Elevation

Maps such as your Jacksonville quadrangle can provide information on many different subjects. Probably their most important function is to reveal the topography, that is, the shape of the land. Topography is shown by contours, lines composed of points with the same elevation. Elevation refers to the vertical height of a point above sea level. Sea level is used as a reference point for elevations because it is more or less the same all over the world.

Every point on the earth's surface has an elevation. However, knowing the elevation of only one point is not enough to describe an area's topography. For example, let's say that your house has an elevation of 250 meters above sea level. Does that tell you if your house is on a hill or valley? No! You need to know the elevations of many points in relation to one another to really describe the landscape. Let's say that your house has an elevation of 250 meters, but that it is surrounded by many other points all with elevations of 100 meters. Does this tell us the topography? Yes, sort of. Your house seems to be on a hill, surrounded by a lower lands, but this still doesn't give us a complete picture.

To describe the topography of an area, we must do two things. First, we must ascertain the elevations of many surrounding points. Secondly, we must relate these points to one another.

With the help of modern high-tech equipment, it is very easy to find the elevation of any point on the earth's surface. All you need is a G.P.S. (Global Positioning System), a tiny electronic instrument you can easily carry in one hand. To find the elevation of a point, such as your house, all you need to do is carry a G.P.S. into your yard, push a button, and the G.P.S. instantly displays how far you are above sea level. It does this by electronically communicating with satellites orbiting hundreds of miles overhead.

To describe the topography of the area around your house, you need to find the elevations of a lot of nearby points. To do this, you would take your G.P.S. and walk around the neighborhood, pushing the button every few minutes. As each elevation was displayed, it could be recorded at the correct location on a map. This map might be very confusing, just a bunch of numbers scattered around. It's really hard to get any idea what the topography looks like from such a random jumble of numbers.

To make sense of all this, we use contour lines. Contours describe the shape of the land by showing how individual points of elevation relate to each other.

Contour lines connect points of equal elevations. To make a contour map, we need to draw lines that "connect the dots" of equal elevations. First, we need to decide which elevations to connect. The points have elevations of 10 meters, 15 meters, 20 meters, and 25 meters above sea level. This being the case, it would be convenient to designate four contour lines, one with a value of 10 meters, one of 15, one of 20 and and one of 25 meters. Contour lines must always be equally spaced, in this case they are 5 meters apart. This spacing is called the "contour interval". A map may have any contour interval depending on what is convenient. The contour interval tells two things: (1) the difference in value between two adjacent lines, and (2) the value of the lines to be shown on the map. The value of the lines shown on the map are always MULTIPLES of the interval.

Now, let's draw some contours through our jumble of points. Let's start with the 15 meter line. To help draw the line in the right place, you need to compare the elevations of nearby pairs of points. In the top right between the 10 and 20 meter elevations no 15 is indicated, yet, we know that as we increase from 10 to 20, there must be a 15 in there somewhere. We assume an even rate of change. This allows us to interpolate (make an educated guess) a 15 meter point somewhere between the 10 and 20. Since 15 is halfway between 10 and 20, we draw the point halfway in between. If we were interpolating an 18 meter point, we would place it closer to the 20. If interpolating a 12 meter point, closer to the 10 meter point. Starting with the 15 meter point on the right side of the map, connect it with a single curving line to all the other 15 meter points, even points not indicated. The result is shown in diagram below. (Note, the contour lines should be solid lines rather than broken or dotted lines. Reducing the size of these images resulted in loss of the continuity of the lines.)

We can finish by adding the 10, 20, and 25 meter contours to get the map shown below.

We can easily check the contours we have drawn to make sure that they are right. Remember, each line represents an elevation. All other points on the map must be either higher or lower than that elevation. The lower points will all be on one side of the line, the higher points on the other. Instead of a mixed-up, meaningless jumble of elevations we now have an easy to read contour map. By looking at the shape of these contours and reading the elevations of the contour lines, we can see that the house sits at an elevation of about 25 meters, on top of a hilltop.

Keep in mind these basic rules about contour lines:

1. Contour lines can never cross. A contour is a line with a single specific elevation. If two different lines cross, that indicates that the intersection has two different elevations. This can't be.

2. A contour line must always eventually meet itself. If the ends of the line do not meet, they must run off the edge of the map. A contour line just can't stop in the middle of a map.