LabFourF20Online

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Santiago Canyon College *

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101

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Geography

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May 5, 2024

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Name _________________________ Lab Four: Topographic Maps, Isolines and Contours Since maps display geographic data, this lab is designed to introduce you to an important method for displaying information on maps- isolines. Isolines are special lines used to connect points of equal value. For example, U.S.G.S. Topographic Maps use contour lines to connect points of equal elevation. Isolines are also used on weather maps to show temperatures (isotherms) and atmospheric pressure (isobars). Contour lines give insight into the topography (difference in relief) of an area without needing a three dimensional model. Materials: Color Pencils or Paint program Screen Capture Software Lab 4 Online Map Objectives: • Identify features on standard U.S. Geological Survey topographic maps. • Interpret & construct isolines • Determine elevation using contour interval • Assess gradient based on contour line spacing • Identify landforms based on contour line patterns Part 1: Interpreting and Constructing Isolines Basic characteristics of isolines: Isolines are closed lines having no ends. Note: The line may close outside the margins of the map. Isolines represent gradations in quantities so they very rarely touch or cross. Isolines are drawn at regular intervals (numeric difference between one isoline and the next). If more than one interval is used on the same map, a different line symbol will be used to display those lines having different intervals. How close together the isolines are drawn depends on the gradient (change in the interval). Isolines drawn close together indicate a rapid horizontal change in the interval (steep gradient). Whereas, isolines drawn far apart, indicate a gradual horizontal change in the interval (gentle gradient). Values inside a closed isoline are either higher or lower than those outside the closed isolines. Cartographers use isolines to make it easier to interpret what otherwise would appear as random data points. Look at Diagram A below; at first glance it seems like randomly place elevations points (expressed in feet). It isn’t until isolines are drawn that an elevation pattern emerges (Diagram B). Note that the elevation incresases as you travel from the right side of the map to the left side. The process by which isolines are constructed is called interpolation. The Interval displayed below is 100 feet. 1
Exercise 4.1: 1. The map below shows temperatures (F ⁰ ) across the United States. The goal of this exercise is to draw isotherms in 10 ⁰ increments . Step 1- Classify the Data Using colored pencils or the Paint program on your computer, lightly circle each data point according to the key below. 70 ⁰ - 79 ⁰ = Red 60 ⁰ - 69 ⁰ = Pink 50 ⁰ - 59 ⁰ = Orange 40 ⁰ - 49 ⁰ = Yellow 30 ⁰ - 39 ⁰ = Green 20 ⁰ - 29 ⁰ = Blue 10 ⁰ - 19 ⁰ = Purple Step 2- Draw isotherm lines in BLACK. Start by locating 70 ⁰ - draw a line representing the 70 ⁰ contour line. Values higher than 70 ⁰ should be on one side of the line and values lower than 70 ⁰ should be on the other side. Repeat this process every 10 ⁰ . I suggest using a pencil first or if using a Paint program, save before drawing each isotherm. Step 3- Color the Bands. Using the colors on the key, color the bands between the contour lines. Optional: If you want to recreate this map on the computer instead of printing it out, download the IsothermsColor.png from Canvas. 2
Part 2: Interpreting Contours The U.S. Geologic Survey is the federal government agency responsible for providing scientific information (i.e., biology, geography, geology and hydrology) about the United States. They are also responsible for providing geospatial information including the mapping of the United States. The U.S.G.S offers several maps series depicting the United States at various scales. Each series is named after its angular dimensions and is often referred to as a quadrangle map because the map depicts a rectangular portion of earth’s surface with each corner of the map representing the intersection of latitude and longitude. For example, 1 ° x 2 ° - Scale: 1:250,000 1” = 4 miles 30- Minute Series – 30’ x 30’Scale: 1:125,000 1” = 2 miles 15 Minute Series -- 15’ x 15’ Scale: 1:62,500 1” = 1 mile 7.5 Minute Series – 7.5’ x 7.5’ Scale: 1:24,000 1” = 2000 feet The most popular and widely used U.S.G.S. quadrangle maps are called topographic maps because they show the shape of the land (slope and relief) through the use of contour lines. While we’ve used quadrangle maps when working with map scale, this lab will focus on contour line interpretation. Throughout the course we will be using U.S.G.S Topographic Maps to interpret Earth’s surface and to take measurements on Earth’s Surface. Sometimes we will use a portion of a physical topographic maps and other times we will look at the topography on an online map. In either medium, topographic maps use contours (lines joining points of equal elevation) to portray the topography or shape of the land. In addition to adhering to the basic characteristics of isolines, here are some general rules to follow when working with contours: Contour lines are never cross since they connect points of equal elevation. Contour Interval - the vertical distance between two successive contours. The contour interval is usually noted at the bottom of a printed U.S.G.S. Topographic Maps. Index Contours – Index Contours are drawn as thicker and darker contour lines and are marked with the elevation. Usually every 5 th contour line is an Index Contour. Supplementary Contours - In flat regions, additional contours will sometimes be drawn. These are usually half the contour interval and are drawn as dotted lines. To determine the elevation of a point on a contour line, find the nearest Index Contour and increase or decrease the elevation by the Contour Interval. Remember, elevations on either side of a contour line will either be higher or lower in elevation. Look at the surrounding contour lines to get a feel for the direction of the slope. To determine the elevation of a point between contours lines, you must estimate the elevation. The spacing of the contour lines tells you about the shape of the land. o Equal spacing indicates uniform slope. o Close spacing indicates a steep slope o Wide spacing indicates a gentler slope. Closed unhachured contours enclose an isolated high area (hill, mountain or peak). o To estimate the elevation on a hill top or ridge top add half the Contour Interval to the elevation of the inner-most closed contour line representing the hill or ridge top. Closed hachured contours (called depression contours) indicate the presence of a closed depression. o To estimate the elevation at the bottom of the depression, subtract half the Contour Interval from the elevation of the inner-most closed, hachured depression contour. Blue lines drawn on U.S.G.S Topographic Maps represent streams. Contour lines crossing rivers or valleys always bend upstream or upvalley creating “V”s. In other words, the V is always points up towards the higher elevation. 3
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