Isopleth Mapping

By Syfujjaman

Isopleth mapping is a cartographic technique used to represent spatial variations of a particular phenomenon or variable across a geographic area. The term "isopleth" is derived from the Greek words "isos," meaning equal, and "plethos," meaning quantity. Isopleth maps are also commonly known as contour maps, contour plots, or isoline maps. It is also a distribution thematic map and known as a quantitating aerial map.

Isopleth maps use lines (isopleths or isolines) to connect points of equal value within a given dataset. These lines help visualise the distribution and patterns of a specific variable, such as temperature, rainfall, elevation, or population density, over a geographical area.

Figure: Example of Isopleth Map

Types of Isopleth Maps:

1. Contour Lines: Representing lines of equal elevation or topographic features.
2. Isotherms: Lines connecting points of equal temperature.
3. Isohyets: Lines connecting points of equal precipitation.
4. Isobars: Lines of equal atmospheric pressure, and many more.

Principle:

• Drawing lines through the points of equal values. Where the  exact value is not present, interpolation is used to determine the value.
• Intensity of colour or shading is proportional to the values of the zones.
• Multiple isolines are drawn to represent the spatial variation.
• Lines of equal value are drawn such that all values on one side are higher than the "isoline" value and all values on the other side are lower.
• Ranges of similar value are filled with similar colours or patterns.

Characteristics:

1. Point based data: Isopleth methods are applicable for point based data. Sometimes data is available at different locations which are represented as points on the map.
2. Equal Value Lines: Isopleth maps consist of lines connecting points with the same value of the variable being mapped. These lines are drawn on the map where the points are located. Isolines are drawn at certain intervals to portray the spatial distribution and variation of the concerned variable.
3. Interpolation: Isopleth maps often involve interpolation between data points, estimating values for locations where data is not available for drawing isolines. Common interpolation methods include linear interpolation, inverse distance weighting, and kriging. These methods help smooth out the transitions between isolines and provide a more continuous representation of the variable across the entire map.
4. Gradual Changes: Isopleths usually represent gradual changes in the variable being mapped rather than abrupt shifts. It means the changes should be continuous through sequential drawing of the isolines.
5. Smooth Transition: Isolines generally form smooth, continuous curves or contours, allowing for easier interpretation of the spatial patterns.
6. More details of spatial variation: Isolines can be drawn on a map with points, either sparsely distributed or densely distributed. More the number of points will be, more detailed spatial variations can be represented through it.

Construction:

The construction of an isopleth map involves the following steps:

1. Data Collection: Gather data on the variable of interest from various locations across the study area.
2. Determination of points: Collected location based data must be plotted on exactly the same location on the base map as points.
3. Selection of Contour Interval: Determine the isoline interval, which is the difference in values between successive isolines. The contour interval is a crucial aspect of constructing isopleth maps. A smaller interval provides more detail but may result in a crowded map, making it visually complex. On the other hand, a larger interval may oversimplify the representation, potentially missing subtle variations. The choice of interval depends on the nature of the data, range and the map's purpose.
4. Drawing Isolines: Connect points with the same value using smooth, continuous lines. In order to draw isolines between two successive points, interpolation methods are adopted where exact isoline values are not present.
5. Colour gradation or shading: Assign colour gradient or shade to regions between successive isolines, keeping in mind the chosen colour gradient or shades. Darker colours typically represent higher values and vice versa.
6. Map Creation: Finalising the map by labelling it. Labelling includes, title, legend, scale of the map, boundary and the coordinate. This can be done manually or using GIS (Geographic Information System) software or other mapping tools.

Figure: Map with point data

Figure: Selected Isolines are drawn

Figure: Colour Gradation

Application:

Isopleth maps find applications in various fields, including:

1. Topography: Illustrating elevation, slope, and terrain features.
2. Geology: Mapping geological features along with topography and for representing topography contour lines of equal elevation are drawn.
3. Meteorology: Representing weather variables like temperature, precipitation, and atmospheric pressure etc.
4. Oceanography: Isohalines are drawn to represent salinity.
5. Demography: Showing population density and distribution.
6. Environment: Isopleth mapping is extensively used in environmental science to represent various phenomena such as air and water quality, soil properties, and ecological patterns. For instance, it can be employed to map concentrations of pollutants, identify regions with similar climate characteristics, or illustrate the distribution of plant and animal species.

Merits:

1. Visual Representation: Isopleth maps provide a visual representation of spatial patterns, making it easier to identify trends and variations.
2. Interpolation: They allow for the interpolation of values between data points, providing a more comprehensive view of the entire area even where values are not present.
3. Comparisons: Isopleth maps facilitate comparisons between different geographic regions.
4. Effective Communication: They are effective in communicating complex spatial data to a diverse audience.
5. Pattern Recognition: Isopleth maps facilitate the identification of spatial patterns and trends.

Demerits:

1. Subjectivity: The choice of interval can be subjective, influencing the perception and interpretation of the spatial patterns.
2. Data Quality: Isopleth maps heavily depend on the quality and distribution of input data. Sparse or inaccurate data can lead to misleading representations.
3. Interpolation Errors: Errors may arise during the interpolation process, especially in areas with sparse data.
4. Scale sensitivity: Scale sensitivity in isopleth mapping refers to how the representation of spatial patterns and the appearance of isolines can be influenced by changes in the scale or size of the map. Isopleth maps can exhibit varying characteristics and visual interpretations when viewed at different scales with changing intervals.
5. Sensitivity to Outliers: Isopleth maps can be sensitive to outliers or extreme values, affecting the overall representation.

Isopleth mapping is a valuable technique for visualising spatial variations in data. Its effectiveness depends on careful consideration of intervals, data quality, and the specific characteristics of the variable being mapped.