Isarithmic Maps

As with many things we see from day to day without technical knowledge of, isarithmic maps are fairly commonplace, and yet their creation and technical details are many.  Weather maps, showing temperature or barometric pressure across a region with differently colored bands, are isarithmic maps.  The prefix "iso-" means "same," and, as such, isarithmic maps display distribution of equal values with lines, like contours on a topographic map depicting differing elevations.

The 2 maps here use the same data, annual precipitation of Washington state, but employ different methods of classification and display.  The first map uses hypsometric tint, which shades the areas between contour lines (lines representing equal values) with different colors according to their values, and the value ranges shown in the legend.  The second map, below the first, uses continuous tones, and has an overlay of the actual contour lines.  Continuous tone symbology shades each point on the surface with a color according to its value.  Thus the main difference between the maps is the top simply shades the areas between the isolines (lines with equal values) with different colors, and the bottom considers, and displays with a corresponding color, each point's value.  Both methods rely on spatial correlation- or the idea that values closer together in space have more in common than values further apart, and both use interpolation to calculate the values between exact places where measurements are taken.  (The exact method of interpolation for this particular data is discussed in more detail in the paragraph in the right corner of each map.)

Creating 2 maps using the same data, but employing different methods of actual mapping, or display, is an excellent way to fully grasp a concept.  The maps above are visually similar, but are clearly not exact copies, and I was able to gain intimate knowledge of their differences through this exercise.  Yet another step taken towards becoming an able and competent GIS professional.