Glossary#
Map elements#
- Layer#
The main elements of a map are “layers”. Layers can be imagined like a an ordered stack of transparencies. Each layer typically represents a single file or web service that provides map data. Layers can provide raster or vector data.
- Raster#
A data Layer composed of a regular grid of pixels. Conceptually, this is similar to an “image”, but the data is often represented in measurement values, not color values.
- Vector#
A data Layer consisting of points, lines, and/or polygons.
- Feature#
In a vector layer, a single point, line, or polygon element. For example, a single earthquake (point), a migration path of an animal (line), or a protected conservation area(polygon).
- Attribute#
Additional data or measurement about a Feature in a vector layer or grid cell in a raster Layer.
For example, the temperature measured at a grid cell, earthquake magnitude on a point feature, the species of an animal on a line feature, or the area in square kilometers of a protected conservation area on a polygon feature.
Symbology#
- Symbology#
How data is visually encoded on the map.
- Symbology Rule#
A set of Symbolizers that define a single representation of a data layer. Symbology rules are ordered; a higher rule will appear “above” a lower rule when rendered. A Rule can have a pre-processor, for example Kernel Density Estimation (KDE), which is applied before its Symbolizers.
- Symbolizer#
A triple of data input, interpolation, and visual encoding. A Rule is composed of many Symbolizers.
- Visual encoding#
How data attributes are matched to visual characteristics like shape, color, or position (Cleveland and McGill[1], figure 1).
- Color map#
A gradient or set of colors used to visually encode data with color on the map.
Sometimes referred to as a “color ramp” or “color scale”.
Important: Color is a fraught visual encoding method. Is your colormap perceptually uniform? Is it usable by people with color vision deficiencies? How does it interact with the other elements on the map (for example, does your basemap include colors that interact poorly with your chosen color map for any form of human color vision)? Also see understanding docs for WCAG SC 1.4.1 “Use of Color”.
- Sequential color map#
A colormap which progresses between two colors. A sequential colormap is useful for representing an ordered scalar value like temperature or magnitude.
- Divergent color map#
A colormap which diverges from a central value to extremes at each end. For example, a white center, dark red on one end, and dark blue on the other end. A divergent colormap is useful for representing scalar values that diverge from a critical value such as gain or loss (divergence from 0), or anomaly (divergence from average).
- Cyclic color map#
A colormap which ends where it starts. A cyclic colormap is useful for representing cyclic values like phase or angle.
- Categorical color map#
A colormap with a small set of discrete colors, without transitions between them. A categorical colormap is useful for representing categorical data, like land surface classification.
Sometimes refered to as a “discrete” or “qualitative” color map, or a “color palette”.