrees, minutes, seconds coordinates for College of Alameda:
Decimal degrees on the other hand are a simplified version of degrees, minutes, seconds. Here is an example of the decimal degree coordinates for College of Alameda. Notice that it is really easy to add, subtract, divide, etc. decimal degrees rather than degrees, minutes, seconds. Something also worth noting is that hemispheres aren’t designated with N, S, E, or W like they are for degrees, minutes, seconds. Instead the hemispheres are determined by positive and negative numbers. For example, with latitude, a positive number represents the northern hemisphere and negative number represents the southern hemisphere. For longitude, a positive number represents the eastern hemisphere and a negative number represents the western hemisphere.
This information is optional (i.e. I will not test you on this): for those of you who are interested in time zones and how the days in each hemisphere are determined read this.
Tools of the Geographer
1. Maps
A map is the fundamental tool of the geographer. With a map, one can illustrate the spatial distribution (i.e., geographic pattern) of almost any kind of phenomena. Maps provide a wealth of information. The information collected to create a map is called spatial data. Any object or characteristic that has a location can be considered spatial data. Maps can depict two kinds of data. Qualitative map data is in the form of a quality and expresses the presence or absence of the subject on a map, like the kind of vegetation present occupying a region. Quantitative map data is expressed as a numerical value, like elevation in meters, or temperature is degrees Celsius. There are many different kinds of maps that serve quite different purposes1.
The science of mapmaking (yes it’s a real science) is cartography.
Reference Maps
Reference or navigational maps are created to help you navigate over the earth surface. These kinds of maps show you where particular places are located and can be used to navigate your way to them. A street map or the common highway road map falls into this category.
Thematic Maps
Thematic maps are used to communicate geographic concepts like the distribution of densities, spatial relationships, magnitudes, movements etc. World climate or soils maps are notable examples of thematic maps. There are five common techniques for depicting geography data on a thematic map. The most common is a choropleth map that uses color to show variations in quantity, density, percent, etc. within a defined geographic area. Each color usually depicts a range of values (see image below).
Figure 1.12 Palmer Drought Index Source: NOAA Climate Prediction Center
Figure 1.13 USDA's Plant Hardiness Zone Map
An isarithmic map uses isolines, lines that connect equal values, to illustrate continuous data such as elevation, air pressure, and precipitation. Topographic maps, such as the one below) use contour lines to show elevation (height above sea level). Contour lines connect points of equal elevation above a specified reference, usually as sea level. The heavy brown contour lines with the elevation printed on them are called index contours. Intermediate contours are the lighter brown lines between index contours. Sometimes dashed lines called supplemental contours are used in areas of very low relief. Benchmarks are locations where the elevation has been surveyed. Benchmarks are denoted on a map with the letters "BM", "X" or a triangle with the elevation printed beside.
Source: USGS Monarch Lake
Not only are natural features like mountains, valleys, streams and glaciers portrayed, but cultural features as well, like houses, schools, streets, and urbanized areas. Examine a topographic symbol sheet from the USGS to see how a variety of features are symbolized on a topographic map.
Major earthquakes felt in Canada. Source: NAISMap WWW-GIS
Dot maps (such as the one below. Source: USDA) use dots to illustrate the presence of the phenomenon on a map. A dot may equate to one or several units of measurement. Dot maps are especially useful in visualizing the frequency of occurrence or density of a mapped variable.
2. Map Projections
All flat maps have distortion. This is because it is mathematically impossible (as you’ll see in the YouTube videos this week) to flatten a 3-D object onto a flat piece of paper without causing the size or shape or distance to become inaccurate. The most accurate representation of Earth and its spatial relationships is on a globe. The challenge to the cartographer (map maker) is to try to combine the geometric exactness of a globe with the convenience of a flat map. This this melding has been attempted for many centuries, and further refinements continue to be made. The fundamental problem is always the same: to transfer data from a spherical surface to a flat map with a minimum amount of distortion. This transfer is accomplished with a map projection.
A map projection is a system in which the spherical surface of Earth is transformed for display on a flat surface. The basic principle of a map projection is simple. Imagine a transparent globe on which are drawn meridians, parallels and continental boundaries; also imagine a lightbulb in the center of this globe. A piece of paper, either held flat or rolled into some shape such as a cylinder or cone is placed over the globe (see image below). When the bulb is lighted, all the lines on the globe are projected outward onto the paper. These lines are then sketched on the paper. When the paper is laid out flat, a map projection has been produced. Due to modern technology, we no longer need to hand draw maps. We can simply click on the projection type we want to use (which is done mathematically using computer software).
Because a flat surface cannot be closely fitted to a sphere without wrinkling or tearing, no matter how a map projection is made, data from a globe (parallels, meridians, continental boundaries, and so forth) cannot be transferred without distortion of shape, relative area, distance, and/or direction. However, a cartographer can choose to control or reduce one or more of these distortions (although all distortions cannot be eliminated on a single map.
There are hundreds of different map projections. Click here to see some of them. Most of these hundreds can be grouped into just a few families. Projections in the same family generally have similar properties and relative distortion characteristics.
The four main map projection families are:
1. Cylindrical Projection
2. Planar Projection
3. Conic Projection
4. Pseudocylindrical Projection
A cylindrical projection is made by mathematically “wrapping” the globe with a cylinder of paper in such a way that the paper touches the globe only at the globes equator. We say that paper positioned this way is tangent to the globe at the equator. The curved parallels and meridians of the globe then form a perfectly rectangular grid on a map. There is no size distortion at the point of tangency, but size distortion does increase progressively with increasing distance from this circle.
