Part 3—Create and Color-Code a Contour Map

The profile you created in Part 2 showed how deep the ocean is along a single east-west line. You can't tell anything about the ocean depth north or south of that line though.

In order to get a sense of the seafloor's three-dimensional (3-D) shape, you would need to take several more virtual cruises across the Atlantic, collecting depth data along other latitude lines. In this way, you could create a whole grid of depth measurements to help you understand the shape of the seafloor.

• If you could use more practice gathering and plotting depth values, follow the instructions in Step 2 to gather and chart a grid of depth data.
• If it is clear to you how a grid of depth measurements would indicate the 3-D shape of the seafloor, go directly to Step 3 to compare an example of a hand-drawn contour map with a computer-generated one.

1. Use a large poster-size piece of paper to create your grid and record your depth measurements.
   • Label the Y axis "Latitude" and show even-numbered latitude lines from 22°N to 40°N.
   • Put distance on the X axis and label "Kilometers". Begin at 80°W on the left and make 15 equally spaced tick marks along the X axis extending to the right side of your paper. Each mark represents a distance of 200 km, so your grid should extend 3000 km east of the 80°W longitude line.
2. Similar to what you did in Part 2, take your cursor on a virtual cruise in GeoMapApp again. Collect depth values along even-numbered latitude lines from 22°N to 40°N. Begin at 80°W for each latitude line, and measure and record the depth every 200 km. Make 15 measurements along each latitude line and record them in the appropriate line on your grid.
Show me an example of a data collection chart
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3. Once you have collected the grid of depth data, draw contour lines to help you visualize the shape of the seafloor. You'll draw them by interpolating where specific depth values are located on your grid.
   Interpolating means estimating the location of a value based on the location of known values. For instance, if you looked at a number line that showed 80 and 90, and you wanted to show where 82 would be, you would interpolate that 82 is two-tenths of the distance from 80 to 90. This technique is commonly used on weather maps to plot isobars or lines of equal pressure. On land, contours are used to show areas of equal elevation. For the seafloor, contour lines show equal depths of the ocean. Unlike contour lines on land, sea floor contour lines are in negative values; they show depth below rather than height above sea level.
4. Using pencil, interpolate contour lines at 500-meter increments. Once you are confident of the lines' locations, use colored pencils to highlight them.

Take a look now and compare visualizations of the seafloor.

• Below is a hand-drawn contour map:



The map is not complete: contours have only been drawn for 500 through 4500 meters depth. Examine the grid carefully; trace your finger along the location where you would draw the 5000 meter contour.

• Below is a contour map of the same area generated by GeoMapApp:
  
  
  

The graphical visualizations constructed by hand show what it takes to make visual sense of ocean depth data. You had to collect, record, and graph the data to see them as a visual representation of the seafloor.

Computer visualization programs such as GeoMapApp perform the same basic steps as you did to produce visualizations. Through the Internet, GeoMapApp has access to huge numbers of ocean depth measurements that have been collected by science teams from many countries. When you generate a visualization through GeoMapApp, the program accesses data that have been collected and shows them in a graphical display designed to make visual sense.

Now that you are familiar with how to construct profile and contour maps from data, you can use GeoMapApp to generate these displays for you. You can quickly and easily produce visualizations of the seafloor anywhere in the world. Using the computational power of visualization software will allow you to focus on what the graphs and visualizations tell you about the geologic and marine processes that have shaped our world.