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What are Isolines

What is an isoline in geography?

Isolines, also known as contour lines, are lines on a map, chart, or graph that connect points of equal value. The term “iso” comes from the Greek word meaning “equal.” The specific name for an isoline depends on the variable being mapped.

For instance:

  1. Isobars are lines on a weather map connecting points of the same atmospheric pressure.
  2. Isotherms are lines on a map or chart depicting equal temperatures.
  3. Isohyets are lines on a map showing areas of equal or constant rainfall.
  4. Isoseismals are lines on a map showing areas of equal intensity of earthquake seismic activity.
  5. Isobaths are lines on a chart of a body of water showing areas of equal depth.
  6. Isodose lines represent areas of equal radiation dose in radiation treatment planning.

These lines are useful in fields such as geography, meteorology, hydrology, geophysics, oceanography, and many more. By providing a visual way to represent variations in a given parameter across a geographic area, they help in identifying patterns, gradients, and relationships between geographical locations and the variable being measured.

How is an isoline different or the same as a contour in geography

In geography, the terms “isoline” and “contour line” are often used interchangeably, but they do have specific differences based on what they represent.

Isoline

An isoline is a general term used for a line on a map or chart that connects points of equal value. This “equal value” could be any measurable quantity, such as temperature, pressure, rainfall, radiation dose, etc. The specific type of isoline is usually named based on what it’s measuring – for example, isobars for pressure, isotherms for temperature, and so on.

Contour Line

A contour line or contour, on the other hand, is a specific type of isoline that connects points of equal elevation. This is most commonly seen on topographic maps, where contour lines provide a way to visually represent the 3D shape of the landscape in a 2D format. The ‘contour interval’ is the vertical height between contour lines.

Therefore, we can say that all contour lines are isolines, but not all isolines are contour lines. Contour lines are a specific type of isoline that deal with elevation. The term “isoline” is more general and can refer to lines representing a variety of different measurable quantities.

What is an Isoline Map?

An isoline map is a type of map that represents data values using lines of constant value, known as isolines or contour lines. These lines connect points on a map with equal values of a particular data set, such as elevation, temperature, or atmospheric pressure.

Isoline maps are often used in geography, meteorology, and geology to visualize topography, weather patterns, and geological features. They are also used in other fields, such as engineering and economics, to represent data on maps.

To create an isoline map, data values are first collected and organized into a grid or a series of points on a map. Then, isolines are drawn to connect points with the same data value, forming a network of lines that highlight areas with similar values.

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How to make an isoline map in QGIS

If you are working with raster data follow these steps

  1. Import your data: The first step is to import your data into QGIS. You can do this by clicking on the “Layer” menu and selecting “Add Layer” > “Add Raster Layer”
  2. Create a contour layer: Click on the “Raster” menu, select “Extraction,” and then “Contour.” In the “Contour” dialog box, select the input raster layer, specify the contour interval or the number of contour levels, and choose an output vector layer. Click “OK” to create the contour layer.
  3. Customize the contour layer: You can customize the appearance of the contour layer by opening the “Layer Properties” dialog box and selecting the “Symbology” tab. Here you can choose the color, line width, and transparency of the contour lines.
  4. Add a base map: To add a base map, click on the “Web” menu and select “OpenLayers plugin.” In the “OpenLayers Plugin” dialog box, select a base map provider and click “OK.”
  5. Save and export the map: Once you have customized the map to your liking, you can save and export it as a PDF, PNG, or another file format by clicking on “Project” > “Save” or “Project” > “Export Map.”

If you are working with point data follow these steps

If you have a point dataset and want to create an isoline map, you will need to interpolate the data to create a continuous surface that can be used to generate the contour lines. Here are the steps to create an isoline map from a point dataset in QGIS:

