4.6. YSLD Workbook Conclusion¶

Navigate to the rest api endpoint for styles:

• view-source:http://localhost:8080/geoserver/rest/styles

Click on one of the styles (for example states.html)

Click on the link to the style contents

• view-source:http://localhost:8080/geoserver/rest/styles/states.sld

Change the URL with ?pretty=true for human readable XML.

• view-source:http://localhost:8080/geoserver/rest/styles/states.sld?pretty=true

Change the URL with yaml to convert to YSLD.

• view-source:http://localhost:8080/geoserver/rest/styles/states.yaml

The original SLD file is convert to YSLD:

name: states
title: Population in the United States
abstract: |-
  A sample filter that filters the United States into three
          categories of population, drawn in different colors
feature-styles:
- name: name
  rules:
  - name: Population < 2M
    title: Population < 2M
    filter: ${PERSONS < '2000000'}
    scale: [min, max]
    symbolizers:
    - polygon:
        fill-color: '#A6CEE3'
        fill-opacity: 0.7
  - name: Population 2M-4M
    title: Population 2M-4M
    filter: ${PERSONS BETWEEN '2000000' AND '4000000'}
    scale: [min, max]
    symbolizers:
    - polygon:
        fill-color: F78B4
        fill-opacity: 0.7
  - name: '> 4M'
    title: Population > 4M
    filter: ${PERSONS > '4000000'}
    scale: [min, max]
    symbolizers:
    - polygon:
        fill-color: '#B2DF8A'
        fill-opacity: 0.7
  - name: State Outlines
    title: State Outlines
    scale: [min, max]
    symbolizers:
    - line:
        stroke-color: '#8CADBF'
        stroke-width: 0.1
  - name: State Abbreviations
    title: State Abbreviations
    scale: ['1.75E7', '3.5E7']
    symbolizers:
    - text:
        label: ${STATE_ABBR}
        font-family: SansSerif
        font-size: 12
        font-style: Normal
        font-weight: normal
        placement: point
        anchor: [0.5, 0.5]
  - name: State Names
    title: State Names
    scale: [min, '1.75E7']
    symbolizers:
    - text:
        label: ${STATE_NAME}
        font-family: SansSerif
        font-size: 12
        font-style: Normal
        font-weight: normal
        placement: point
        anchor: [0.5, 0.5]
        x-maxDisplacement: 100
        x-goodnessOfFit: 0.9

Challenge: Create a new style adjust road appearance based on type.

Hint: The available values are ‘Major Highway’,’Secondary Highway’,’Road’ and ‘Unknown’.

Here is an example:

define: &common
  stroke-opacity: 0.25

rules:
- filter: ${type = 'Major Highway'}
  symbolizers:
  - line:
      stroke-color: '#000088'
      stroke-width: 1.25
      <<: *common
- filter: ${type = 'Secondary Highway'}
  symbolizers:
  - line:
      stroke-color: '#8888AA'
      stroke-width: 0.75
      <<: *common
- filter: ${type = 'Road'}
  symbolizers:
  - line:
      stroke-color: '#888888'
      stroke-width: 0.75
      <<: *common
- filter: ${type = 'Unknown'}
  symbolizers:
  - line:
      stroke-color: '#888888'
      stroke-width: 0.5
      <<: *common
- else: true
  symbolizers:
  - line:
      stroke-color: '#AAAAAA'
      stroke-width: 0.5
      <<: *common

Challenge: Have a look at the documentation for putting a graphic on a text symbolizer in SLD and reproduce this technique in YSLD.

The use of a label shield is a vendor specific capability of the GeoServer rendering engine. The tricky part of this exercise is finding the documentation online ( i.e. TextSymbolizer - Graphic).

symbolizers:
- line:
    stroke-color: '#000000'
    stroke-width: 3
- line:
    stroke-color: '#D3D3D3'
    stroke-width: 2
- text:
    label: ${name}
    fill-color: '#000000'
    font-family: Ariel
    font-size: 10
    font-style: normal
    font-weight: normal
    placement: point
    graphic:
      size: 18
      symbols:
      - mark:
          shape: square
          stroke-color: '#000000'
          stroke-width: 1
          fill-color: '#FFFFFF'

When we rendered our initial preview, without a stroke, thin white gaps (or slivers) are visible between our polygons.

