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# Exploring the Depths of the Great Lakes with ArcGIS Online, or "A GIS Lesson as a Tribute to Gordon Lightfoot."

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06-12-2023 03:47 PM
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In this new lesson I invite you to explore the depths of the Great Lakes with ArcGIS Online using several straightforward yet powerful tools.  One could also consider this lesson my tribute to musician Gordon Lightfoot, who passed away in 2023 after a lifetime of songwriting and performing.  I love his stories-set-to-music about space, place, and time (my particular favorite is the Canadian Railroad Trilogy).  Here we will include an investigation of his song, "The Wreck of the Edmund Fitzgerald".  However, the lesson has broader aims--to encourage spatial and critical thinking with a focus on understanding the physical geography of lakes--specifically, the Great Lakes. Perhaps it is even more fittingly a tribute to all who work on and under our oceans and lakes.

To examine the lesson in video form, watch these 3 videos:

Through this lesson, skills in Geography, GIS, and mathematics are fostered through inquiry and hands-on work.  This lesson is most suitable for secondary students and university students.  Some familiarity with the ArcGIS Online Map Viewer and 3D scene interfaces is helpful but not required.  The activity requires no sign in--only a web browser with an internet connection is needed.  If you wish to save your results, you will need to sign in to an ArcGIS Online organizational account.

Representing Numbers as Isolines

Many ways exist to represent and analyze data and numbers using maps—choropleth maps, dot density maps, graduated symbol maps, and more. Another way to represent the world through maps is through isolines (iso="same"). These are lines that connect variables having the same value. You have seen weather maps, which often show isolines that connect areas that currently have the same temperature or pressure. You may have gone hiking and taken topographic maps so you know how steep your trail will be--on such maps, contours are isolines that show land with the same elevation.  Isolines can also show frequency of crime in a city, types of diseases in a region, or earthquakes around the world. The manner in which these isolines are symbolized affects the map reader's perception of how that variable changes across a particular area of the Earth. Think about that as you investigate depths in these magnificent lakes using isolines--in this case, lines of bathymetry.

Open this linked map of Great Lakes bathymetry (shown below). This map shows data resulting from a cooperative effort between investigators at the NOAA National Geophysical Data Center's Marine Geology & Geophysics Division (NGDC/MGG) and the NOAA Great Lakes Environmental Research Laboratory (GLERL) showing the five Great Lakes plus Lake St Clair.

Only one of the lakes is entirely within the territory of the USA.  Which one is it?   What 2 countries do the rest of the lakes share territory of?

Make the legend visible.  Zoom in until you clearly see the individual contours—each of these lines is an isoline. These lines are similar to contour lines on land, but here, they represent the depth of the water. They are called bathymetric contours, showing the bathymetry (depth) of the water bodies. Each is a line representing equal depth, in meters. The lake floor is steeper where the bathymetric contours are closer together and flatter where they are farther apart.

Expand the layer to see the sub-layer underneath > click to make it active > on the right, > enable popups >
click on contours to view the depth in different parts of the lakes (as shown below).

Just like land contours, the shape of bathymetric contours in lakes and oceans may reveal plains, canyons, hills, and other features across these floors—even volcanoes!

Investigating the floors of the Great Lakes

Based on your observations of the bathymetric contours in the Great Lakes, answer the following questions:

Why do you suppose these lakes are so important to commerce in the USA and Canada?

Which lake is the deepest?  The shallowest?

Because a certain lake in the Great Lakes is so deep, it contains more than half of the total amount of water in all of the Great Lakes.  According to Michigan.org, the Great Lakes hold over 5,400 cubic miles of water. Of this amount, the lake you identified above as the deepest holds 2,900 cubic miles, or 3 quadrillion gallons (over 11 quadrillion liters). The rest is distributed among the other four lakes, with Lake Michigan holding the second most, Huron third, Ontario fourth, and Erie holding the least.  Because it is so large, to raise the water level of the deepest lake that you identified above by one inch, you would need to pour more than 500 billion additional gallons into it!

Name the part of each lake (use cardinal directions in your answer) that is the deepest.

Can you detect any underwater canyons or cliffs in the Great Lakes?  If so, where are they?

What are the 3 largest towns or cities on the shores of each of the lakes?   Which of the cities have the deepest harbors? The shallowest?

Between Lake Erie and Lake Ontario are the famous Niagara Falls.  Ships obviously cannot go over it as it is 57 m (187 ft) tall (at Horsehoe Falls)!  How do ships get around it?

Search for and zoom to the city of Port Colborne, Ontario, Canada, noting the canal between this city on Lake Erie (shown below) north to the city of Port Dalhousie.  Ships use this canal to avoid Niagara Falls.  How long is this canal?

Change the base map to USA topo map.  This map shows elevations of the land at selected places on the base map.  Careful!  The elevations on the largest scale maps here are in feet, the medium scale are in meters, and the small scale are in feet.  Choose your desired scale--ideally, the largest, so you can see the elevations most clearly.  Zoom and pan the map on the shoreline of Lake Ontario near Rochester New York.  Note the elevation of the shoreline.  Do the same thing for the shoreline around Duluth Minnesota at Lake Superior.  Based on your observation, which of the two lakes is the highest in elevation?  The lowest?

