To view and set the properties for a raster layer, double click on the layer name in the map legend or right click on the layer name and choose Properties from the context menu:
This will open the Raster Layer Properties dialog, (see figure_raster_1).
There are several menus in the dialog:
Figure Raster 1:
The General menu displays basic information about the selected raster, including the layer source path, the display name in the legend (which can be modified) and the number of columns, rows and No-Data Values of the raster.
Below you find the coordinate reference system (CRS) information printed as a PROJ.4-string. If this setting is not correct, it can be modified by clicking the [Specify] button.
Additionally Scale Dependent visibility can be set in this tab. You need to check the checkbox and set an appropriate scale where your data will be displayed in the map canvas.
At the bottom you can see a thumbnail of the layer, its legend symbol, and the palette.
QGIS offers four different Render types. The renderer chosen is dependent on the data type.
With the multiband color renderer three selected bands from the image will be rendered, each band representing the red, green or blue component that will be used to create a color image. You can choose several Contrast enhancement methods: ‘No enhancement’, ‘Stretch to MinMax’, ‘Stretch and clip to MinMax’ and ‘Clip to min max’.
Figure Raster 2:
This selection offers you a wide range of options to modify the appearance of your rasterlayer. First of all you have to get the data range from your image. This can be done by choosing the Extent and pressing [Load]. QGIS can Estimate (faster) the Min and Max values of the bands or use the Actual (slower) Accuracy.
Now you can scale the colors with the help of the Load min/max values section. A lot of images have few very low and high data. These outliers can be eliminated using the Cumulative count cut setting. The standard data range is set from 2% until 98% of the data values and can be adapted manually. With this setting the gray character of the image can disappear. With the scaling option Min/max QGIS creates a color table with the whole data included in the original image. E.g. QGIS creates a color table with 256 values, given the fact that you have 8bit bands. You can also calculate your color table using the Mean +/- standard deviation x . Then only the values within the standard deviation or within multiple standard deviations are considered for the color table. This is useful when you have one or two cells with abnormally high values in a raster grid that are having a negative impact on the rendering of the raster.
All calculation can also be made for the Current extend.
Viewing a Single Band of a Multiband Raster
If you want to view a single band (for example Red) of a multiband image, you might think you would set the Green and Blue bands to “Not Set”. But this is not the correct way. To display the Red band, set the image type to ‘Singleband gray’, then select Red as the band to use for Gray.
This is the standard render option for singleband files that already include a color table, where each pixel value is assigned to a certain color. In that case, the palette is rendered automatically. If you want to change colors assigned to certain values, just double-click on the color and the Select color dialog appears.
Figure Raster 3:
When adding GRASS rasters the option Contrast enhancement will be always set to automatically to stretch to min max regardless if the QGIS general options this is set to another value.
This renderer allows you to render a single band layer with a Color gradient ‘Black to white’ or ‘White to black’. You can define a Min and a Max value with choosing the Extend first and then pressing [Load]. QGIS can Estimate (faster) the Min and Max values of the bands or use the Actual (slower) Accuracy.
Figure Raster 4:
With the Load min/max values section scaling of the color table is possible. Outliers can be eliminated using the Cumulative count cut setting. The standard data range is set from 2% until 98% of the data values and can be adapted manually. With this setting the gray character of the image can disappear. Further settings can be made with Min/max and Mean +/- standard deviation x . While the first one creates a color table with the whole data included in the original image the second creates a colortable that only considers values within the standard deviation or within multiple standard deviations. This is useful when you have one or two cells with abnormally high values in a raster grid that are having a negative impact on the rendering of the raster.
This is a render option for single band files including a continous palette. You can also create individual color maps for the single bands here.
Figure Raster 5:
Three types of color interpolation are available:
In the left block the button Add values manually adds a value to the individual color table. Button Remove selected row deletes a value from the individual color table and the Sort colormap items button sorts the color table according to the pixel values in the value column. Double clicking on the value-column lets you insert a specific value. Double clicking on the color-column opens the dialog Change color where you can select a color to apply on that value. Further you can also add labels for each color but this value won’t be displayed when you use the identify feature tool. You can also click on the button Load color map from band, which tries to load the table from the band (if it has any). And you can use the buttons Load color map from file or Export color map to file to load an existing color table or to save the defined color table for other sessions.