This effect is made more pronounced by the rendering engine making use of the Java 2D sub-pixel accuracy. This technique is primarily used to prevent an aliased (stair-stepped) appearance on diagonal lines.

Explore: Experiment with fill and stroke settings to eliminate slivers between polygons.

The obvious approach works - setting both values to the same color:

symbolizers:
- polygon:
    stroke-color: 'lightgrey'
    stroke-width: 1
    fill-color: 'lightgrey'

An exciting use of the GeoServer shape symbols is the theming by changing the size used for pattern density.

Explore: Use the Categorize function to theme by datarank.

Example:

symbolizers:
- polygon:
    stroke-color: 'black'
    stroke-width: 1
    fill-color: 'gray'
    fill-graphic:
      size: ${Categorize(datarank,'4','4','5','6','8','10','10')}
      symbols:
      - mark:
          shape: shape://slash
          stroke-color: 'darkgray'
          stroke-width: 1

The halo example used the fill color and opacity for a muted halo, while this improved readability it did not bring attention to our labels.

A common design choice for emphasis is to outline the text in a contrasting color.

Challenge: Produce a map that uses a white halo around black text.

Here is an example:

symbolizers:
- polygon:
    stroke-color: 'gray'
    stroke-width: 1
    fill-color: '#7EB5D3'
- text:
    label: ${name}
    fill-color: 'black'
    halo:
      fill-color: 'white'
      radius: 1
    font-family: Arial
    font-size: 14
    font-style: normal
    font-weight: normal
    anchor: [0.5, 0.5]

A powerful tool is theming using multiple attributes. This is an important concept allowing map readers to perform “integration by eyeball” (detecting correlations between attribute values information).

Challenge: Combine the mapcolor9 and datarank examples to reproduce the following map.

This should be a cut and paste using the recode example, and categorize examples already provided.

symbolizers:
- polygon:
    stroke-color: 'black'
    stroke-width: 1
    fill-color: ${Recode(mapcolor9,
      '1','#8dd3c7',
      '2','#ffffb3',
      '3','#bebada',
      '4','#fb8072',
      '5','#80b1d3',
      '6','#fdb462',
      '7','#b3de69',
      '8','#fccde5',
      '9','#d9d9d9')}
- polygon:
    stroke-color: 'black'
    stroke-width: 1
    fill-color: 'gray'
    fill-graphic:
      size: ${Categorize(datarank,'6','4','8','6','10','10','12')}
      symbols:
      - mark:
          shape: shape://slash
          stroke-color: 'black'
          stroke-width: 1
          fill-color: 'gray'

Using multiple feature-styles to simulate line string casing. The resulting effect is similar to text halos - providing breathing space around complex line work allowing it to stand out.

Challenge: Use what you know of LineString feature-styles to reproduce the following map:

This is much easier when using YSLD, where z-order is controlled by feature-style order. In this instance, multiple symbolizers within a feature-style will not work, as the order within a feature-style is only consistent per-feature (not per-layer).

feature-styles:
- rules:
  - symbolizers:
    - polygon:
        stroke-width: 1.0
        fill-color: 'lightgrey'
- rules:
  - symbolizers:
    - polygon:
        stroke-width: 1.0
        fill-color: 'gray'
        fill-graphic:
          size: 8
          symbols:
          - mark:
              shape: shape://slash
              stroke-color: 'black'
              stroke-width: 0.75
- rules:
  - symbolizers:
    - line:
        stroke-color: 'lightgrey'
        stroke-width: 6
- rules:
  - symbolizers:
    - line:
        stroke-color: 'black'
        stroke-width: 1.5

The structure of the legend graphic provides an indication on what is going on.

The mark property can be used to render any geometry content.

Challenge: Try this yourself by rendering a polygon layer using a mark property.

This can be done one of two ways:

• Changing the association of a polygon layer, such as ne:states_provinces_shp to point_example and using the layer preview page.

• Changing the Layer Preview tab to a polygon layer, such as ne:states_provinces_shp.

The important thing to notice is that the centroid of each polygon is used as a point location.

SLD Mark and ExternalGraphic provide an opportunity for dynamic symbolization.

