"EXTRACTING" A COASTLINE

OVERVIEW

Students will access an interactive web page allows the user to create a data file or an image of any coastline in the world. If they have appropriate software available, they may also import coastline data and make their own "X-Y" plots.

 

CONCEPTS

  • Standard geographic maps are made up of many sets of coordinates: each set has a value for latitude and another value for longitude.
  • Each coordinate is given in degrees. This is because all circles (including our global Earth) are made up of 360 degrees.
  • The students will "extract" and use the coastline data in six steps:
    1. At an interactive website, students will enter latitude and longtiude data for an area of coastline
    2. After submitting their data, a plot of the coastline will be "extracted"
    3. They may then download the coastline data onto their computer
    4. The latitude and longitude data may be imported into an plotting program
    5. The data will then be sorted and formatted
    6. Data will be plotted in a "X-Y" coordinate grid.
      • More coordinates -- such as buoy locations in the Gulf of Maine -- can be added during this step.
      • The concept of "aspect ratio" -- the relationship between the width and the height of a rectangular map -- is introduced.
 
 

MATERIALS

  • World map for reference
  • (optional): Software program that makes X-Y plots (e.g., Microsoft Excel, Cricket Graph)
  
 

PREPARATION

  • Briefly discuss how Earth's geography is given in a coordinate system made up of latitude and longitude.
  • After this discussion, visit the U.S. Geological Survey's "Coastline Extractor" website at http://crusty.er.usgs.gov/coast/getcoast.html.
  • If you do not have an "X-Y" plotting program:
    • Complete the Activity and Questions for Steps 1 and 2
    • Read the description of Step 4, and answer the Questions for Step 4
    • Use the provided example "X-Y" plot in Step 6 to answer the Questions for Step 6.
  • If you do have an "X-Y" plotting program, complete the Activity and Questions for all six Steps.
 
 

PROCEDURE

 

Background

  • Latitude is relative to Earth's equator. In terms of degrees latitude, north of the equator is positive and south is negative.
  • Longitude is relative to zero degrees, along which Greenwich, England is located. The International Date Line is located half-way around the world (i.e., 180 degrees away) in the Pacific Ocean.
 
 

Activity - Step 1 "Visit Website & Enter Data"

1. Visit the "Coastline Extractor" website ("Preview" at right >>>).

2. Choose the area of your map under "Geographic Range of Extracted Coastline."

3. Type in the northermost latitude as "45".

4. Type in the westernmost longitude as "-72". Note that for this map-making tool, negative longitude is west of Greenwich

5. Type in the easternmost longitude as "-62".

6. Type in the southernmost latitude as "40".

7. Using the "pull down menu" under "Coastline data base," choose the option: "WCL (World Coast Line) (designed for 1:5,000,000)"

8. Leave the rest of the "clickable" buttons alone

9. Hit "SUBMIT" to "extract the coast."

Coastline Extractor

Geographic Range of Extracted Coastline (decimal degrees, west negative) :
 
Northernmost latitude
45
 
 
Westernmost longitude:
-72
-62
Easternmost longitude
 
Southernmost latitude
40
 

 

Coastline data base :
WCL (World Coast Line) (designed for 1:5,000,000) V

Compression method for extracted ASCII data:
None GNU GZIP UNIX Compressed ZIP

Coast Format options:
Mapgen Arc/Info Ungenerate Matlab Splus

Coast Preview options:
No preview Quick Plot GMT Plot

 

Extract the Coast:

SUBMIT
 

Questions - Step 1

  1. Based on these coordinates, is your map going to be:
    • North or south or the equator?
    • West or east of Greenwich, England?
    • West or east of the International Date Line?
  
 

Activity - Step 2 "Extract Coastline Data"

1. After you've hit "SUBMIT," you may get a "warning message" that says "Any information that you submit is insecure..." Hit "OK" to continue.

2.Your map will appear as an image on top. (See example at right >>>.)

3. Latitude and longitude data, formatted as two columns, are called "XXXX.dat." Note that "XXXX" is a number generated by the program itself.
Extracted Coastline Data

 

Here is the coastline data you extracted: XXXX.dat (4650 bytes).

Note: If you don't want to list the file to your screen, you might want to select "load to local disk" (or the equivalent) before selecting. In Netscape, this is accomplished by holding down the shift key while you click.

Questions - Step 2

  1. The scale tells you how the size of any feature compares to its real size. A map with a scale of 1:12 would be one-twelfth the area of the map's coverage zone. On such a map each square-inch would represent only one square-foot... this is not a very practical scale for large areas. To show a larger area, a map's scale needs to be smaller than 1:12. Think of 1:12 as the ratio 1/12. Any number larger than 12 will give a ratio that's smaller than 1:12.
    • Would you rather be given 1/12th of a medium pizza or 1/100th of a medium pizza?
    • Would you rather split your lottery winnings with 12 people or 100 people?
  2. A map's scale is usually quite small in order to show large areas. For example, to fit the state of Maine (30,995 square miles) on a typical road map, the scale needs to be around 1:679,000.
    • Can you approximate what the scale of a road map of Alaska (570,833 square miles) might be ? How about Rhode Island (1,054 square miles)?
  3. The file that you generated is from the "World Coast Line" data set and is good for maps with scales of about 1:5,000,000. The "World Data Bank II" data set is appropriate for making maps with scales of about 1:2,000,000.
    • The map you made using the "World Coast Line (WCL)" data base has 220 coordinates (i.e., 220 latitude / longitude pairs).
    • On the other hand, the same area mapped using the "World Data Bank II" data base has almost 10,000 coordinates. Click here to see this map.
      • Do the coastlines look the same?
      • Which has more detail?
      • Which data set might be easier to work with?
    • These images demonstrate the "trade-off" between the size of the data set and the relative detail of the data.
      • What type of user might prefer the "World Coast Line" data set?
      • What type of user might prefer the "World Data Bank II" data set?
  
