4.5. Rasters¶

4.5.1. Image¶

The raster symbolizer controls the display of raster data. By default, the raster symbolizer automatically selects the appropriate red, green and blue channels for display.

1. Navigate to the Styles page.

2. Click Add a new style and choose the following:

   Name:	image_example
   Workspace:	No workspace
   Format:	YSLD

3. Choose raster from the Generate a default style dropdown and click generate.

4. Replace the initial YSLD definition with:

   symbolizers:
   - raster:
       opacity: 1.0
   

5. And use the Layer Preview tab to preview the result.

   

6. The channels property can be used to provide a list three band numbers (for images recording in several wavelengths) or a single band number can be used to view a grayscale image.

   symbolizers:
   - raster:
       opacity: 1.0
       channels:
         gray:
           name: '2'
   

7. Isolating just the green band (it will be drawn as a grayscale image):

   

4.5.2. DEM¶

A digital elevation model is an example of raster data made up of measurements, rather than color information.

The usgs:dem layer used for this exercise:

1. Return to the Styles page.

2. Click Add a new style and choose the following:

   Name:	raster_example
   Workspace:	No workspace
   Format:	YSLD

3. Choose raster from the Generate a default style dropdown and click generate.

4. The rendering engine will select our single band of raster content, and do its best to map these values into a grayscale image. Replace the content of the style with:

   symbolizers:
   - raster:
       opacity: 1.0
   

5. Use the Layer Preview tab to preview the result. The range produced in this case from the highest and lowest values.

   

6. We can use a bit of image processing to emphasis the generated color mapping by making use of contrast-enhancement.

   symbolizers:
   - raster:
       opacity: 1.0
       channels:
         gray:
           name: '1'
           contrast-enhancement:
             mode: histogram
   

7. Image processing of this sort should be used with caution as it does distort the presentation (in this case making the landscape look more varied then it is in reality.

   

4.5.3. Explore Contrast Enhancement¶

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

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

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

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

   

   Note

   Discussion provided at the end of the workbook.

4.5.4. Challenge Intervals¶

1. 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.

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

   Note

   Answer provided at the end of the workbook.

4.5.5. Explore Image Processing¶

Additional properties are available to provide slight image processing during visualization.

Image processing can be used to enhance the output to highlight small details or to balance images from different sensors allowing them to be compared.

1. The contrast-enhancement property is used to turn on a range of post processing effects. Settings are provided for normalize or histogram or none;

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

1. Producing the following image:

   

2. The raster-gamma property is used adjust the brightness of contrast-enhancement output. Values less than 1 are used to brighten the image while values greater than 1 darken the image.

symbolizers:
- raster:
    opacity: 1.0
    contrast-enhancement:
      gamma: 1.5

1. Providing the following effect:

   

4.5.6. Challenge Clear Digital Elevation Model Presentation¶

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

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

   Note

   Answer provided at the end of the workbook.

4.5.7. Challenge Raster Opacity¶

1. 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.

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

   Note

   Discussion provided at the end of the workbook.