Isolines and Contour Lines

Isolines

• To map particular characteristics of an area, such as elevation, the amount of rainfall, or the temperature isolines are often used.

• An isoline is a line on a map that connects points of equal value.

Isolines

• For example contour lines on topographic maps are isolines that show elevation.

• When we study weather and climate, we will use several kinds of isolines, such as isotherms, to show temperatures, and isobars to show atmospheric pressure

Basic Rules of isolines

1. An isoline connects points on a map where the value of some phenomenon is the same.

2. Isolines are drawn at regular intervals.• For example every 5° of temperature difference.

3. Isolines are always closed lines, although the often close beyond the margins of a map.

4. Isoline NEVER cross each other.5. When isolines are close together, they show a rapid

horizontal change in the phenomenon; where they are far apart, the show a gradual horizontal change.

6. Values inside a closed isoline are either higher or lower than those outside the close isoline

• It is usually clear which is the case based on the pattern of adjacent isolines.

Figure One

• This drawing will help illustrate how isotherms are drawn.

• Figure One shows a simple map with temperatures plotted for 17 different cities.

http://www.ux1.eiu.edu/~cfjps/1400/chapter3.html

• We will draw isotherms at 5° intervals (15°, 20°, 25°, etc.) for problems part two.

• An isotherm will pass through any point with the same value as the isotherm, but between higher and lower values.

• On one side of the line, the temperatures will be higher than the value of the isotherm, while on the other side, temperatures will be lower.

Drawing Isolines

• Drawing isolines involves interpolation (estimating values between two known values).– For example, the 15° isotherm passes between the

14° and 16° locations, while the 27° location is about half way between the 25° and 30° isotherms.

• Figure two shows the completed isotherm map. – Notice that isotherms show the spatial pattern of

temperature more clearly than the temperatures of the cities alone.

http://www.middleschoolscience.com/isotherms.htm

Isolines: More Information

• Isolines are a graphical tool used to denote geographic lines of equal value.

• Isolines may also be thought of as contour lines showing increasing/decreasing trends of a value of interest i.e. rainfall, elevation, temperature.

• Any points falling on the same isoline will have the same value associated with that isoline.

• Isolines may be used to depict many values which may be of geographic importance.

• Typically isolines of a certain value will be named using the iso- prefix followed by the name of the feature being illustrated.

• Some of the more important types are listed on the following slide.

http://www.clas.ufl.edu/users/kastofan/isolinenotes.htm

Types of Isolines

• Isoheights – lines of equal elevation or topographic contours.– Isoheights are lines of equal elevation.

• Typically found on topographic maps like USGS topo quadrangles.• Isoheights are useful for observing elevation variations and profiles.

• Isobars – lines of equal atmospheric pressure (weather maps).

• Isotherms – lines of equal temperature.• Isobaths – lines of equal depth or bathymetry.• Isohaline – lines of equal salinity.• Isopycnals – lines of equal rainfall.• Isotachs – lines of equal wind speed.

http://www.clas.ufl.edu/users/kastofan/isolinenotes.htm

Isoline Trends

• Spatial trends may be interpreted through isolines.• Increases/decreases in the values of interest may be

determined from isolines.• The relative increase/decrease of the values of interest

may also be determined.• The value difference between any two consecutive

isolines is the contour interval.• The relative increase/decrease of a certain value may

also be determined.• Tight spacing between isolines depict relatively sharp

increases/decreases in values.• Wide spacing between isolines depict relatively small

increases/decreases in values.

http://www.clas.ufl.edu/users/kastofan/isolinenotes.htm

Contour Lines

• Isotherms and Isobars are used to show equal lines of temperature and pressure. Studying landforms involves another kind of isoline, contour lines.

• Contour lines are lines that connect points of equal elevation.

• Contour lines enable us to study the topography of a region from a two-dimensional map.– Figure One show a simple contour line map and a

profile cross section through the landscape.

http://raider.muc.edu/~mcnaugma/Topographic%20Maps/contour.htm

Simple Contour Line Map and Profile

Figure Two

• Figure Two shows a fictitious landscape and a contour line map of the same landscape with various elevations and features labeled.

Contour Line Rules1. A contour line connects points of equal elevation.2. The difference in elevation between two contour lines is known as the contour

interval. 3. Usually every fifth contour line is a darker index contour. 4. Elevations on one side of a contour line are higher than on the other side.5. Contour lines never cross one another although they may touch at a vertical cliff.6. Contour lines have no beginning or end, every line closes on itself either on or off

the map.7. Uniformly spaced contours indicate a uniform slope.8. If spaced far apart, contour lines indicate a gentle slope. If spaced closed

together, they represent a steep slope.9. When crossing a valley or gully, a contour line makes a “v” pointing uphill.10. When crossing a spur or a ridge running down the side of a hill, a contour line

makes a “v” pointing downhill.11. A contour line that closes within the limits of the map represents a hill or rise. The

land within the closed contour is higher than the land outside the closed contour.12. The top of a hill shown with closed contour lines is higher than the uppermost

closed contour, but lower than the next highest contour that hasn’t been shown on the map.

13. A small depression is represented by a closed contour line that is hachured on the side leading into the depression. Hachured contours are called depression contours.