 

Activity - Step 3 "Download Coastline Data"

There are two simple ways to dowload these data:

1. Click on the link "XXXX.dat" and the data will appear on the screen. From here you can copy and paste onto your own harddisk.

  • This is the top instruction given (shown at right >>).

2. Hold down your shift key while clicking on the link "XXXX.dat". A "pop up" menu should appear that says "Save this link as..." You may want to change the name of this file from "XXXX.dat" to "coast.dat".

  • This is the bottom instruction given (shown at right >>).
Extracted Coastline Data

 

Here is the coastline data you extracted: XXXX.dat (4650 bytes).

Note: If you don't want to list the file to your screen, you might want to select "load to local disk" (or the equivalent) before selecting. In Netscape, this is accomplished by holding down the shift key while you click.

 
 

Activity - Step 4 "Import Data into a Plotting Program"

1. Load your latitude and longitude data into an X-Y plotting application. Open the application first and then import the data. [Note that the numbers shown at right will not necessarily match the ones you import.]

For this area's data, each column has a specific data format:

  • LEFT: 10 characters
  • RIGHT: 9 characters
  • "Tab-delimited" (i.e., columns are separated by tabs)

2. Knowing these two facts about data format, you should be able to pull these numbers into your X-Y plotting program without "breaking up" the data incorrectly.

  • For example, if you just looked at the 2nd line of data:
     
    		-65 43.699862
     

You might think that the first column had 3 characters and your other column had 9 characters. If you assumed this and formatted the rest of your data accordingly, your data would look like**:

 
-65 43.699862
 
  -65 .09886 43.735619
  -65 .131715 43.745007
  -65 .232629 43.705111
Data table will look like:

 

Questions - Step 4

  1. Look at the data columns (and ignore lines with "# -b").
    • Which column has latitude data?
    • Which column has longitude data?
    • Are the data sorted in any way?
  2. Let's say you "mis-sorted" your data as shown above** and you interpreted the first column as longitude, the second column as latitude, and "threw away" the third column.
    • About where would your "mis-sorted coastline" (i.e., bold numbers) be found?
  3. You may notice that there is room for an "extra" character in each column: The left column could hold up to 11 characters and the right up to 10 characters.
    • Can you think of any geographic location that might use all possible characters in these columns?
  
 

Activity - Step 5 "Sorting Coastline Data"

1. You'll notice that some lines have only one entry: "# -b". These are page breaks and not part of the coordinate data set. It is recommended that you remove these page breaks as follows:

  • Select both columns of data. This will ensure that the coordinate pairs do not get separated from one another.
  • Use the "sorting" function to sort the left-hand column into descending (or decreasing) numbers. By doing this, all the page breaks should end up at the top.
  • Delete all the lines that contain page breaks (should be about 6).
  • The "leftover" data will be used to made a map.
Data table will look like:

 

Questions - Step 5

  1. If the data in the left column are sorted into descending (or decreasing) numbers, then why is "62" above "63"?
  
 

Activity - Step 6 "Creating an X-Y Plot"

1. Plot the data as an "X-Y scatter plot." The lefthand column should be plotted along the horizontal ("X") axis and the righthand column data along the vertical ("Y") axis.

  • The "X" and "Y" axes are also known as the "abscissa" and "ordinate," respectively, in the Cartesian coordinate system.

2. At this point, you can also easily add your own data points. For example, you can add coordintes that show the locations of buoys in the Gulf of Maine.

  • To prepare your map for the buoy data, add the following three coordinates to data base:
     
    		-70.14 43.53
     
      -68.94 42.9  
      -66.58 41.08  

3. Your map will look something like the plot (without the buoy names marked) shown below VVV:

Questions - Step 6

  1. The term "aspect ratio" refers to the relationship between the width and the height of a rectangular image or map. For example, you may notice that when a few movies are shown on television there is a black strip at the top and bottom of the image. This is done to provide a match between the viewing area of your television's screen and the aspect ratio of a movie theater's screen.
    • The the aspect ratio of a typical television screen (light blue) and that of a typical movie screen (dark blue) are represented in the image below (VVV).
      • Which has a higher width-to-height, or aspect, ratio?
  • Compare the aspect ratio of your "X-Y" plot*** with the one created within the "Coastline Extractor" website. (For example, for each map measure 10 degrees longitude and 5 degrees latitude. Then calculate the ratio for each map.)
    • Do the two charts have the same aspect ratio?
    • If not, which map more closely represents the way your coastline really looks?
    • How do you know what the coastline really looks like?

***If you did not created your own"X-Y" plot, use the example "X-Y" plot above.

2. The "Coastline Extractor"-generated map is in a format called "Mercator projection." This is a common way to represent our "3-D" globe-shaped Earth on flat, two-dimensional map.

  • Click here to learn more about Mercator Projection maps.
 

EXTENSION

 
 

VOCABULARY

  • aspect ratio: the proportion between its longer and shorter sides of a rectangle; for example, the aspect ratio of a square is 1:1.
  • coordinates: any of a set of magnitudes by means of which the position of a point, line, or angle is determined with reference to fixed elements.
  • latitude: location on Earth's surface based on angular distance north or south of the equator. Equator, 0°; North Pole, 90°N; South Pole, 90°S.
  • longitude: distance (expressed in degrees) east or west of the prime meridian, along which Greenwich, England is located. The International Date Line is 180° away from the prime meridian.
  • scale: the proportion that map, etc. bears to the thing it represents (e.g., a scale of one inch to a mile).