Knowing this, which direction does the water flow over Niagara Falls--from Lake Ontario to Lake Erie, or from Lake Erie to Lake Ontario.  Does water flow from Lake Superior to Lake Ontario or from Lake Ontario to Lake Superior?  Zoom out until you can see the Atlantic Ocean.  Which lake--Ontario or Superior--is closer to the ocean?  Does this confirm your earlier answer about the elevation of each lake, and which direction the water flows in the lakes?  Although modern-day water is constrained in the lakes through human intervention (locks), some does indeed flow to the ocean.  Indeed, 3,160 tons of water flows over Niagara Falls every second, or 75,750 gallons of water per second over the American and Bridal Veil Falls (286,745 liters/second) and 681,750 gallons per second (2580704 liters/second) over the Horseshoe Falls!  To get a sense for the volume, see my video that I filmed there.

Suppose you are assigned the task of identifying the deep places in the lakes where shipwrecks may have occurred, at specified depths. Open the data table behind the contours via the ellipsis to the right of the data layer > click on the Depth (m) field > sort descending. Each of the 119,813 records in the table represents one bathymetric contour on the map. Scroll down in the table, looking at the deepest contours. In which lake are all these deepest places?

Consider the following:

Maps are rich sources of information, but sometimes they contain too much information to be easily
understood. Filtering maps or data tables can make specific data clearer. Use the Filter tool to the right of the map > Add an Expression > Depth is at least 100 meters (as shown).

Examine the resulting patterns: Which of the Great Lakes do you believe has the highest percentage of its depths at 100 meters or more? Which one(s)of the five Great Lakes have no depths more than 100 meters? Where is the deepest part of Lake Huron? Open the table. How many records exist in the newly filtered table? Jot this number down. Then, clear the filter and examine the original table to be able to compare the number of records in the original table versus the filtered table. What percentage of the original data set (as measured by the number of records) do you have in the filtered table of only the deep contours?

The Wreck of the Edmund Fitzgerald

As you are learning from your study of the lake floors, these lakes are large, and deep.  They have sadly claimed many ships and the lives of people on those ships, over the centuries.  Probably the most famous Great Lakes shipwreck of all occurred in relatively modern times.  This was the sad tale of the SS Edmund Fitzgerald, which sank on 10 November 1975.

Tribute to the Edmund Fitzgerald.  I took this picture at the maritime museum in Duluth Minnesota.

The Edmund Fitzgerald sank at a longitude and latitude of -85.11017, 46.998500.

Use the search tool to search for this shipwreck's location and add it to your map as a sketch. Zoom to this
location.

In which lake did this shipwreck occur?

Is the shipwreck located in waters over 100 meters deep?

Use the measure tool to answer this question:  How far is the wreck from the nearest shoreline?

Conduct research on the shipwreck (such as with this source) and speculate on the theories as to why it sank.  Which do you find most likely?   How could it have sunk despite being so close to shore?  What were the final words that the captain sent over the wireless?

Lake Superior can indeed be a dangerous place.  I took this photograph in October on the lakeshore on a blustery and rainy day.

Lakes are 3D objects, after all, and the ArcGIS 3D scene viewer might be very useful for further investigation.  Examine the same set of bathymetric contours in this 3D scene.  How does this 3D visualization affect your understanding of the floors of the Great Lakes?

Next, examine this 3D web mapping application showing Lake Superior shipwrecks.  Note that here, the bathymetric contours are colored differently than the 2D map you were examining up to this point:  Can you determine which colors signify the greatest depths?  In this 3D scene, note how many ships have been wrecked near where the Edmund Fitzgerald went down.  How does this 3D visualization affect your understanding of shipwrecks?  Did most of the wrecks occur in the middle of the lake or close to shore?  Why do you suppose this is the case?

While the wreck was located a few days after the ship sank, using magnetic anomaly detection, shortly thereafter by sonar, and afterward by unstaffed and staffed submersibles, it wasn't until 1995 that a scuba dive resulted in the first people to touch the ship since 1975.  Examine the table showing the deepest technical scuba dives for shipwrecks around the world, in this article.  What rank in shipwreck dive depths was the 1995 dive to the Edmund Fitzgerald?   Read about this 1995 dive in this article.  Why was the dive controversial?

Suppose you just received word of new budgetary constraints on your shipwreck project that means that you can now only focus on the lake where you found the deepest contours and where you located the above wreck.

Return to your original 2D map investigation in ArcGIS Online.  Again access the Filter tool.  You will need to re-run your filter on the deepest parts of the Great Lakes (100 meters or more).  Staying inside the Filter tool, now > Add an expression where the Lake Name is the name of the lake that meets both of the above criteria.

What does your expression look like?

After running your expression, open the data table again. Now how many records exist in the newly filtered table?

What percentage of the original data set (as measured by the number of records) do you now have?

As you have seen, using mathematical expressions in filtering aids in analysis and decision-making.  Mathematical expressions can also be used in interactive maps to customize the appearance of the popups, for labeling features, for symbolization, for spatial analysis, and much more.

If time permits, add data for other Great Lakes shipwrecks, using ArcGIS Online as a starting point. What patterns do you notice?

Now consider another way of visualizing Great Lakes bathymetry using a technique that results in an etched wood block effect, from my Esri colleague John Nelson, here: