Rasters

Rasters are geographic data displayed in a grid. They are similar to image files such as PNG files, except that instead of each point containing visual information, each point contains geographic information in numerical form. Rasters can be thought of as a georeferenced table of numerical values.

One example of a raster is a Digital Elevation Model (DEM) layer, which has elevation data encoded numerically at each georeferenced data point.

Example raster

The raster layer that is used in the examples below contains elevation data for a fictional world. The data is stored in EPSG:4326 (longitude/latitude) and has a data range from 70 to 256. If rendered in grayscale, where minimum values are colored black and maximum values are colored white, the raster would look like this:

../../../_images/raster3.png

Raster file as rendered in grayscale

Download the raster shapefile

Two-color gradient

This example shows a two-color style with green at lower elevations and brown at higher elevations.

../../../_images/raster_twocolorgradient2.png

Two-color gradient

Code

Download the "Two-color gradient" YSLD

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title: 'YSLD Cook Book: Two color gradient'
feature-styles:
- name: name
  rules:
  - symbolizers:
    - raster:
        opacity: 1.0
        color-map:
          type: ramp
          entries:
          - ['#008000',1,70,'']
          - ['#663333',1,256,'']

Details

There is one rule in one feature style for this example, which is the simplest possible situation. All subsequent examples will share this characteristic. Styling of rasters is done via the raster symbolizer (lines 2-7).

This example creates a smooth gradient between two colors corresponding to two elevation values. The gradient is created via the color-map on lines 8-12. Each entry in the color-map represents one entry or anchor in the gradient. Line 11 sets the lower value of 70 and color to a dark green ('#008000'). Line 12 sets the upper value of 256 and color to a dark brown ('#663333'). Line 9 sets the type to ramp, which means that all data values in between these two quantities will be linearly interpolated: a value of 163 (the midpoint between 70 and 256) will be colored as the midpoint between the two colors (in this case approximately '#335717', a muddy green).

Transparent gradient

This example creates the same two-color gradient as in the Two-color gradient as in the example above but makes the entire layer mostly transparent by setting a 30% opacity.

../../../_images/raster_transparentgradient2.png

Transparent gradient

Code

Download the "Transparent gradient" YSLD

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title: 'YSLD Cook Book: Transparent gradient'
feature-styles:
- name: name
  rules:
  - symbolizers:
    - raster:
        opacity: 0.3
        color-map:
          type: ramp
          entries:
          - ['#008000',1,70,'']
          - ['#663333',1,256,'']

Details

This example is similar to the Two-color gradient example save for the addition of line 7, which sets the opacity of the layer to 0.3 (or 30% opaque). An opacity value of 1 means that the shape is drawn 100% opaque, while an opacity value of 0 means that the shape is rendered as completely transparent. The value of 0.3 means that the the raster partially takes on the color and style of whatever is drawn beneath it. Since the background is white in this example, the colors generated from the color-map look lighter, but were the raster imposed on a dark background the resulting colors would be darker.

Brightness and contrast

This example normalizes the color output and then increases the brightness by a factor of 2.

../../../_images/raster_brightnessandcontrast2.png

Brightness and contrast

Code

Download the "Brightness and contrast" YSLD

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title: 'YSLD Cook Book: Brightness and contrast'
feature-styles:
- name: name
  rules:
  - symbolizers:
    - raster:
        opacity: 1
        color-map:
          type: ramp
          entries:
          - ['#008000',1,70,'']
          - ['#663333',1,256,'']
        contrast-enhancement:
          mode: normalize
          gamma: 0.5

Details

This example is similar to the Two-color gradient, save for the addition of the contrast-enhancement parameter on lines 13-15. Line 14 normalizes the output by increasing the contrast to its maximum extent. Line 15 then adjusts the brightness by a factor of 0.5. Since values less than 1 make the output brighter, a value of 0.5 makes the output twice as bright.

As with previous examples, lines 8-12 determine the color-map, with line 11 setting the lower bound (70) to be colored dark green ('#008000') and line 12 setting the upper bound (256) to be colored dark brown ('#663333').

Three-color gradient

This example creates a three-color gradient in primary colors.

