MARINE GEOLOGY (GEOL 5533) - Fall 2001

LABORATORY 3 - OCEAN & EARTH GEOMETRY

Due 15 October 2001

You have had some experience downloading data from publicly available databases on the Internet. In our first lab of the semester, you were asked to download data from the ETOPO5 gridded topographic data from the National Geophysical Data Center. To complete this week's lab, follow the instructions below and answer the questions that are presented.

1. What is the ETOPO5 database? How was it compiled? What is the areal distribution of data points within the database? When you download data from the coordinates below, what are those data?

All of the bathymetric/topographic profiles that we have plotted so far used degrees of latitude or longitude for the horizontal axis. However, you are all aware that the linear distance represented by 1-degree of longitude varies with latitude, being largest at the equator and smallest (0) at both poles. Thus, though we have been plotting profiles using degrees, these profiles represent very different linear distances across Earth's surface. This week, we will learn to convert profile data from degrees to absolute linear distances based on the variation of longitudinal distance north or south of the equator.

Before you can proceed with this exercise, you will need to determine a few facts. First, we will need to know the linear distance represented by 1-degree of longitude at the equator. There are a number of ways to determine this value. Let's assume that Earth has an equatorial radius of 6378.14 km.

2. Calculate the circumference of Earth.

3. Determine the linear distance represented by 1-degree of this circumference.

Here is another way to make this determination. By definition, 1 nautical mile is equal to 1-minute of latitude. In practice, this equates to 6,080 feet.

4. How many kilometers are in one nautical mile?

5. How many kilometers are in one minute of latitude?

6. From the above, how many kilometers are in one degree of latitude?

7. How many kilometers are in one degree of longitude at the equator?

8. How well does this value correspond to that you calculated in #3 above?

Now that we have established the linear distance represented by one degree of longitude at the equator, it remains to determine how linear distance changes as we proceed north or south of the equator. To a first approximation, this distance varies as the cosine of latitude. Precisely,

linear distance (km) per degree of longitude = 111.12 x cos q where q is latitude in degrees.

Note that since cos (0) = 1, at the equator, this equation resolves to 111.12 x cos (0) = 111.12 x 1 = 111.12 km.

Whereas at the poles, cos (90) = 0, and thus 111.12 x cos (90) = 111.12 x 0 = 0 km.

9. How many kilometers per degree of longitude at 45 N?

10. How many kilometers per degree of longitude at 60 S?

For this lab, recalculate the distance of each of the profiles below in kilometers and replot them at the same scale to examine their true spatial characteristics.

11. Are there any noticeable differences among profiles when you plot depth against distance versus degrees latitude or longitude?