Lesson 2 - Measuring, Monitoring and Describing the Earth
|
To do:
- Check the schedule for this week's reading & upcoming assignments
- Read the lecture and assigned reading in the text
- Participate in discussions
- Take the Week 2 Quiz
By the end of this lesson you should be able to:
- List the ways in which Geographers use maps
- Demonstrate how longitude relates to time on earth
- Describe how scale and map projection can be used to distort ideas
- Relate electromagnetic radiation to remote sensing
- Define a Geographic Information System
|
Cartography
How do we know exactly where we are? How do we represent our world? Cartography is the science of map making. Maps are representations of the world, usually two-dimensional, graphic relationships that use lines, symbols and colors to convey information or ideas about spatial relationships. Geographers use maps for four reasons:
- To locate phenomena- such as places, disease, peoples, etc.
- To show relationships - using maps to prove that some traits are correlated, such as population growth and infant mortality rates, or air pollution and auto usage.
- To prove ideas - maps illustrate relationships (as mentioned above) and statistics which allow us to support our ideas with location examples
- To ask questions - at times maps may reflect areas of contradiction from basic relationships. Such anomalies enable us to question the causal factors we are addressing.
These are four thematic maps of the United States. Each communicates different information, each could be used for different purposes. The most familiar maps are a combination choropleth (where colors have values) and vector (line) maps. Your standard road atlas is also usually a combination choropleth and vector map, where colors represent land cover (green for forested areas, blue for water, and sometimes yellow for urban areas), and roads are represented by lines. A third type of map is an isopleth map, where lines represent changes of a continuous value. A USGS topographic map is an example of an isopleth map.
In a standard choropleth map, the colors represent value added by manufacture. This is a graduated color scheme, commonly used to convey a range of values. Without even looking at the legend in this graduated color scheme choropleth map, you can probably figure out that the darker colors (reds and dark oranges) represent high values, and the lighter colors represented lower values.
Another type of map uses pie charts to convey labor structures. While the divisions of the pie chart represent the types of labor in each city, the size of the pie chart represents the total size of the labor force. In this way, the map is conveying both the distribution of the labor force by type and also by size. Such a map would answer questions like: Where is the majority of the labor force in the US located? Where is the labor force not located? What reasons can you think of for this distribution? These are the types of questions that a good map should prompt you to contemplate.
Browse through more examples of maps.
Location and Time on Earth
We define where we are on earth by several grid systems. The most common of these is latitude and longitude. Latitude and longitude divide the earth up into a series of parallel lines (latitude) and vertical lines running from the north pole to the south pole (longitude). Your book has a very good description of latitude and longitude, so read it carefully.
The base lines for measuring latitude and longitude are the equator and the prime meridian (see the graphic below).The equator goes around the middle of the earth, or it is the longest circumference. The prime meridian is an arbitrary line of longitude, but it was picked to go through Greenwich, England, the location of the Royal Observatory. Latitude is measured from 0 to 90 degrees north of the equator (0 to 90N), and 0 to 90 degrees south of the equator (0 to 90S), with 0 being the equator. It is easy to remember this by thinking about 180 degrees in a hemisphere, or half circle. Longitude is measured from 0 to 180 degrees east of the Prime Meridian (0 to 180E) and 0 to 180 west of the Prime Meridian (0 to 180W). Therefore, everything in the United States has a northerly latitude (it is in the northern hemisphere), and a westerly longitude (it is in the western hemisphere).
Increments of latitude and longitude are called ‘degrees’. One degree is divided into 60 minutes (‘). One minute is divided into 60 seconds (“). So a location might look something like this: 37º15’ 23”N, 122º14’8”W (where is this?).
Conduct some research about the relationship between time and longitude. Each 15º increment in longitude equals one hour of time on earth. We have roughly divide the earth up in this manner by time zones. You can therefore roughly estimate (ignoring daylight savings time and the peculiarities of political gerrymandering of time zones) how many hours earlier or later one location is from another. For example, if you were in a ship at 20ºW and you sailed to 80º W, what time would it be in your new location if it was 6pm in your old location? You would have traveled 60º to the west, so 60/15 = 4, and west equals earlier, so it would be 2pm in your new location.
Check out these web sites on time and location on earth:
Map Scale and Projection
Map scale is defined as: The ratio of distance on a map to distance on the ground Map scale is generally expressed as a ratio, such as 1:100,000. This means that one unit on the map is equal to 100,000 units on the ground, or the map representation of a widget is 1/100,000th of the actual size of the widget.
One way that I find useful to visualize map scale is to think about a map of the world. The length of the equator at different scales is a good way to think about the actual size of a map at that scale. The table below lists the distance on the map (as if you were to lay a ruler along the map and measure the equator) for each map scale. As you can see, a 1:400,000,000 scale map would probably fit across two pages of your textbook, while a 1:10,000,000 scale map would take up a wall of your classroom. At 1:1,000 scale, a map of the world would stretch across the county!
