A contour line is a curve that joins points of equal value. In cartography, contour lines join the points of equal elevation above a given standard level like the mean sea level. A contour map is a map to illustrate the topographical features of an area using contour lines. It is often used to show the heights, slopes and depths of valleys and hills. The space between two consecutive contour lines in a contour map is called as the contour interval which shows the difference in the elevation.
The best feature of using contour lines in mapping is that it can represent the 3-dimensional surface of the any terrain in a 2-dimensional contour map. By interpreting the Contour map the user is able to infer the relative gradient of the surface. Be it the depth or the height of a landscape, a contour map can help to represent the topography of the area.
Contour lines are curved, straight or a mix of both lines which do not cross each other in a map. The space between consecutive contour line determines the gradient of the surface that is being represented and is termed as the “interval”. If the contour lines are spaced very closely then they represent a steep slope and if they are spaced far apart then it represents a gentle slope. Streams and rivers in a valley are represented by a “v” or a “u” in a contour map.
Contour lines are often typified with the prefix “iso” which means “equal’ in Greek, as per the type of variable being mapped. The prefix “iso” can be replaced with “isoallo” which specifies that the contour line joins points where a given variable change at the same rate over a given period of time. Although the term contour line is commonly used, specific names are often used in meteorology where there is a greater possibility of viewing maps with different variables at a given time.
History of contour lines
The use of lines joining points of equal value has been existent since a long time although they were known by names other than contour lines. The first recorded use of contour lines were made to illustrate the depth of the river Spaarne located near Haarlem by a Dutchman named Pieter Bruinsz in the year 1584. Contour lines denoting constant depth are now known as “isobaths”. Throughout the 1700s contour lines have been used in numerous charts and maps to illustrate depths and heights of water bodies and landscapes.
Edmond Halley in 1701 used contour lines on a chart of magnetic variation. Nicholas Cruquius used isobaths at an equal interval of 1 fathom to draw the bed of the river Merwede in the year 1727, while Philippe Buache used an interval of 10 fathoms for the English Channel in the year 1737. In 1746 contour lines were used to map land surface by Domenico Vandelli who drew a map of the Duchy of Modena and Reggio. In 1774 conducted the Schiehallion experiment to measure the mean density of the Earth. The concept of contour lines was used in surveying the mountainside for the experiment. Thereafter the use of contour lines for cartography became a standard method. This method was used in 1791 by J.L Dupain-Treil for a map of France and in 1801 Haxo used it for his projects in Rocca d’Aufo. Since then there has been a widespread use of contour lines for mapping and other applications.
In 1889 Francis Galton proposed the term “isogram” as a reference for lines indicating equality or similarity in qualitative or quantitative features. The terms “isogon”, “isoline” and “isarithm” are commonly used to describe contour lines. The term “isoclines” refers to a contour line that joins points with equal slope.
Types and Applications of contour lines
Contour lines have been used extensively in maps and representation of graphical and statistical data. Contour lines can be drawn as plan view or as a profile view. Plan view allows the representation of the map in a way an observer would see it from above. The profile view is often a parameter that is mapped vertically. For e.g. terrains of a location can be mapped as a plan view while the air or noise pollution in the area can be represented as a profile view.
Contour lines are used in the various fields of study to represent a set of data over a region. However, the terms that is used to denote the contour lines may vary with the variation in the data type that is represented.
1. Ecology: Isopleth is used for contour lines that represent a variable which cannot be measured at a point but is a derivative of a data that is collected over a larger area for example population density.
Similarly, in ecology Isoflor, an isopleth is used to connect regions with similar biological diversity showing distribution patterns and trends of a species.
2. Environmental Science: There are varied applications of contour lines in environmental science. Pollution density maps are useful to indicate areas of higher and lower contamination levels allowing the possibility of scaling pollution in the area.
Isoplats are used to indicate acid precipitation in a map while isobels are used to indicate levels of noise pollution in the area.
The concept of contour lines have been used in contour planting and contour ploughing which is known to reduce soil erosion by a great extent in areas along river banks or other water bodies.
3. Social sciences: Contour lines are frequently used in social sciences to demonstrate variations or to display a comparative study of a variable over a particular area. The name of the contour line varies with the type of information that it represents. For example in Economics contour lines are used to describe features that may vary over an area, like an isodapane represents cost of travel time, isotim refers to the cost of transport from the source of raw materials, isocost curve represents equal production from alternative usage and Isoquant represents equal quantity of production from alternative input usages.
4. Statistics: In statistical studies contour lines are used to join the points with the same value of probability density, these are called as isodensity lines or as isodensanes.