This is accomplished by embedding a small CQL expression in the string passed to symbol or url. This sub-expression is isolated with ${ } as shown:

- point:
    symbols:
    - mark:
        shape: ${if_then_else(equalTo(FEATURECLA,'Admin-0 capital'),'star','circle')}

Challenge: Use this approach to rewrite the Dynamic Styling example.

Example available here point_example.css :

Use a Layer Group to explore how symbology works together to form a map.

• ne:NE1

• ne:states_provincces_shp

• ne:populated_places

This background is relatively busy and care must be taken to ensure both symbols and labels are clearly visible.

Challenge: Do your best to style populated_places over this busy background.

Here is an example with labels for inspiration:

This is opportunity to revisit label halo settings from Polygons:

symbolizers:
- point:
    size: ${'5' + '10' - SCALERANK / '3'}
    symbols:
    - mark:
        shape: circle
        stroke-color: 'white'
        stroke-width: 1
        stroke-opacity: 0.75
        fill-color: 'black'
        x-labelObstacle: true
    - text:
        label: ${name}
        fill-color: 'black'
        font-family: Arial
        font-size: 14
        anchor: [0.5, 1]
        offset: [0 ${'-12' + SCALERANK}]
        halo:
          fill-color: `lightgray`
          radius: 2
          opacity: 0.7
        priority: ${`0` - LABELRANK}
        x-maxDisplacement: 90

Using a lightgray halo, 0.7 opacity and radius 2 fades out the complexity immediately surrounding the label text improving legibility.

A special effect that is effective with grayscale information is automatic contrast adjustment.

Make use of a simple contrast enhancement with usgs:dem:

symbolizers:
- raster:
    opacity: 1.0
    contrast-enhancement:
      mode: normalize

Can you explain what happens when zoom in to only show a land area (as indicated with the bounding box below)?

What happens is insanity, normalize stretches the palette of the output image to use the full dynamic range. As long as we have ocean on the screen (with value 0) the land area was shown with roughly the same presentation.

Once we zoom in to show only a land area, the lowest point on the screen (say 100) becomes the new black, radically altering what is displayed on the screen.

The color-map type property dictates how the values are used to generate a resulting color.

• ramp is used for quantitative data, providing a smooth interpolation between the provided color values.

• intervals provides categorization for quantitative data, assigning each range of values a solid color.

• values is used for qualitative data, each value is required to have a color-map entry or it will not be displayed.

Challenge: Update your DEM example to use intervals for presentation. What are the advantages of using this approach for elevation data?

By using intervals it becomes very clear how relatively flat most of the continent is. The ramp presentation provided lots of fascinating detail which distracted from this fact.

Here is style for you to cut and paste:

symbolizers:
- raster:
    opacity: 1.0
    color-map:
      type: intervals
      entries:
      - ['#014636', 0, 0, null]
      - ['#014636', 1.0, 1, null]
      - ['#016C59', 1.0, 500, null]
      - ['#02818A', 1.0, 1000, null]
      - ['#3690C0', 1.0, 1500, null]
      - ['#67A9CF', 1.0, 2000, null]
      - ['#A6BDDB', 1.0, 2500, null]
      - ['#D0D1E6', 1.0, 3000, null]
      - ['#ECE2F0', 1.0, 3500, null]
      - ['#FFF7FB', 1.0, 4000, null]

Now that you have seen the data on screen and have a better understanding how would you modify our initial gray-scale example?

Challenge: Use what you have learned to present the usgs:dem clearly.

The original was a dark mess. Consider making use of mid-tones (or adopting a sequential palette from color brewer) in order to fix this. In the following example the ocean has been left dark, allowing the mountains stand out more.

symbolizers:
- raster:
    opacity: 1.0
    color-map:
      type: ramp
      entries:
      - ['#000000', 1.0, 0, null]
      - ['#444444', 1.0, 1, null]
      - ['#FFFFFF', 1.0, 3000, null]

There is a quick way to make raster data transparent, raster opacity property works in the same fashion as with vector data. The raster as a whole will be drawn partially transparent allow content from other layers to provide context.

Challenge: Can you think of an example where this would be useful?

This is difficult as raster data is usually provided for use as a basemap, with layers being drawn over top.

The most obvious example here is the display of weather systems, or model output such as fire danger. By drawing the raster with some transparency, the landmass can be shown for context.