ISOLINE MAPS

Geography 101 Lab                Name_______________________________

Purpose: Introduce students to one of the most common and useful types of maps used in studying the natural environment. When completed, the student should understand the basic meaning of isolines maps, how to construct them, and how to derive information from them.


Many surfaces in nature are represented with isolines. They provide a way to represent a 3-Dimensional surface on a 2-Dimensional map. An isoline is simply a generic term for any line on a map that connects points of equal value. Some examples of isolines are:

Name Connects Equal Values of
Contour Elevation
Isohyet Rainfall
Isobar Air Pressure
Isotach Wind Speed
Isohaline Salinity
Isoseismal Earthquake Intensity
Isoflor Number of Plant Species

Part 1: Basic Rules

Some basic rules regarding isoline maps are:

The map below is an isoline map showing isobars (in millibars) for the area Hawaiian Islands area. After reading the information above, answer the following questions:

  1. Isobars show equal values of what?
  2. Do any isobars touch or overlap?
  3. What is the pressure interval between isobars?
  4. What is the pressure at point A?
  5. What is the difference in pressure between B and A?
  6. Where is the highest pressure located?
  7. What direction will air want to flow, from B to C or from C to B?
  8. Estimate the pressure at point D.
  9. Estimate the pressure at point C.
  10. Name three other environmental variables not mentioned in this lab exercise than could be mapped using isolines.


Part 2: Drawing Isolines

Isoline maps show patterns in an easily recognizable way. Usually they are constructed from scattered observations that, by themselves, are difficult to interpret. The mapmaker (or the computer) must convert these isolated observations into a complete isoline map by interpolating between observed values and then connecting points of equal value.

Sound confusing? Well, consider the map to the left showing isohyets of annual rainfall over East Maui in millimeters.

Notice how the blue isohyets pass between observations of higher and lower value. The 2000 mm isohyet, for example, passes between the 2345 mm and 1799 mm stations just north of Hana and the 6000 mm isohyet passes between the 5920 mm and 6231 mm isohyets further inland.

To extend the isohyets into the bottom half of the map, you need to determine where they will pass with respect to the observed rainfall values. Look at the orange dashed lines. Notice that the 2000 and 3000 mm isohyets pass between the 3549 and 1791 mm stations. Determining how far from each station they pass is called interpolation.

11. Now you try. Click for a printable map. Try and complete the bottom half of the isohyet map to answer the questions below. To begin, figure out approximately where the 4000, 5000, and 6000 mm isohyets would pass between the 3549 and 6083 mm stations just inland of the orange lines I drew, then extend the isohyet lines. Once you have the technique, try and complete the entire bottom half of the map. Remember, each isoline must pass between observations with higher and lower values, never between two lower values or between two higher values.

 

QUESTIONS:

12. What is the approximate rainfall value at A?

14. What is the approximate rainfall value at C?

15. What is the approximate rainfall value at D?

16. What is wrong with line G?

17. What is wrong with line I?

Look at the final pattern of the map in which you completed the isolines:

18. Does rainfall tend to increase or decrease as you move inland (westward) from Hana?



Part 3: Gradient and Profile

Isolines show surfaces whose values differ with location. Is there is no change, there would be no isolines. Therefore, there is always a gradient on an isoline map, say from high to low pressure, high to low elevation, high to low concentration of a pollutant, high to low density of termites, and so on. The amount of the gradient is shown by the distance between isolines. Very close together means a steep gradient, far apart means a gradual gradient. This can be quite meaningful. In hurricanes, the isobars are very close together and so the air pressure gradient is said to be high, or steep. This drives strong winds. We will look at gradient and profile use the example of contour maps, also called topographic maps, which show surface elevation.

Upper images courtesy USGS

The image to the left show a perspective view of a river mouth at the head of a valley between a steep cliff and a smoother slope to the right. The rendition of this topography as an isoline (contour) map from an overhead perspective is shown directly below.

Notice that the contours (isolines) are very close together where they represent the steep cliff and further apart in the flatter river valley. The elevation gradient, then, is very high for the cliff area and very low for the river valley. In practical terms, a high elevation gradient means that a rock will roll down the hill very fast. A low elevation gradient means the rock may not move downhill at all, or at best, very slowly. This metaphor applies to many gradients in nature.

So: Gradient refers to how rapidly the value of the isolines varies with distance.

Profile refers to a cross-section, or horizontal view, of some part of the isoline map. For example, if I walked from A to B on the map, my path would look like the drawing at the bottom. First I walk up a gradual slope, then down a cliff, then across a flat river valley, then up slope that becomes increasingly steep..

Below is a topographic map of the area above Makawao, Maui showing isolines of elevation called contour lines. Using the information given in the explanation above, answer the questions and draw a profile. (Click for printable exercise sheet).

QUESTIONS:

19. What is the interval between contours in feet?
20. What (in feet) and where is the highest elevation point on the map?
21. What is the approximate elevation at point E?
22. What is the approximate elevation at point D?
23. What is the approximate elevation at point C?
24. From point C, which direction is downhill (NE, SE, NW, or SW)?
25. Which point lies on the highest elevation gradient, C, D, or E?
26. In your own words, what is meant by "highest elevation gradient?"

27. Which point lies on the lowest elevation gradient, C, D, or E?
28. In your own words, what is meant by "lowest elevation gradient?"

29. On the exercise sheet that you printed out (see link right above the map), draw an elevation profile along the line that connects point A and point B.

30. Topographic maps show many features besides just elevation contour lines. Look at the map of Makawao and name four other features that it shows the location of (these will not be isolines).

 

Part 4: Stereoscope Viewing

31. Use a stereoscope to view any of the photo pairs in the Stereo Atlas (Diamond Head is number 50). Were you able to see the 3-D effect? Was the terrain more or less exagerated than reality? Comment on what you see.