Changing Raster Layer Style

Display of raster data in the map canvas can be enhanced using linear histogram stretching. Useful Raster tools are not activated by default. Right click on the main QGIS menu and activate the checkbox as seen on the screenshot. A new Raster Toolbar is now available on the main menu.

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Render type: multiband file

1. Click the Open Data Source Manager and then Add raster layer and select a virtual stack or the file /geodata/lucc/s2/Subset_S2A_MSIL2A_20170619T_MUL.tif.
2. After clicking Open, the raster layer appears on the QGIS canvas.
3. Right click the layer name in the Layer Panel and select Properties --> Symbology.
4. Change the assignment of Red, Green, Blue to band numbers and create color composites.

Attention: usually the assignment refers to the original spectral band numbers. Pick the corresponding band number in the multiband file.

• True Color composite.
  • Red band: Original B4, BandNo. 3
  • Green band: Original B3, BandNo.2
  • Blue Band: Original B2, BandNo. 1

In case of the Sentinel-2 example file this is a typical True Color composite.

Try other band combinations and produce false color composites. Band numbers refer to the original Sentinel-2 band names:

• Natural Colors: (B4, B3, B2)
• False-Color Infrared: (B8, B4, B3)
• False-Color Urban: (B12, B11, B4)
• Agriculture: (B11, B8, B2)
• Atmospheric Penetration: (B12, B11, B8A)
• Healthy Vegetation: (B8, B11, B2)
• Land/Water: (B8, B11, B4)
• Natural with Atmospherical Removal: (B12, B8, B3)

1. Color composites and single band rster files may be enhanced in the Style menu by unfolding the 'Load min/max values. Select Stretch to MinMax. As default a tail clip of the lower 2% and upper 2% counts of the cumulative band histograms is activated. Activate the checkbox Clip extent to canvas. Only the values inside the image extent shown in the map canvas are now taken into account. Click on Load and Apply. The image display is enhanced by a linear stretch with tail clipping.

1. Select the radio button Mean +/- standard deviation x radio button. After adjusting the standard deviation factor (a value between 1 and 2), click Load and confirm with Apply. Evaluate the image display in the map canvas.

Note, same effects but much faster can be reached by using the Raster Toolbar of the main Menu:

1. • Linear stretch using 2% cumulative count
   • Linear stretch clipping the mean +/- 2 * standard deviation

Render Type: Singleband pseudocolor

Pseudocolor is not to be mistaken for false color, the latter one being a composite of three bands. See also: Pseudocolor in Wikipedia

Pseudocolor is a way to display a raster map in which the color scale is derived from a single grayscale band. It can be a good alternative to grayscale if it comes to display metric data like elevation or temperature.

To change map color and symbology, again select the layer in the TOC with a right click and select Properties --> Style. From the pulldown menu Render type, select Singleband pseudocolor. A different menu will come up, where you may select the style of the color map in the section Generate new color map. Click classify and confirm with Apply or OK. The map should now appear in the selected style (figure C).

Figure C: Landsat map displayed in pseudocolor