Topographic Maps. Contour Lines

Transcription

1 Topographic Maps Our first task today will consist of locating ourselves with the help of a topographic map. GPS units that can tell you the location of any Dunkin' Donuts within a five mile radius. Paper maps are still an important tool in geology. They are cheap, don't use any batteries, do not depend on any satellites and are a great way to represent the lay of the land and lots of other information in a convenient and efficient way. The United States Geological Survey (USGS) is the nation's prime maker and publisher of topographic maps. Topographic maps display the shape of the Earth's surface, its topography. All information printed on a map is a fairly straightforward and relatively intuitive way. The USGS has an excellent website about its maps, the symbols commonly used and a whole lot more. Contour Lines Contour lines are one way to represent topography on a map. They are the light brown lines that snake all over most topographic maps. Contour lines, or contours, represent lines of constant elevation as shown in the classic (and often copied) figure below. In order to interpret contour lines one has to know the elevation difference between two contours and which way goes up or down. The elevation difference between two contours is called the contour interval and it is usually printed somewhere on the map (most often at the bottom, near to the scale bar and other information). To figure out which way is up (or down) some (generally every fifth) contours are labeled in feet or meters above sea level. That can be the only way to figure out what is a mountain or a depression in the landscape. Often, however, it is pretty obvious what constitutes a hill or a valley. Hills have often closed contours, valleys or depressions are often filled with water or have streams running in them. Check out the example image below. The spacing of contours tells us something about the steepness of a slope. Tightly spaced contours indicate steep slopes, while gentle slopes are represented by widely spaced contours. The images below illustrate this point. Contours do not necessarily have to represent topography. You might have noticed them in your newspaper, where they might represent daily temperatures on the weather page or the concentration of pesticides in groundwater. If you have taken a few math classes you might have come across contours when studying functions. Math folks generally call them level curves.

2 Detail from USGS topographic map, showing Dead Horse Point State Park in Utah. (view from Dead Horse Point) Note the wide contour spacing on the top of the plateau, where the road and campsite are, and the steep canyon walls indicated by narrowly spaced contours.

3 Map Scale Another important thing we have to know when interpreting maps is its scale. Map scale refers to the ratio between the distance on the map to the distance on the Earth's surface. Map scales can be given in several ways. The map you are using for this exercise, for example, has a scale of 1: 24,000 which is sometimes called the representative fraction. It means that one unit on the map (and any unit: in, ft, cm, mi, km will work) represents 24,000 units on the ground. In this case the scale is "one to twenty four thousand". A different way of expressing scale is by stating it explicitly as in : 1 inch equals 1 mile, which would correspond to a scale of 1:63360 (I had to look that up). Finally, scale can also be represented by a scale bar as the one shown in the figure below. Most USGS maps have both a scale bar and the scale printed on the bottom of the map sheet. The image below also gives you information regarding the contour interval and which coordinate system (map datum) was used in constructing the map. Maps come in different scales and people talk about large scale and small scale maps. Large and small refer to the numerical value of the representative fraction. The value of the ratio1:5,000 is larger than 1:2.5 Million. Therefore a map printed at a scale of 1:5000 would have a larger scale than a map printed at a scale of 1:2.5 Million. The images below show examples of large and small scale maps covering the city of Chicago. Which one shows more detail, which one covers more ground?

4 Profiles and Cross-sections You've learned in part a) how contours are used to represent topography on maps. It is sometimes useful to construct a cross-section or topographic profile through a map in order to fully understand the landforms and rock formations associated with it. The following images will guide you through the process of constructing a topographical profile from a contour map. We start out with a 3D drawing of a hill (Fig. to right). It has contours drawn on top of it and the bottom of the figure shows an approximate (I drew this thing by hand). Also indicated on the figure is the trace of the cross-section we intend to construct. On the map view this cross-section is labeled A-B. A cross-section shows us what we could see if we sliced through the hill as indicated in the figure to the right. In geography we are mostly interested in the shape of the topography. In geology we will use this simple cross-section and add geological information as it is done in this example.

5 We start constructing our crosssection by taking a paper strip (a 8.5 x 11 sheet, folded lengthwise works great) and align it with the trace of our cross-section as shown in the figure to the right. Then we mark the contact of each contour map with the paper strip by a small tick mark. It is also helpful to label as many of these tick marks as possible and add the position of prominent features to your paper strip. This will help you later to figure out the position of your cross-section. Now that all the important information is recorded on your paper strip we can construct our cross-section. Draw a horizontal line. This is the baseline of your cross-section. Its scale is determined by the scale of the map (unless you enlarge or shrink your paper strip). The choice of a vertical scale is up to you. If your vertical relief is small you might have to choose some vertical exaggeration. Its up to you, but you should keep in mind that vertical because exaggeration can complicate the addition of geological features later angular measurements have to be adjusted to the change in vertical scale. Instructors often tease vertical exaggerators that they want to impress their friends with the steep mountains they climbed in geology class... We are almost done. Align your paper strip with the baseline of your cross-section and carefully plot the contour lines at the appropriate elevation as shown in the figure to the right. A profile of the hill should slowly emerge. Make sure you don't skip any of the ticks and keep your vertical elevations straight. Finally, connect the dots and you are done. There is one small, almost philosophical, aspect to be considered. In the figure to the right the dots are connected by straight lines. Sticklers will tell you that is all the information you recorded on your paper strip and that's the way to do it. However, you might have noticed that those edgy mountains are rarely observed in nature. So it is perfectly OK to

6 smooth out the corners a bit to make it look more realistic. Mt. St. Helens cross-section (from USGS)