  1. Import your point data: The first step is to import your point data into QGIS. You can do this by clicking on the “Layer” menu and selecting “Add Layer” > “Add Delimited Text Layer.” In the “Add Delimited Text Layer” dialog box, browse to your data file and select the appropriate options for your data format.
  2. Interpolate the data: Click on the “Raster” menu, select “Interpolation,” and then “Interpolation.” In the “Interpolation” dialog box, select your point layer as the input layer, choose a method for interpolation (e.g., IDW, Kriging, or Spline), and specify the output raster layer.
  3. Create a contour layer: Click on the “Raster” menu, select “Extraction,” and then “Contour.” In the “Contour” dialog box, select the input raster layer (the one you just created in step 2), specify the contour interval or the number of contour levels, and choose an output vector layer. Click “OK” to create the contour layer.
  4. Customize the contour layer: You can customize the appearance of the contour layer by opening the “Layer Properties” dialog box and selecting the “Symbology” tab. Here you can choose the color, line width, and transparency of the contour lines.
  5. Add a base map: To add a base map, click on the “Web” menu and select “OpenLayers plugin.” In the “OpenLayers Plugin” dialog box, select a base map provider and click “OK.”
  6. Save and export the map: Once you have customized the map to your liking, you can save and export it as a PDF, PNG, or another file format by clicking on “Project” > “Save” or “Project” > “Export Map.”

These are the basic steps to create an isoline map from a point dataset in QGIS. The key difference is that you need to interpolate the data first to create a continuous surface that can be used to generate the contour lines.

The advantages of an isoline map include:

  1. Easy to visualize patterns: Isoline maps are excellent for visualizing patterns in a data set. By connecting points with equal values, isolines highlight areas of similar data values and allow viewers to see the distribution and trends in the data.
  2. Useful for spatial analysis: Isoline maps are particularly useful for spatial analysis, such as identifying areas of high or low elevation, temperature, or atmospheric pressure. They can also be used to analyze changes over time, such as the spread of a disease or the growth of a city.
  3. Simplifies complex data: Isoline maps can simplify complex data sets by reducing the information to a series of lines. This makes it easier to understand the data and communicate it to others.
  4. Applicable in many fields: Isoline maps are widely used in many fields, including geography, meteorology, geology, engineering, and economics. This means that the skills and knowledge gained from creating and interpreting isoline maps are transferable across different disciplines.
  5. Compatible with different data types: Isoline maps can be used to represent different types of data, such as elevation, temperature, atmospheric pressure, population density, and more. This versatility makes them a valuable tool for a wide range of applications.

The disadvantages of an isoline map:

  1. Interpolation limitations: Isoline maps are based on interpolation, which can result in inaccuracies and inconsistencies in the data. The accuracy of the isolines depends on the density and distribution of the data points, as well as the interpolation method used.
  2. Subjectivity in contour interval selection: The selection of the contour interval can be subjective and may influence the interpretation of the data. Different contour intervals can result in different patterns, so it is important to choose an appropriate interval based on the data being represented.
  3. Difficulty in interpretation: Isoline maps can be difficult to interpret for individuals who are unfamiliar with the data being represented or the methods used to create the map. The complexity of the data may require additional explanation or contextual information to fully understand the patterns and trends in the map.
  4. Limited ability to represent discrete data: Isoline maps are best suited for continuous data sets, where the values change smoothly across space. They may not be appropriate for representing discrete data or data with abrupt changes, such as political boundaries or land use classifications.
  5. Limited to two-dimensional representation: Isoline maps are typically represented in two dimensions, which can limit the ability to fully represent the complexity of the data in three-dimensional space. This can result in distortions or misrepresentations of the data.

Choosing the appropriate contour interval for an isoline map?

Choosing the appropriate contour interval for your isoline map can be challenging because it requires balancing the level of detail you want to show with the clarity of the map. Here are some steps to follow:

  1. Determine the range of values in your data: Look at the range of values in your data set. Determine the minimum and maximum values and calculate the range of the data.
  2. Consider the purpose of the map: Think about the purpose of the map and the information you want to communicate. If you want to show fine detail, choose a small contour interval. If you want to show larger trends or general patterns, choose a larger contour interval.
  3. Use a standardized interval: Consider using a standardized interval, such as 10%, 20%, or 25% of the data range. This can help ensure consistency across maps and make it easier for others to compare maps.
  4. Experiment with different intervals: Try using different contour intervals and compare the resulting maps. Look for a balance between detail and clarity. You may need to adjust the interval several times before finding the best one.
  5. Consider the audience: Finally, consider the audience for your map. Choose an interval that is appropriate for their level of understanding and familiarity with the data. If the audience is not familiar with the data, it may be best to use a larger contour interval to simplify the map.

