Background:

Navigational charts show the depth of water at different points using bathymetry (ba-thim-et-tree) by showing depth contours in the same way that contours are used on topographic maps to show the relative height of land. In this way navigators can tell if water contains hazards to travel.

For Parks Victoria the bathymetry of a park is important because it gives the shape of the sea floor and identifies important features that are located in a Marine National Park such as reefs, canyons, or simply sandy plains. Bathymetry can be used predict where human activities are likely to be most frequent, such as where good SCUBA diving locations may occur, as well as helping to improve understanding of the influences of adjacent land activities on the marine environment.

Originally recorded using a “lead line” (a long length of string with a lead weight attached and various objects tied to the rope to indicate the depth), sailors took “soundings” by throwing the lead line over the side and recording the length of line needed for the weight to reach the bottom.

In the Marine Habitat Mapping project the depth of the water was obtained by using multibeam sonar that bounced many pulses of sound off the bottom which were collected and analysed by powerful computers on board. This provides enough information to accurately map the entire sea floor in sufficient detail to construct three dimensional views of the sea floor.

The information collected in the first of the marine mapping projects at Point Addis Marine National Park actually updated the some of the data previously collected by Flinders in the early 1800s.

You are provided with a comprehensive bathymetry image of Point Addis Marine National Park on which there are lines representing points of equal depth with areas of the same depth colour coded for easy interpretation.

1. Select two points A and B on a print out of the depth image and mark these.
   
2. Using the edge of a piece of paper make a line joining the two points. Mark the two points on the paper and add in further marks to show where each of the depth contours are crossed. Note also the names of the contour lines that are crossed.
   
3. Place the piece of paper along the bottom axis of a grid marked on graph paper and carefully transfer the marks onto this.
   
4. Place a vertical scale on the graph showing 1 cm = 10 m depth along the side of the grid with the surface (0m) at the top of the scale. Remember that depth contours are below the water surface and the greater the depth, the lower it will appear to be.
   
5. Finally mark the points on the graph that represent the depth and the corresponding distance across the Marine National Park to see the shape of the sea floor.