3.5. Rasters¶

3.5.1. Image¶

The raster-channels is the key property for display of images and raster data. The value auto is recommended, allowing the image format to select 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:	CSS

3. Replace the initial CSS definition with:

   * {
     raster-channels: auto;
   }
   

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

   

5. If required a list three band numbers can be supplied (for images recording in several wavelengths) or a single band number can be used to view a grayscale image.

   * {
     raster-channels: 2;
   }
   

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

   

3.5.2. DEM¶

A digital elevation model is an example of raster data made up of measurements, rather than colors 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:	CSS

3. When we use the raster-channels property set to auto 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:

   * {
     raster-channels: auto;
   }
   

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

   

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

   * {
     raster-channels: 1;
     raster-contrast-enhancement: histogram;
   }
   

6. 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.

   

3.5.3. 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:

   * {
       raster-channels: auto;
       raster-contrast-enhancement: normalize;
   }
   

3. 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.

3.5.4. Challenge Intervals¶

1. The raster-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.

   Instructor Notes

   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:

   * {
     raster-channels: auto;
     raster-color-map:
        color-map-entry(#014636,   0,0)
        color-map-entry(#014636,   1)
        color-map-entry(#016c59, 500)
        color-map-entry(#02818a,1000)
        color-map-entry(#3690c0,1500)
        color-map-entry(#67a9cf,2000)
        color-map-entry(#a6bddb,2500)
        color-map-entry(#d0d1e6,3000)
        color-map-entry(#ece2f0,3500)
        color-map-entry(#fff7fb,4000);
     raster-color-map-type: intervals;
   }
   

   

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

   * {
       raster-channels: auto;
       raster-contrast-enhancement: normalize;
   }
   

2. Producing the following image:

   

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

   * {
      raster-channels: auto;
      raster-contrast-enhancement: none;
      raster-gamma: 1.5;
   }
   

4. Providing the following effect:

   

3.5.6. 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.

   Instructor Notes

   The original was a dark mess, students will hopefully make use of the mid-tones (or even check color brewer) in order to fix this. I have left the ocean dark so the mountains can stand out more.

   * {
     raster-channels: auto;
     raster-color-map: color-map-entry(#000000, 0)
                       color-map-entry(#444444, 1)
                       color-map-entry(#FFFFFF, 3000);
   }
   

   

3.5.7. 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.