5. Meteorology: Contour lines have a significant usage in meteorology. Data received from weather stations and weather satellites help in making the meteorological contour maps showing weather conditions like precipitation, air pressure over a period of time. Isotherms and isobars are used in multiple overlapping contour sets to present various thermodynamic factors affecting the weather conditions.
Temperature study: Contour line that connects points in a map with equal temperatures is called as an isotherm and those connecting areas with equal solar radiation is called as isohel. A contour line connecting areas with equal mean annual temperature is called as isogeotherms and that connecting areas with equal mean winter temperature is called as isocheim while that connecting equal mean summer temperature is called as isothere.
Study of wind: In meteorology, a contour line joining points with constant wind speed is called as isotach. An isogon refers to a constant wind direction.
Rainfall and humidity: Various terms are used to refer to the contour lines that join areas with similar precipitation and humidity content.
- Isohyet or isohyetal line refers to equal rainfall regions in a map.
- Isochalaz joins points in a map representing the areas receiving constant frequency of hailstorms.
- Isobront joins points in map representing areas which experienced thunderstorm activity simultaneously.
- Isoneph indicates equal cloud cover.
- Isohume refers to the contour line joining areas with constant relative humidity.
- Isodrostherm refers to constant or equal dew point areas.
- Isopectic contour line denotes regions with identical or similar dates of ice formation while isotac refers to the thawing dates.
Barometric pressure: In meteorology, the study of atmospheric pressure is important to predict future weather patterns. The barometric pressure is reduced to sea level when represented in a map. An isobar is a contour line that joins the regions with constant atmospheric pressure. Isoallobars joins points in the map with equal pressure change over a specific period of time. Isoallobars in turn can be divided into the ketoallobars and the anallobars which represents the decrease and increase in the pressure change respectively.
6. Thermodynamics and Engineering: Although these fields of study rarely involves a map contour lines finds usage in graphical representation of data and phase diagrams some of the common types of contour lines used in these fields of study are:
- Isochor represents constant value of volume
- Isoclines are used in differential equation
- Isodose refers to absorption of equal dose of radiation
- Isophote is constant illuminance that is received.
7. Magnetism: Contour lines are extremely helpful in studying the magnetic field of the earth. It helps in the study of the magnetic dip and magnetic declination.
The isogon or isogonic contour lines represents the line of constant magnetic declination. The contour line that joins the points of zero magnetic declination is called as Agonic line. A contour line that joins all the points with constant magnetic force is called as isodynamic line. An isoclinic line joins all regions with equal magnetic dip while an aclinic line joins all regions with zero magnetic dip. An isoporic line joins all the points with constant annual variation of magnetic declination.
8. Geographical studies: The most common usage of the contour lines is in the representation of elevation and depth of an area. These contour lines are frequently used in the topographic maps to show elevation and bathymetric charts to show depths. These topographic or bathymetric maps can be used to represent a smaller area or can be used to represent larger areas like a continent. The space between consecutive contour lines called as the interval refers to the difference in the elevation or depth between the two points. The interval is usually noted in the map key.
While representing a terrain with contour lines, close contours represent a steep slope or gradient while distant contours represent a shallow slope. The closed loops on the inside represent uphill while the outside shows downhill. The innermost loop on a contour map shows the highest area however if the map is that of a depression and not an elevation then short lines called “hachures” radiate from the inside of the loop.
9. Geology and Oceanography: Contour maps are used in the study of structural geology and physical and economic features of the earth’s surface. Isopach are contour lines that join points with equal thickness of geologic units.
Similarly in Oceanography areas of equal water density is represented by contour lines called as isopycnals and isohalines join areas with equal ocean salinity. Isobathytherms joins points with equal temperatures in an ocean.
10. Electrostatics: Electrostatics in space is often depicted with the isopotential map. The curve joining the points with constant electric potential is called as he isopotential or the equipotential line.
Features of contour lines in contour maps
Contour maps not only allow a representation of the elevation or depths of the terrains, the features of the contour lines that are drawn in the map but also allows a greater understanding of the terrains that is being mapped. Here are some features that are frequently used in mapping:
- Line type can be dotted, solid or dashed. A dotted or dashed line is frequently used when there is data in the base map that could be covered by a solid line.
- Lineweight refers to how bold or thick the contour line has been drawn. Contour maps are often drawn with lines of varying thickness to show different numerical values or variation in terrain elevations
- LineColor of the Contour line color is varied in a map to distinguish it from the base map. Line color is also used to represent different set of numerical values as well.
- Numerical marking is very important on all contour maps. It can be done alongside the contour line or it can be shown in a map key. The numerical value helps in identifying the direction of the gradient.