Lab 2.3 – Bubble charts

Fundamental concept: Variability in the data, finding trends
Preparation for: Geology (Lab 3)
Estimated time to complete: 20 minutes
Materials needed (none)

Maritime disasters, such as marine pollution or shipwrecks, are a reality of human interactions with the ocean. To study such events, scientists or historians may need to display data linked to spatial locations on a map. In this lab you will add to your knowledge of bathymetric charts (Lab 2.2) by interpreting bubble charts, a way to visualize spatial data.

Bubble charts

Bubble charts display data with three variables by using marker size and/or color. These are commonly used for data collected at different geographic locations. The map below (Figure 2.3.1) shows the size of several oil spills in the Gulf of Mexico. The “bubbles” (blue circles) are located at the latitude and longitude of each spill. The size of each bubble represents the number of gallons of oil spilled. The legend on the right side of the map shows the gallons of oil represented by different sized bubbles. The two largest bubbles are the 1979 Ixtoc I and 2010 Deepwater Horizon oil rig disasters.

Figure 2.3.1. Oil spills in the Gulf of Mexico. Marker diameter represents the size of the spill in gallons.

Instead of bubble size, we could use a color scale to represent this same geographic data. The map below (Figure 2.3.2) displays the same oil spills, but with gallons of oil represented by color. Light pink markers indicate spills that were smaller than 20,000 gallons. The darkest red markers indicate spills that were larger than 100 million gallons of oil.

Figure 2.3.2. Oil spills in the Gulf of Mexico. Marker color indicates size of spill in millions of gallons.

 

Now compare the two oil spill maps to answer this question:

  1. Which version, bubble size or bubble color, do you think is a better way to show differences in oil spill size? Why?

 

What if we have more than one variable to display on a map?

We can combine size and color! We will see how this works in the next maritime disaster example.

Numerous shipwrecks found along the Outer Banks of North Carolina, sometimes nicknamed the “Graveyard of the Atlantic”, were caused by treacherous shifting sand bars, storms, and military battles. The locations of some of these shipwrecks appear in the map below (Figure 2.3.3), color coded by the year in which the ship sank.

bubble map of shipwreck locations, color coded by year

Figure 2.3.3. Shipwrecks off the coast of North Carolina. Marker color indicates year of wreck, as shown on the right-hand color scale.

  1. Examine the color of the shipwreck markers in Figure 2.3.3. In which decade did the largest number of shipwrecks occur? (Look closely, as some of the markers are so close together that they overlap)
  2. Are the most recent shipwrecks in this dataset closer to land or further out at sea compared to the oldest shipwrecks?

Let’s say we are also interested in investigating the size of the ships compared to the location and date that they sank. Figure 2.3.4 shows size as the length of the ship from bow to stern.

bubble map showing year of wreck by color and size of ship by bubble size

Figure 2.3.4. Shipwrecks off the coast of North Carolina. Marker color indicates year of wreck, as shown on the right-hand color scale. Marker size indicates the length of the ship in feet, as indicated by the legend in the lower right corner of the map.

  1. Find the shipwreck that occurred in the 1970’s. What was the approximate length of this ship?
  2. Do you see a pattern in the ship length compared to the proximity of the shipwreck to the coastline? If so, what is the pattern that you see? If not, why do you think these two variables are unrelated?

Reflection questions

  1. Treacherous shallow waters are marked in a variety of ways to assist boaters in making a safe passage.  These markers, which include lighthouses and bell buoys, are located on the water or on land, and are also marked on navigation charts. Navigational markers do not exist in the deep ocean, although maritime disasters can still happen there.  What might cause ships to sink in deep water?
  2.  Can you think of a way to indicate the cause of a disaster on a bubble chart? If so, how would you show this information? If not, why is it difficult to display that type of information in a bubble chart format?