Isolines
Definition
Isolines are continuous lines drawn on a map that connect locations of equal value for a given variable. Familiar forms include contours for elevation, isotherms for temperature, isobars for pressure, isochrones for travel time, and isohyets for precipitation totals. In GIS, isolines are usually derived from interpolated rasters using specified intervals and smoothing rules to enhance readability. Choice of interval controls visual density and interpretability; too coarse hides nuance, too fine causes clutter. The topology of isolines carries information: nested shapes, saddles, closed loops, and breaks can reveal ridges, basins, fronts, or barriers. Label placement, index lines, and subtle hypsometric tints or hachures improve legibility. As analytical objects, isolines support surface profiling, watershed delineation, and threshold mapping such as “areas above 2 meters of flood depth.”
Application
Cartographers use isolines to build topographic maps. Transport planners generate isochrones to show the reach of a site under different travel modes or departure times. Meteorologists broadcast isobars and isotherms to communicate fronts and heat waves. Public-health teams map contours of walking access to clinics. In hazard work, isolines provide simple boundaries for evacuation triggers and design levels.
FAQ
What’s the difference between isolines and isopleths?
Isolines connect equal values from point-based or continuous surfaces. Isopleths are similar but conceptually represent averaged values over areas (like population density). In practice, the terms are often used interchangeably, but noting provenance avoids confusion.
How do you choose contour intervals?
Base intervals on data range, map scale, and intended use. Provide index contours every fifth line, and avoid awkward values that make mental arithmetic hard. For dynamic maps, allow users to change intervals to explore detail.
Do isolines mislead on flat or noisy data?
They can. Over-smoothing suggests nonexistent structure; under-smoothing produces spaghetti. Inspect residuals, and suppress contours where gradients are below a visibility threshold to avoid implying precision.
Can isolines be used legally for zoning thresholds?
Yes, but attach metadata on sources, interpolation, and date. Provide buffers for positional uncertainty and align updates with regulatory cycles.
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