The Length of the Earth's Equator at Different Map Scales
|
Map Scale
|
Distance of the Equator on the Map (m)
|
1:400,000,000 |
0.10002 |
1:40,000,000 |
1.0002 |
1:10,000,000 |
4.0008 |
1:1,000,000 |
40.008 |
1:100,000 |
400.078 |
1:10,000 |
4000.78 |
1:1,000 |
40,007.8 |
Large Scale Maps vs. Small Scale Maps
“Large Scale”
Small features are large
- A map of this room
- A map of this campus
- A map of this city
|
“Small Scale”
Large features are small
- A map of this country
- A map of this world
|
Resolution is defined as: The smallest feature that is represented on the map
- A city such as Los Altos Hills probably would not be represented on a map of the USA (at a scale 1:10,000,000). The resolution of this map is therefore too coarse to represent our city.
- A city such as Los Altos Hills probably would be represented on a map of the Bay Area (scale 1:500,000). The resolution of this map is therefore fine enough to represent our city.
Map projection is the way in which we represent the spherical earth on a flat map (see below). You may want to do some searches to find different types of map projections. The most important thing to remember about map projections, however, is that there will always be some distortion. Some map projections preserve relative areas, some projections preserve shape (such as the shape of coast lines), some try to do both and end up doing neither. But the result is that there will always be some error in a projected map.
One example of error in map projection can be seen in maps of the United States. The graphic below shows three different projections of the United States overlain. Note that the further you get from the center of the projection (the hot pink dot in the middle of the figure below) the bigger the distortion gets. Look in particular at Florida, Washington and Maine.
Another example of distortion caused by map projection is the represented area of Australia and Greenland. The landmass of Greenland is 1/3 that of Australia. That is to say, Australia is three times as big as Greenland. The map on the left is a Mollweide Equal Area projection which preserves the relative area of landforms. The projection on the right is a Gall projection (very similar to a Mercator projection), and it preserves the shape of landforms but not the area.
Remote Sensing
Remote sensing is very important to physical geography. It allows geographers to look at change over a large area over time. In addition, remote sensing takes advantage of the interaction of earth materials with various wavelengths of the electromagnetic spectrum (recall last week’s lecture). The chemical properties of some materials preferentially absorb or reflect certain wavelengths. For example, the iron in chlorophyll (the ‘energy’ producing element of all green plants) reflects very strongly in the near infrared (the wavelength slightly longer than red, and just outside the range of human vision). Chlorophyll also absorbs very strongly in the blue wavelengths. Therefore by looking at the near infrared wavelength and comparing it with the blue, you can pick out the vegetated areas by looking for areas that reflect strongly in red, and not in blue.
The following web pages are excerpts from a NASA tutorial on remote sensing. Some of the material is pretty technical, but read it carefully, and pay particular attention to the graphs:
In the first graph, which compares Pinewood, Grassland, Red Sand Pit and Silty water, if you were looking at a remotely sensed image at 0.8 micrometers wavelength, which would you expect to appear the brightest? Which would appear the least bright?
The Grasslands would appear the brightest, as it reflects the most electromagnetic energy at that wavelength. The silty water would appear the darkest, as it would reflect the least at that wavelength. An example of this type of remote sensing is the Normalized Difference Vegetation Index or NDVI. NDVI is a ratio of near infrared wavelengths to visible wavelengths, and can be thought of as ‘vegetation vigor’. The figure below is an example of global NDVI for January and July. Recall that December and June are the solstices or the height of summer or winter. The figures below are selected as January and July because vegetation generally has a lag-time between when the conditions are at a maximum (height of summer or winter) and when they are at their greenest. In the figure below, light colors indicate low vegetation vigor, while dark colors indicate high vegetation vigor.
In addition to optical remote sensing (remote sensing that uses the visible and near infrared portion of the electromagnetic spectrum), another common type of remote sensing is radar. Radar remote sensing measures the ‘roughness’ of a surface. It is very useful for studies which look at land-use change because man-made features tend to be very smooth compared to vegetation which is rough. Think about if you were in an airplane looking down at a city and a forest. The forest looks ‘fuzzy’ and ‘soft’ while the buildings and roads of the city have large flat, ‘smooth’ surfaces. See a radar image of the San Francisco Bay Area. Note that the urban areas (smooth) reflect very strongly (have a bright color) and the vegetated hills surrounding the Bay Area (rough) do not reflect (appear dark).
Geographic Information Systems (GIS)
A geographic information system (GIS) is a suite of map layers or ‘themes’ that are all georeferenced to the same location on earth, and their associated tabular spreadsheet data. A GIS is a map, a spreadsheet and an information management system all rolled into one. Conduct an internet search for a common rendition of a GIS. The map layers of a GIS are all stacked on top of one another in the computer, allowing the user to query all the layers at once. GIS are used in numerous professional fields, including Geography, Archaeology, Biology, Business, Forestry, Geology, Emergency Management and Law Enforcement.
The best way to experience GIS is to experiment with a GIS. Go to Ice Maps , at the UC Davis Information Center for the environment. Start out by selecting several layers to view. Some suggested combinations are listed below. Look at the legend, zoom in and out and pan around the state then discuss the following prompt in the DC.
Suggested theme combinations:
- Shaded Relief, Major California Rivers, Lakes, Jurisdictional Dams, Non Jurisdictional Dams
- Shaded Relief, Major Cities, State Highways, Federal Highways, Proposed Significant Natural Areas
Review learning outcomes.
Please complete the Assignments and Exams section for each lesson before proceeding to the next lesson.
Back to Top
