ISOPLETH MAPS


An isopleth map generalizes and simplifies data with a continuous distribution. It shows the data as a third dimension on a map, thus isopleth maps are more common for mapping surface elevations, amounts of precipitation, atmospheric pressure, and numerous other measurements that can be viewed statistically as a third dimension.

The third dimension is shown by a series of lines called isopleths which connect points of equal value. The isopleth interval is the difference in value between two adjacent isopleths. Note, the values of the isopleths drawn on the map are ALWAYS multiples of the interval. Isopleths never cross or divide and always form enclosed circles, however, this occurrence may not be in the mapped area.

To read the value of a point on an isopleth map, answer the following questions.

What is the isopleth interval?

Where is the point?

Is the point on an isopleth? If so, what is the value of that isopleth?

If it is NOT on an isopleth, what are the values of the TWO isopleths it is between? (You must always give both values!)

NOTE: If the point is NOT on an isopleth, the map only gives a range for the value, but an approximate value can be derived by assuming an even rate of change between the two isopleths and interpolating this APPROXIMATE VALUE. For simplicity in GY 214, Dr. Hill will NEVER examine you on the approximate values.

Practice

Answer the following questions using MAP A below.

1. What is the value of point A?
2. What is the value of point B?
3. What is the value of point C?
4. What is the value of point D?
5. May the exact value of point E be determined from the map?
6. What is the value of point E?
7. What is the approximate value of point E?
8. What is the value of point F?
9. What is the approximate value of point F?
10. What is the value of point G?
11. What is the value of point H?
12. What is the value of point I? (Note: What two isopleths would it be between?)
13. What is the value of point J?
14. What is the value of point K? (Be careful!)
15. How do the values of point H and point K compare?
isopleth maps

Determine the value of points A through Z on MAP B. Be sure that your answers match exactly

Determine the value of points A through W on MAP C.

Answers: 1. 50; 2. 70; 3. 70; 4. 60; 5. no; 6. greater than 40 but less than 50; 7. approximately 46; 8. greater than 60 but less than 70; 9. approximately 65; 10. greater than 40 but less than 50; 11. greater than 50 but less than 60; 12. greater than 80 but less than 90; 13. greater than 30 but less than 40; 14. greater than 50 but less than 60; 15. the same. MAP B--A 70; B 60; C 50; D 50; E 90; F 80; G 80; H 90; I greater than 50 but less than 60; J 80; K greater than 80 but less than 90; L greater than 60 but less than 70; M greater than 50 but less than 60; N greater than 90 but less than 100; O greater than 50 but less than 60; P greater than 70 but less than 80; Q greater than 70 but less than 80; R greater than 70 but less than 80; S greater than 70 but less than 80; T greater than 70 but less than 80; U greater than 30 but less than 40; V greater than 90 but less than 100; W greater than 100 but less than 110; X greater than 70 but less than 80; Y greater than 70 but less than 80; Z greater than 60 but less than 70. MAP C--A greater than 230 but less than 240; B 230; C 230; D greater than 230 but less than 240; E 220; F greater than 200 but less than 210; G greater than 220 but less than 230; H greater than 230 but less than 240; I 240; J greater than 250 but less than 260; K greater than 230 but less than 240; L greater than 260 but less than 270; M 250; N 230; O 220; P greater than 210 but less than 220; Q greater than 220 but less than 230; R greater than 230 but less than 240; S greater than 240 but less than 250; T greater than 240 but less than 250; U 240; V greater than 240 but less than 250; W greater than 230 but less than 240.


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