Understanding Topographic Maps

Transcription

1 Understanding Topographic Maps 1. Every point on a contour line represents the exact same elevation (remember the glass inserted into the mountain). As a result of this every contour line must eventually close on itself to form an irregular circle (in other words, the line created by the intersection of the glass with the mountain cannot simply disappear on the backside of the mountain). Contour lines on the edge of a map do not appear to close on themselves because they run into the edge of the map, but if you got the adjacent map you would find that, eventually, the contour will close on itself. 2. Contour lines can never cross one another. Each line represents a separate elevation, and you can t have two different elevations at the same point. The only exception to this rule is if you have an overhanging cliff or cave where, if you drilled a hole straight down from the upper surface, you would intersect the earth s surface at two elevations at the same X,Y coordinate. In this relatively rare case, the contour line representing the lower elevation is dashed. The only time two contour lines may merge is if there is a vertical cliff. 3. Moving from one contour line to another always indicates a change in elevation. To determine if it is a positive (uphill) or negative (downhill) change you must look at the index contours on either side (see figure). 4. On a hill with a consistent slope, there are always four intermediate contours for every index contour. If there are more than four index contours it means that there has been a change of slope and one or more contour line has been duplicated. This is most common when going over the top of a hill or across a valley. 5. The closer contour lines are to one another, the steeper the slope is in the real world. If the contour lines are evenly spaced it is a constant slope, if they are not evenly spaced the slope changes. A series of closed contours (the contours make a circle) represents a hill. If the closed contours are hatchured it indicates a closed depression. 6. Contour lines crossing a stream valley will form a "V" shape pointing in the uphill (and upstream) direction.

2 Understanding Topographic Maps

3 Questions about Math State Park topo. Determine the elevations of points 1-5: 1) ft., 2) ft., 3) ft., 4) ft., 5) ft.

4 How do I calculate slope/gradient? "Rise over run" in the geosciences Many of us know that the slope of a line is calculated by "rise over run". However, the application of slope calculation can seem a little more complicated. In the geosciences, you may be asked to calculate the slope of a hill or to determine rate by calculating the slope of a line on a graph. This page is designed to help you learn these skills so that you can use them in your geoscience courses. Why should I calculate slope or gradient? In the geosciences slope can play an important role in a number of problems. The slope of a hill can help to determine the amount of erosion likely during a rainstorm. The gradient of the water table can help us to understand whether (and how much) contamination might affect a local well or water source. How do I calculate slope (or gradient) in the geosciences? Gradient in the case of hillslope and water table is just like calculating the slope of a line on a graph - "rise" over "run". But how do you do that using a contour (or topographic) map?! 1) Measure the elevation change using the contour lines. This is the rise.! 2) Determine the distance with ruler or string and map scale. This is the run.! 3) Divide the rise by the run. This is the gradient. The units are usually ft/mi or m/km. Use the enlarged version of the map on the next page to determine the gradient of the red line from the creek to the top of the hill feet per mile.

5 More Questions About the Main Map. 1. What is the height of the picnic table at the end of road (black line)? 2. Is the creek flowing east, west, north or south below that picnic table (at the location of the word creek ), and how do you know for sure which way it is flowing? 3. Use a string to follow the stream and determine how long it is from where it enters the map and where it leaves. What is this distance?!!! What is the elevation change (rise) over this distance?!!! What is the gradient of the stream?!!! Don t forget the units for these answers.