In the right block Generate new color map allows you to create newly categorized colormaps. For the Classification mode ‘Equal interval’ you only need to select the number of classes and press the button Classify. You can invert the colors of the the color map by clicking the Invert checkbox. In case of the Mode ‘Continous’ QGIS creates classes depending on the Min and Max automatically. Defining Min/Max values can be done with the help of Load min/max values section. A lot of images have few very low and high data. These outliers can be eliminated using the Cumulative count cut setting. The standard data range is set from 2% until 98% of the data values and can be adapted manually. With this setting the gray character of the image can disappear. With the scaling option Min/max QGIS creates a color table with the whole data included in the original image. E.g. QGIS creates a color table with 256 values, given the fact that you have 8bit bands. You can also calculate your color table using the Mean +/- standard deviation x . Then only the values within the standard deviation or within multiple standard deviations are considered for the color table.
For every Band rendering a Color rendering is possible.
You can achieve special rendering effects for your raster file(s) using one one of the blending modes (see blend_modes).
Further settings can be made in modifiying the Brightness, the Saturation and the Contrast. You can use a Grayscale option where you can choose between ‘By lightness’, ‘By luminosity’ and ‘By average’. For one hue in the color table you can modiy the ‘Strength’.
The Resampling option makes it appearance when you zoom in and out of the image. Resampling modes can optimize the appearance of the map. They calculate a new gray value matrix through a geometric transformation.
While applying the ‘Nearest neighbour’ method the map can have a pixelated structure when zooming in. This appearance can be improved by using the ‘Bilinear’ or ‘Cubic’ method. Sharp features are caused to be blurred now. The effect is a smoother image. The method can be applied e.g. to digital topographic raster maps.
QGIS has the ability to display each raster layer at varying transparency levels. Use the transparency slider to indicate to what extent the underlying layers (if any) should be visible though the current raster layer. This is very useful, if you like to overlay more than one rasterlayer, e.g. a shaded relief map overlayed by a classified rastermap. This will make the look of the map more three dimensional.
Additionally you can enter a rastervalue, which should be treated as NODATA in the Additional no data value menu.
An even more flexible way to customize the transparency can be done in the Custom transparency options section. The transparency of every pixel can be set here.
As an example we want to set the water of our example raster file landcover.tif to a transparency of 20 %. The following steps are neccessary:
You can repeat the steps 5 and 6 to adjust more values with custom transparency.
As you can see this is quite easy to set custom transparency, but it can be quite a lot of work. Therefore you can use the button Export to file to save your transparency list to a file. The button Import from file loads your transparency settings and applies them to the current raster layer.
Large resolution raster layers can slow navigation in QGIS. By creating lower resolution copies of the data (pyramids), performance can be considerably improved as QGIS selects the most suitable resolution to use depending on the level of zoom.
You must have write access in the directory where the original data is stored to build pyramids.
Several resampling methods can be used to calculate the pyramids:
If you choose ‘Internal (if possible)’ from the Overview format menu QGIS tries to build pyramids internally. You can also choose ‘External’ and ‘External (Erdas Imagine)’.
Please note that building pyramids may alter the original data file and once created they cannot be removed. If you wish to preserve a ‘non-pyramided’ version of your raster, make a backup copy prior to building pyramids.
The Histogram menu allows you to view the distribution of the bands or colors in your raster. It is generated automatically when you open the Histogram menu. All existing bands will be displayed together. You can save the histogram as an image with the button. With the Visibility option in the Prefs/Actions menu you can display histograms of the individual bands. You will need to select the option Show selected band. The Min/max options allow you to ‘Always show min/max markers’, to ‘Zoom to min/max’ and to ‘Update style to min/max’. With the Actions option you can ‘Reset’ and ‘Recompute histogram’ after you have chosen the Min/max options.
The Metadata menu displays a wealth of information about the raster layer, including statistics about each band in the current raster layer. From this menu entries are made for the Description, Attribution, MetadataUrl and Properties. In Properties statistics are gathered on a ‘need to know’ basis, so it may well be that a given layers statistics have not yet been collected.