An isoline map visualizes continuous spatial change by connecting points with the same value. Contour lines, isotherms, and isobars are familiar examples. Unlike choropleth maps, which color administrative areas, isoline maps emphasize the continuity of the phenomenon itself.
Historical Background
The idea of isolines is old. Edmond Halley produced an early scientific isoline map in 1701 showing magnetic declination in the Atlantic. In the eighteenth and nineteenth centuries, contour lines and isotherms became important tools in topography, meteorology, oceanography, and geology.
With GIS, isolines can now be generated from observation points using interpolation methods such as kriging, inverse distance weighting, and splines.
Data Structure
| Data | Role |
|---|---|
| Observation points | Locations with measured values |
| Values | Temperature, elevation, pressure, travel time, etc. |
| Interpolation method | Generates a continuous surface |
| Contour interval | Determines which values are drawn as lines |
Purpose
The purpose is to reveal gradients and spatial structure. Isolines show how values change across space, where peaks and valleys occur, and how smoothly a phenomenon varies.
Use Cases
- Elevation contours
- Weather maps
- Sea-level or ocean temperature maps
- Air pressure and wind analysis
- Travel-time accessibility maps
Design Notes
- Choose contour intervals carefully.
- Label lines when exact values matter.
- Avoid drawing too many lines.
- Combine with color only when it improves clarity.
- Note the interpolation method when values are estimated.
Summary
Isoline maps are effective for continuous phenomena. They help readers see gradients and patterns, but the result depends strongly on interpolation and contour interval choices.