We'll be making a bathymetric map, which is a map of depths to the seafloor. A bathymetric map is the opposite of a topographic map, which shows elevations above sea level. If you've never looked at a topographic map or understood what one conveys, you might want to look at the first figure and the text below it on a US Geological Survey web page and/or the text and first illustration on an Idaho State University web page.

The handout says to begin in the northeast, so let's start there. We find points with depths in the range of 4.3 to 5.2 kilometers. Our objective is to draw lines where the depth would reasonably be 5.0 kilometers, 4.5 kilometers, 4.0 kilometers, and so on.

To do that, we can look at pairs of points, one shallower than 5.0 kilometers and one deeper, and envision where depth would be 5.0. For example, below we can make the following marks:

A: Between 4.92 and 5.14, the number 5.0 is closer to 4.92, so we'll put a dot closer to 4.92.
B: Between 4.92 and 5.02, the number 5.0 is closer to 5.02, so we'll put a dot closer to 5.02.
C: Between 4.45 and 5.02, the number 5.0 is much closer to 5.02, so we'll put a dot very close to 5.02.
D: Between 4.87 and 5.14, the number 5.0 is halfway between (0.13 more and 0.14 less), so we'll put a dot halfway between.

So we have these dots:

If we now join our dots as shown below, we have a 5.0 kilometer or 5000 meter contour. Another way to look at is that we've drawn a line between depths greater than 5.0 and depths less than 5.0.

Now we can work on the next contour, and use the same approach:

E: Between 4.27 and 4.52, the number 4.50 is much closer to 4.52, so we'll put a dot close to 4.52.
F: Between 4.41 and 4.68, the number 4.50 is about halfway between, so we'll put a dot halfway between.
G: Between 4.49 and 4.55, the number 4.50 is much closer to 4.49, so we'll put a dot close to 4.49.
H: Between 3.95 and 4.55, the number 4.50 is much closer to 4.55, so we'll put a dot closer to 4.55.
I: Between 4.23 and 4.74, the number 4.50 is about halfway in between, so we'll put a dot halfway in between.
J. Between 4.45 and 4.87, the number 4.50 is much closer to 4.45, so we'll put a dot closer to 4.45.
K. Between 4.46 and 4.65, the number 4.50 is closer to 4,45, so we'll put a dot closer to 4.46.
L: Between 4.44 and 4.58, the number 4.50 is halfway between, so we'll put a dot halfway between.
M: Between 4.39 and 4.55, the number 4.50 is closer to 4.55, so we'll put a dot closer to 4.55.

We can now connect these dots, and thus we have a 4.5 kilometer or 4500 meter contour.

Again, another way to look at is that we've drawn a line between depths greater than 4.5 and depths less than 4.5, as shown below.

Having done the 5.0 and 4.5 contours, you would do the same for 3.5, 3.0, 2.5, etc. You'll probably find you don't need to always make the dots and connect them - you can just look ahead as you bring your pencil along and see where you should go next. It's not obligatory to color the bathymetric intervals as I have - in fact, it's probably better not to do so.

Finally, note that our 4500 meter contour wraps around, or mimics, the 5000 contour. That's typical of topographic and bathymetric maps. As you work on your map and come to difficult areas, one good rule is to see if you can make a contour like the adjacent contour that you just made - the previous contour is commonly a good clue as to the shape of the next contour!

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