Projection

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A cartographic projection maps longitude and latitude pairs to x, y coordinates. As with Vega, one can use projections in Vega-lite to layout both geographic points (such as locations on a map) represented by longitude and latitude coordinates, or to project geographic regions (such as countries and states) represented using the GeoJSON format. Projection’s are specified at the unit specification level, alongside encoding. Geographic coordinate data can then be mapped to longitude and latitude channels (and longitude2 and latitude2 for ranged marks).

For example, this example chart shows all airports in the United States by projecting latitude, longitude as x, y coordinates using the albersUsa projection.

See the example gallery for more examples with geographic projection.

Projection Properties

Property Type Description
type ProjectionType

The cartographic projection to use. This value is case-insensitive, for example "albers" and "Albers" indicate the same projection type. You can find all valid projection types in the documentation.

Default value: mercator

clipAngle Number

Sets the projection’s clipping circle radius to the specified angle in degrees. If null, switches to antimeridian cutting rather than small-circle clipping.

clipExtent Number[]

Sets the projection’s viewport clip extent to the specified bounds in pixels. The extent bounds are specified as an array [[x0, y0], [x1, y1]], where x0 is the left-side of the viewport, y0 is the top, x1 is the right and y1 is the bottom. If null, no viewport clipping is performed.

center Number[]

Sets the projection’s center to the specified center, a two-element array of longitude and latitude in degrees.

Default value: [0, 0]

rotate Number[]

Sets the projection’s three-axis rotation to the specified angles, which must be a two- or three-element array of numbers [lambda, phi, gamma] specifying the rotation angles in degrees about each spherical axis. (These correspond to yaw, pitch and roll.)

Default value: [0, 0, 0]

precision String

Sets the threshold for the projection’s adaptive resampling to the specified value in pixels. This value corresponds to the Douglas–Peucker distance. If precision is not specified, returns the projection’s current resampling precision which defaults to √0.5 ≅ 0.70710….

If you want to explore the various available properties in more depth, Vega’s projection documentation hosts a useful demo

In addition to the shared properties above, the following properties are supported for specific projection types in the d3-geo-projection library: coefficient, distance, fraction, lobes, parallel, radius, ratio, spacing, tilt.

Note: All properties of projections are optional with defaults as defined in the Vega projection properties. Because of this, marks that don’t have explicitly defined projections may implicitly derive a projection. Implicit projections will be added for any geoshape mark, any encoding with field of geojson type, and encoding with latitude or longitude channels.

Projection Types

Vega-lite includes all cartographic projections provided by the d3-geo library.

Type Description
albers The Albers’ equal-area conic projection. This is a U.S.-centric configuration of "conicEqualArea".
albersUsa A U.S.-centric composite with projections for the lower 48 states, Hawaii, and Alaska (scaled to 0.35 times the true relative area).
azimuthalEqualArea The azimuthal equal-area projection.
azimuthalEquidistanct The azimuthal equidistant projection.
conicConformal The conic conformal projection. The parallels default to [30°, 30°] resulting in flat top.
conicEqualArea The Albers’ equal-area conic projection.
conicEquidistant The conic equidistant projection.
equirectangular The equirectangular (plate carrée) projection, akin to use longitude, latitude directly.
gnomonic The gnomonic projection.
mercator The spherical Mercator projection. Uses a default clipExtent such that the world is projected to a square, clipped to approximately ±85° latitude.
orthographic The orthographic projection.
stereographic The stereographic projection.
transverseMercator The transverse spherical Mercator projection. Uses a default clipExtent such that the world is projected to a square, clipped to approximately ±85° latitude.

Projection Configuration

// Top-level View Specification
{
  ...,
  "config": {          // Configuration Object
    "projection": { ... },   // - Projection Configuration
    ...
  }
}

The projection property of the config object determines the default properties and transformations applied to different types of projections. The projection config can contain any of the projection properties as specified above.