How to Read the Depth & Height of Bathymetric Contours

Bathymetric charts document the surface of the ocean floor. These types of maps show the topography of the seabed just like a topographic map shows the surface of the Earth above water. Bathymetric maps provide accurate depth information to seagoing vessels, geologists, oil riggers and scientific agencies. Learning how to read the depth and height of bathymetric contours requires a basic knowledge of how topography is depicted on the flat surface of a map. Let's look at some ways to analyze depths and heights of bathymetric contours.

Difficulty: Moderate

Instructions

Things You'll Need:

• Bathymetric chart
• Ruler

1. Prepare yourself by obtaining a rudimentary understanding of map contouring. Bathymetric maps simply document the surface of the sea floor in reverse of land features. Flip the land upside down and you can visualize upside-down mountains being the equivalent of deep-sea depths.

2. Locate your colored depth guide near the legend of the chart. This guide depicts the colored gradations applied to each contour to differentiate one depth from another.

3. Be aware that there are both depths and heights under water. Depths would be deep-sea trenches and canyons. Heights would be sea mounts or mid-ocean ridges.

4. Determine the value of sea level and the units of measure used on the chart (feet or meters). Sea level datum measures exact elevation based on specific vertical surveying points and applies these fixed positions to obtain the most accurate underwater surveying measurements possible. Some charts require the application of datum shifts to account for time since the survey and magnetic variations.

5. Pick a point on the chart and search for the nearest number that breaks the contour line. This number indicates depth from mean sea level. All points along this continual line are equal depths. Use your ruler to measure the width between lines of equal contour. Compare this measurement against the scale on the chart (located near the legend) to get a grasp of how large underwater features can be.

6. Evaluate the seafloor structures of the map. Contours that bunch together indicate an incline and contours spaced apart denote a gentle slope.

7. Note that bathymetric contours indicate depth with notations of units of measure. These units of measure break the contour line in the form of a textual notation. If a bathymetric contour contains tick marks pointing toward the center of a circular-shaped contour, this indicates an indentation or hole in the seafloor surface.

References

• Intergovernmental Committee on Surveying and Mapping