Making a Bathymetric Map
Class Handout

Background:

Hydrographic surveys combine topographic surveying with remote sensing. Hydrographic surveyors cannot see the bottom that they are mapping; instead they measure the depth of the water over the bottom. Using many points, they can interpret the features on the bottom, by connecting the lines of equal depth.

Before the advent of sonar, measurements were made using probes or lead lines. In this activity, you will measure the depth to a simulated ocean bottom, record your measurements, plot the points, and create a map.

Instructions:
Recording the data:

1. Your teacher will provide you with the ocean box and the probe. Assume that the probe is calibrated in 1-foot increments; that is, each heavy line is equal to 1 foot. The smaller lines between the 1-foot increments are fractions of a foot.

2. On a separate piece of paper, make a table to keep track of your measurements. What pieces of information will you need to record?

3. Insert the probe until it hits the bottom. Record the depth from the top surface to the ocean bottom at each of the grid holes.

Making a bathymetric map:

1. Use a piece of graph paper. Recreate the grid of measurements, recording the measured depth at each point.

2. Look at the results. Do you see a pattern to any to the measurements? Pick out areas that look as if they will be related. These areas may represent a specific feature on the bottom; for instance, a hill, a depression, or a rock outcropping.

3. Draw lines to connect areas of similar depth for each related group of measurements. Draw lines to connect all the areas (see example).

Discussion:

1. Share your maps with the other students in the room.

2. Do they all look the same? Have some students interpreted the data differently?

3. Take the lid off of the box. Did you miss any features? Why or why not?