Common interpolation methods used to create isoline maps

There are several interpolation methods used to create isoline maps, including:

  1. Inverse Distance Weighting (IDW): This method calculates the value of a point based on the values of surrounding points. The closer a point is to the target point, the more influence it has on the final value.
  2. Kriging: Kriging is a statistical method that models the spatial autocorrelation of the data. It calculates the values of unknown points based on the values of nearby points and their spatial relationships.
  3. Spline: This method fits a mathematical function to the data, creating a smooth surface. The interpolated values are based on the coefficients of the function.
  4. Natural Neighbor: This method calculates the value of a point based on the values of its nearest neighbors. The weighting of each neighbor is based on the Voronoi polygon around the target point.

The choice of interpolation method depends on the type of data, the distribution of the data points, and the desired level of accuracy. Some methods may work better for certain data types or patterns, while others may be more suitable for data with irregular distributions. It is important to consider the advantages and limitations of each method when choosing which one to use.

How to interpret an isoline map

Interpreting an isoline map involves understanding the patterns and trends in the data that the map is representing. Here are some steps to follow:

  1. Understand the data: First, understand the type of data being represented and what the isolines are showing. For example, if the map represents elevation, the isolines will show areas of equal elevation.
  2. Look at the spacing and direction of the isolines: The spacing and direction of the isolines can provide valuable information about the data. Areas with closely spaced isolines indicate steep changes in the data, while areas with widely spaced isolines indicate flatter or more uniform changes.
  3. Look for trends: Look for trends in the data, such as areas with consistently high or low values, or areas with abrupt changes in value. Trends can help identify patterns in the data that may be useful for analysis or decision-making.
  4. Look for clusters: Clusters of isolines can indicate areas with similar data values. These clusters may represent natural features, such as valleys or ridges, or human-made features, such as cities or roads.
  5. Compare multiple maps: If you have multiple isoline maps, compare them to identify changes over time or differences between areas. This can help identify areas of growth or decline, or areas that may require further investigation.
  6. Consider the limitations of the map: Finally, consider the limitations of the isoline map, such as the accuracy of the interpolation method used or the resolution of the data. These limitations can affect the interpretation of the map and should be taken into account when making decisions or drawing conclusions based on the data.

In conclusion

Isoline maps are a valuable tool for visualizing and analyzing spatial data. By representing data as lines of equal value, isoline maps can reveal patterns and trends that may not be immediately visible in other types of maps or data representations.

Creating and interpreting isoline maps requires careful consideration of factors such as interpolation methods, contour intervals, and data accuracy. However, with the right knowledge and tools, isoline maps can be an effective way to communicate complex data to a wide range of audiences.

Whether you are a geographer, meteorologist, economist, or engineer, isoline maps can help you gain insights into your data and make informed decisions based on spatial trends and patterns. So, start exploring the world of isoline maps and discover the power of visualizing data in a new and exciting way.

Frequently Ask Questions about Isolines

What is the purpose of isolines on a map?

Isolines help to represent a 3-dimensional quantity on a 2-dimensional surface (like a map or chart). They can provide a clear visual representation of variations in a specific parameter across a geographical area.

How are isolines drawn on a map?

Isolines are drawn by first measuring the parameter of interest at a variety of locations. Then, lines are drawn to connect points with the same value. These lines should never cross, as each line represents a specific, consistent value.

What do the different shapes and patterns of isolines signify?

The shape and pattern of isolines can show geographic and environmental features, like mountains, valleys, or atmospheric fronts. Closely spaced isolines often indicate steep gradients, while widely spaced lines indicate gentle changes.

What’s the difference between isobars, isotherms, and contour lines?

Topographic maps commonly use contour lines to represent changes in elevation. By following the lines, you can see the changes in elevation. Closer lines mean steeper slopes, while further apart lines indicate more gentle slopes.

How do I interpret a topographic map with isolines?

Topographic maps commonly use contour lines to represent changes in elevation. By following the lines, you can see the changes in elevation. Closer lines mean steeper slopes, while further apart lines indicate more gentle slopes.

Can there be two isolines for the same value in the same area?

Typically, isolines do not cross each other as they represent the same value of a specific variable. However, in certain situations, you might have closed loops representing a specific value, for example in areas of high or low pressure in meteorological charts or at the peak of a hill or the bottom of a valley in topographic maps.

About the Author
I'm Daniel O'Donohue, the voice and creator behind The MapScaping Podcast ( A podcast for the geospatial community ). With a professional background as a geospatial specialist, I've spent years harnessing the power of spatial to unravel the complexities of our world, one layer at a time.