The most famous map projection, the Mercator projection, originated in 1569 by a Flemish geographer and cartographer. The Mercator projection is a type of cylindrical projection (i.e. it belongs in the cylindrical family). The Mercator projection is designed to facilitate oceanic navigation. The easiest way to tell that you are looking at a cylindrical or Mercator projection is to look at Greenland. In actuality, Greenland is only about the size of Latin America, but on a cylindrical projection it looks almost the same size as Africa.
A planar projection is obtained by projecting the markings of a center-lit globe onto a flat piece of paper that is tangent to the globe at one point (see image on the following page) – usually the North or South Pole, or some point along the equator. The is no distortion immediately around the point of tangency, but the distortion increases progressively away from this point. Typically, planar projections only show one hemisphere.
A conic projection is obtained by projecting the markings of a center-lit globe onto a cone wrapped tangent to, or intersecting, a portion of the globe (see image below). Normally the apex of the cone is positioned above a pole, which means that the circle of tangency coincides with a parallel. Distortion is least in the vicinity of this parallel and increases progressively as one moves away from it. Consequently, conic projections are best suited for regions of east-west orientation in the midlatitudes, being particularly useful for maps of the United States, Europe, and China.
A pseudocylindrical projection is a roughly football-shaped map, usually of the entire world (see image on the following page), although sometimes only the central section of a pseudocylindrical projection is used for maps of lesser areas. Mathematically, a pseudocylindrical projection wraps around the equator like an ordinary cylindrical projection, but then further “curves” in toward the poles, effectively conveying some of the curvature of the Earth. In most pseudocylindrical projections, a central parallel (usually the equator) and a central meridian (often the prime meridian) cross at right angles in the middle of the map, which is a point of no distortion. Distortion in size and/or shape normally increases progressively as one moves away from this point in any direction. All of the parallels are drawn parallel to each other, whereas all meridians, except for the central meridian, are shown as curved lines.
3. Remote Sensing
Review this page regarding aerial photographs and remote sensing and imagery. You do not need to click “continue”, just review the one page only.
4. Global Positioning System (GPS)
Review this page regarding how geographers utilize GPS. You do not need to click “continue”, just review the one page only.
5. Geographic Information Systems (GIS)
Review this page regarding the uses and significance of GIS. You do not need to click “continue”, just review the one page only.
If you are all tech savvy or interested in map making, I strongly encourage you to consider learning more about GIS. GIS and other mapping technicians is a continuously growing industry. EVERYONE NEEDS A MAP. Click here and here to learn more about the software and what can be done with it (hint: the applications are endless!).
The Annual March of the Seasons
The last topic I want to discuss for this chapter is what is known as the annual march of the seasons. This will set up our discussion for weather and climate (which will be introduced in Ch. 3). During the year, the changing relationship of the Earth to the Sun results in variations in day length and in the angle at which the Sun’s rays strike the surface of Earth. This is where those seven important parallels come back into play. These changes are most obvious in the mid- and high latitudes (towards the North and South Poles), but important variations take place in the tropics as well. We will focus on four special days of the year: The December solstice, the March equinox, the June solstice, and the September equinox (see image on pg. 17).
The December solstice occurs on or about December 21 (the exact date varies depending on the calendar year). The date this year happens to be this week (!) – Thursday, December 21. On this day, Earth reaches the position in its orbit where the North Pole is oriented most directly away from the Sun; vertical rays of the Sun now strike 23.5°S, the Tropic of Capricorn. The circle of illumination (which is the great circle/dividing line between the daylight half of Earth and the nighttime half of Earth), reaches to the far side of one pole and falls short on the near side of the other pole. Areas north of the Arctic Circle (66.5°N) are in continuous darkness, whereas areas south of the Antarctic Circle (66.5°S) are in daylight for 24 hours. The December solstice is called the winter solstice (or first day of winter) in the Northern Hemisphere and the summer solstice (or first day of summer) in the Southern Hemisphere.
On approximately March 20 (the exact date varies depending on the calendar year), Earth experiences the March equinox. The vertical rays of the Sun are striking the equator. This means that the circle of illumination just touches both poles. On this day all locations on Earth receive 12 hours of daylight and 12 hours of darkness. When you hear “equinox”, think equal (equal length of day and night)! The March equinox is called the vernal or spring equinox in the Northern Hemisphere (or the first day of fall) and the fall or autumnal equinox in the Southern Hemisphere (or the first day of spring).
On the June Solstice, which occurs on or about June 21 (the exact date varies depending on the calendar year), the Earth reached the position in its orbit where the North Pole is oriented most directly toward the Sun; vertical rays of the Sun now strike 23.5°N, the Tropic of Cancer. The circle of illumination (which is the great circle/dividing line between the daylight half of Earth and the nighttime half of Earth), reaches to the far side of one pole and falls short on the near side of the other pole. Areas south of the Antarctic Circle (66.5°S) are in continuous darkness, whereas areas south of the Arctic Circle (66.5°N) are in daylight for 24 hours. The June solstice is called the summer solstice (or first day of summer) in the Northern Hemisphere and the winter solstice (or first day of winter) in the Southern Hemisphere.
On approximately September 20 (the exact date varies depending on the calendar year), Earth experiences the September equinox. The vertical rays of the Sun are striking the equator. This means that the circle of illumination just touches both poles. On this day all locations on Earth receive 12 hours of daylight and 12 hours of darkness. When you hear “equinox”, think equal (equal length of day and night)! The September equinox is called the fall or autumnal equinox in the Northern Hemisphere (or the first day of fall) and the vernal or spring equinox in the Southern Hemisphere (or the first day of spring).
1 Hess, D. (2014). “Introduction to Earth”. McKnight’s Physical Geography, 3rd California Edition.
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