../../../_images/raster_threecolorgradient2.png

Three-color gradient

Code

Download the "Three-color gradient" YSLD

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title: 'YSLD Cook Book: Three color gradient'
feature-styles:
- name: name
  rules:
  - symbolizers:
    - raster:
        opacity: 1
        color-map:
          type: ramp
          entries:
          - ['#0000FF',1,150,'']
          - ['#FFFF00',1,200,'']
          - ['#FF0000',1,250,'']

Details

This example creates a three-color gradient based on a color-map with three entries on lines 8-13: line 11 specifies the lower bound (150) be styled in blue ('#0000FF'), line 12 specifies an intermediate point (200) be styled in yellow ('#FFFF00'), and line 13 specifies the upper bound (250) be styled in red ('#FF0000').

Since our data values run between 70 and 256, some data points are not accounted for in this style. Those values below the lowest entry in the color map (the range from 70 to 149) are styled the same color as the lower bound, in this case blue. Values above the upper bound in the color map (the range from 251 to 256) are styled the same color as the upper bound, in this case red.

Alpha channel

This example creates an “alpha channel” effect such that higher values are increasingly transparent.

../../../_images/raster_alphachannel2.png

Alpha channel

Code

Download the "Alpha channel" YSLD

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title: 'YSLD Cook Book: Alpha channel'
feature-styles:
- name: name
  rules:
  - symbolizers:
    - raster:
        opacity: 1
        color-map:
          type: ramp
          entries:
          - ['#008000',1,70,'']
          - ['#008000',0,256,'']

Details

An alpha channel is another way of referring to variable transparency. Much like how a gradient maps values to colors, each entry in a color-map can have a value for opacity (with the default being 1.0 or completely opaque).

In this example, there is a color-map with two entries: line 11 specifies the lower bound of 70 be colored dark green ('#008000'), while line 13 specifies the upper bound of 256 also be colored dark green but with an opacity value of 0. This means that values of 256 will be rendered at 0% opacity (entirely transparent). Just like the gradient color, the opacity is also linearly interpolated such that a value of 163 (the midpoint between 70 and 256) is rendered at 50% opacity.

Discrete colors

This example shows a gradient that is not linearly interpolated but instead has values mapped precisely to one of three specific colors.

../../../_images/raster_discretecolors2.png

Discrete colors

Code

Download the "Discrete colors" YSLD

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title: 'YSLD Cook Book: Discrete colors'
feature-styles:
- name: name
  rules:
  - symbolizers:
    - raster:
        opacity: 1
        color-map:
          type: intervals
          entries:
          - ['#008000',1,150,'']
          - ['#663333',1,256,'']

Details

Sometimes color bands in discrete steps are more appropriate than a color gradient. The type: intervals parameter added to the color-map on line 9 sets the display to output discrete colors instead of a gradient. The values in each entry correspond to the upper bound for the color band such that colors are mapped to values less than the value of one entry but greater than or equal to the next lower entry. For example, line 11 colors all values less than 150 to dark green ('#008000') and line 12 colors all values less than 256 but greater than or equal to 150 to dark brown ('#663333').

Many color gradient

This example shows a gradient interpolated across eight different colors.

../../../_images/raster_manycolorgradient2.png

Many color gradient

Code

Download the "Many color gradient" YSLD

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title: 'YSLD Cook Book: Many color gradient'
feature-styles:
- name: name
  rules:
  - symbolizers:
    - raster:
        opacity: 1
        color-map:
          type: ramp
          entries:
          - ['#000000',1,95,'']
          - ['#0000FF',1,110,'']
          - ['#00FF00',1,135,'']
          - ['#FF0000',1,160,'']
          - ['#FF00FF',1,185,'']
          - ['#FFFF00',1,210,'']
          - ['#00FFFF',1,235,'']
          - ['#FFFFFF',1,256,'']

Details

A color-map can include up to 255 entries. This example has eight entries (lines 11-18):

Entry number Value Color RGB code
1 95 Black '#000000'
2 110 Blue '#0000FF'
3 135 Green '#00FF00'
4 160 Red '#FF0000'
5 185 Purple '#FF00FF'
6 210 Yellow '#FFFF00'
7 235 Cyan '#00FFFF'
8 256 White '#FFFFFF'
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