6.1 Visual Variables (Bertin's Semiology of Graphics) and Map Symbology

Cartography is the disciplined synthesis of art, science, and spatial communication. A GIS professional may conduct sophisticated geoprocessing and database modeling, but if the resulting visual output fails to communicate spatial patterns clearly and honestly, the analysis loses its utility. The foundation of modern cartographic symbolization traces back to French cartographer Jacques Bertin's 1967 landmark treatise, Sémiologie Graphique (Semiology of Graphics). Bertin established that visual marks on a map communicate through systematic graphic variations that human perception decodes according to predictable psychological rules.

Bertin's Visual Variables

Bertin identified seven primary visual variables (often expanded by modern cartographers to include saturation and arrangement) that alter graphic marks on a display:

1. Position: The geographic $(X, Y)$ coordinates of the mark on the map. Position is the most powerful visual variable; human vision detects differences in position more rapidly and accurately than any other graphic property.
2. Size: The physical dimensions or area occupied by a symbol (e.g., symbol diameter, line width, bar height). Size is uniquely suited to communicating quantitative, ordered, and ratio data.
3. Shape: The external geometric form of the mark (e.g., circle, square, triangle, star, cross). Shape communicates qualitative, nominal differences without implying order or hierarchy.
4. Value (Lightness / Tone): The relative lightness or darkness of a symbol, ranging from white to black through intermediate grays. Value conveys a powerful visual impression of order and magnitude; darker symbols are instinctively perceived as "more" or "greater density."
5. Color Hue: The spectral wavelength identity of a color (e.g., red, green, blue, yellow). Hue communicates categorical distinctions (e.g., soil types, political boundaries) and should never be used alone to represent ordered numerical quantities.
6. Orientation: The angular direction of an elongated mark (e.g., vertical, horizontal, diagonal, compass angle). Used effectively for directional flow, wind direction, or geological fault strikes.
7. Texture (Grain): The coarseness or frequency of a repeating graphic pattern (e.g., cross-hatching, stippling, dots). Texture can represent either categorical classes (coarse vs. fine hatching) or uncertainty.

Matching Visual Variables to Levels of Measurement

One of the most frequently tested concepts on the GISP exam is the strict matching of visual variables to Stanley Smith Stevens' four levels of measurement: Nominal, Ordinal, Interval, and Ratio.

Exam Trap: A common exam question presents a scenario where an analyst maps five categorical soil types (Nominal) using graduated circle sizes (Size). The question asks why this cartographic choice is invalid: Size communicates quantitative magnitude, falsely signaling to the map reader that one soil type has greater numerical value than another. Nominal data must be displayed using Shape or Hue.

Visual Hierarchy and Figure-Ground Contrast

Visual hierarchy is the deliberate graphic arrangement of map elements to establish a reading order. A well-designed map presents information in distinct perceptual planes:

1. The Figure (Thematic Focus): The primary subject of the map (e.g., proposed highway alignment, epidemiological disease clusters) must occupy the foreground plane. It should exhibit high contrast, saturated colors, bold line weights, or prominent labels.
2. The Ground (Basemap and Context): Reference layers (e.g., county boundaries, water bodies, relief shading, road grids) provide essential geographic context but must recede into the background plane using muted tones, lower contrast, thin lines, and desaturated colors.
3. Map Marginalia (Supporting Furniture): Title, legend, scale bar, and citations must support the map without competing with the thematic figure for visual dominance.

Achieving Strong Figure-Ground Separation

• Contrast: Place dark figures on light backgrounds, or bright symbols against dark basemaps.
• Drop Shadows and Feathers: Applying subtle drop shadows or vignette edges lifts the primary study area off the surrounding basemap.
• White Space (Inter-Element Spacing): Generous, balanced spacing around layout elements prevents visual collision and reduces cognitive fatigue.

Gestalt Principles of Perceptual Organization

Cartographers rely on Gestalt psychology principles to understand how human vision groups disparate graphic elements into coherent patterns:

• Proximity: Features positioned close to one another are automatically perceived as belonging to a common group.
• Similarity: Features sharing identical visual properties (same color hue, shape, or size) are perceived as related across the map extent.
• Continuity: The eye naturally follows smooth, continuous curves or lines rather than abrupt angular breaks (essential for tracing contour lines and stream networks).
• Closure: Human vision mentally completes incomplete geometric boundaries, allowing cartographers to use subtle dashed lines or vignettes without losing regional definition.