*Note: From time to time old posts will be resurfaced on the blog. This one is from Sept. 2010. The watercolor illusion is similar to, but not exactly the same as, the vignette concept. A vignette would create a subtle illusion as well. A simple example would be this banding effect created in TileMill:

I was digging around the cartography literature yesterday and came across something called the watercolor illusion.*

The illusion is thus: if you have a dark line (the more squiggly the better) next to a light colored line, your eyes will fill in the missing white space with the lighter color, albeit in a lighter tone, and give a washed-out effect. In this example your eye may think that the outside rectangle is green but in fact it is white. It is just the light green fine line next to the purple that is causing that illusion:

Perhaps you all have some better examples of how this plays out on a map, but here is my attempt. You can see I am trying to create a major definition between the water and the land (these are geology polygons). The geology polygons are in dark purple and I’ve used a bright, light blue next to the outer edge by first creating a buffer of the polygon and then symbolizing the buffer:

There are other ways to produce a gradual fading of a line in your GIS and in your graphics programs, of course. In ArcGIS, for example,  you can use a gradient fill, though sometimes, especially with large datasets, this taxes the renderer quite a bit. However, this is a fairly easy and handy way to create the effect within the GIS and without exporting to graphics software.

*Pinna, Baingio and Gavino Mariotti, “Old Maps and the Watercolor Illusion: Cartography, Vision Science and Figure-Ground Segregation Principles,” Systemics of Emergence: Research and Development. 2006, 3, 261-278.

(1) Map Measles: Too many points at too low of a zoom level. Caused by lack of zoom-level specific coding and/or lack of simplification pre-processing. The solution is to pre-process the data to rid it of the least important points, or to on-the-fly select by attribute (e.g., “population > 500,000”), or on-the-fly binning (e.g., hexagonal binning). Rx: TileMill’s Styling For Zoom Levels tutorial, Hexbin discussion on indiemaps.

(2) Blotchy Skin: Jarring colors interrupt an otherwise flawless complexion. Caused by lack of pre-determined color palette. The solution is to choose a color palette prior to beginning the project and adjust it as needed. Rx: Color Brewer, Cartographer’s Toolkit, Colour Lovers, your own art collection.

(3) Abnormal Cell Walls: Differentiation of polygonal features via varying polygon line-widths and colors, cataclysmically colliding at shared boundaries. Caused by rapid output needs and resulting sloppiness. The solution is to use inner-buffering to provide a continuous color for each polygon while allowing for neighboring polygons to have a different color. Rx: view how it’s done, buffer with QGIS using negative distance.

(4) ADHD: No clear hierarchy of elements, features jumbled in an inappropriate order. Caused by misuse or ignorance of established standards. Also caused by human error and not acquiring critical feedback from outside sources prior to publication. The solution is to be aware of appropriate layer order (e.g., state borders under interstates, park shading under local roads). Rx: study Google Maps, seek critique.

(5) Over Accessorizing: Putting everything on the map that can be put on the map. Caused by GIS analysts, who have a multitude of information at their fingertips, wanting to squeeze every potential drop of wisdom possible out of the data by presenting it all at the same time, resulting in less information transfer rather than more. Also caused by meddling bosses. The solution is to drill down to only the salient details, to separate data into comprehensible segments, to present different maps in different tiles, or to put in a huge effort in getting the data massaged together (e.g., road atlas). Rx: study road atlases, NYTimes map graphics, MIT Senseable City Lab products, winning maps from various map competitions (e.g., NACIS 2012 narrative, first place GISCI 2012).