Renderers used to display raster data
Raster datasets can be displayed, or rendered, in your map in many different ways. Rendering is the process of displaying your data. How a raster dataset is rendered depends on what type of data it contains and what you want to show. Some rasters have a predefined color scheme—a color map—that ArcMap automatically uses to display them. For those that don't, ArcMap chooses an appropriate display method that you can adjust as needed.
You can change display colors, group data values into classes, or stretch values to increase the visual contrast. For multiband rasters, you can display three bands together as a red, green, blue (RGB) composite. If you have a raster dataset digital elevation model (DEM), you can choose to display it with hillshading.
When a raster dataset is displayed or previewed (or a layer is created for a raster dataset), it is displayed using the most appropriate renderer. Generally, there are particular ways to display a raster dataset to take advantage of all its data. Individual raster datasets, mosaic datasets, and raster catalogs provide similar display methods. ArcMap allows you to choose from different rendering methods based on your display and analysis needs.
You can change these methods using the Image Analysis window or by opening the layer's Properties dialog box and editing the parameters on the Symbology tab. To preserve the rendering properties you've set, you can save a layer (*.lyr) file.
Methods of rendering raster data
The Stretched renderer displays continuous raster cell values across a gradual ramp of colors. Use the Stretched renderer to draw a single band of continuous data. The Stretched renderer works well when you have a large range of values to display, such as in imagery, aerial photographs, or elevation models. Below is an example of the Stretched renderer on a single band in a multiband raster dataset:
You can choose from several different automatic stretches and a manual option when deciding exactly how to stretch the values.
The RGB Composite renderer uses the same methods as the Stretched renderer but allows you to combine bands as red, green, blue composites. When viewing color aerial photography, you are often viewing a three-band raster dataset, and this renderer is applied by default. You will also want to use this renderer to display different combinations of bands when working with multiband raster datasets, such as satellite or aerial imagery.
Below is an example of a multiband raster dataset displayed using three bands. The image above uses the Stretched renderer to display one of the bands in the raster dataset. The image below was created with the RGB Composite renderer and displays with color:
The Classified renderer is used with a single-band raster layer. The Classified method displays thematic rasters by grouping cell values into classes. Use this type of thematic classification on continuous phenomena, such as slope, distance, or suitability, where you want to classify the range into a small number of classes and assign colors to those classes. Below is an example of an elevation raster dataset displayed using the Classified renderer:
You can choose one of these classification methods:
- Manual—Lets you set the class breaks. Use this choice if, for example, you want to emphasize particular patterns by placing breaks at important threshold values or if you need to comply with a particular standard that demands certain class breaks.
- Equal interval—The range of cell values is divided into equally sized classes where you specify the number of classes. Use this method to emphasize the relative amount of attribute values compared to other values. It is best applied to familiar data ranges such as percentages and temperature.
- Defined interval—You specify an interval to divide the range of cell values, and ArcMap determines the number of classes.
- Quantile—Each class contains an equal number of cells. Use this method with linearly distributed data.
- Natural breaks (Jenks)—The class breaks are determined statistically by finding adjacent feature pairs between which there is a relatively large difference in data value.
- Standard deviation—Shows you the amount a cell's value varies from the mean. It is best applied on normally distributed data.
Geometrical interval—Creates class breaks based on class intervals that have a geometric series. The geometric coefficient in this classifier can change once (to its inverse) to optimize the class ranges. The algorithm creates geometric intervals by minimizing the sum of squares of the number of elements in each class. This ensures that each class range has approximately the same number of values with each class and that the change between intervals is fairly consistent.
This algorithm was specifically designed to accommodate continuous data. It is a compromise method between equal interval, natural breaks (Jenks), and quantile. It creates a balance between highlighting changes in the middle values and the extreme values, thereby producing a result that is visually appealing and cartographically comprehensive.
One example for using the geometrical interval classification would be with a rainfall dataset in which only 15 out of 100 weather stations (less than 50%) have recorded precipitation and the rest have no recorded precipitation, so their attribute values are zero.
The Unique Values renderer is used to display each value in the raster layer individually. For instance, you might have discrete categories representing particular objects on the earth's surface, such as those in a thematic raster layer, which could display soil types or land use. Below is an example of a classified raster dataset showing land use:
The Unique Values renderer displays each value as a random color. If your data has a color map, you can use the Colormap renderer to display your data with assigned colors.
Use the Colormap renderer as you would the Unique Values renderer or when you want the values in the raster layer to be represented by a prespecified color.
The Colormap renderer appears automatically in the list of available renderers on the Symbology tab if a colormap is present with the raster dataset.
Use Discrete Color when you want to display the values in the raster dataset using a random color. This renderer is similar to the Unique Values renderer, but it is more efficient when there are a large number of unique values as it does not have to calculate how many unique values exist. The Discrete Color renderer assigns a color to each unique value until it reaches the maximum number of colors that you chose. The next unique value starts at the beginning of the color scheme; this process continues until each unique value has a color assigned to it.
A legend displaying which color is mapped to which value is not generated or displayed in the table of contents.
Rendering single-band datasets
When a single-band raster dataset is drawn, the rules for determining the default renderer are as follows:
- If your dataset has a color map, it will be displayed with the Colormap renderer using the colors stored in the color map.
- If your dataset contains 1-bit data and the dataset does not have pyramids, the Unique Values renderer will be used with 0 set to white and 1 set to black. When 1-bit data has pyramids, the Stretched renderer will be used. Pyramid files for 1-bit data are created as 8-bit grayscale to achieve improved display when viewing the raster at its full extent.
- If your dataset contains 25 or fewer unique values, the Unique Values renderer will be used with random colors assigned to the values.
- If your dataset has an internally stored custom stretch, the Stretched renderer will be used with a color ramp from black to white.
- If your dataset has statistics, the Stretched renderer will be used with a color ramp from black to white, and a value of 2 will be applied for a standard deviation stretch. If there are no statistics for your dataset, the Stretched renderer with a color ramp from black to white will still be used, but the values won't be stretched.
Rendering multiband datasets
When a multiband raster dataset is drawn using a default RGB composite renderer, it uses the default band combination defined on the Raster tab of the Options dialog box. If your dataset has statistics, a standard deviation stretch will be used, with a value of 2. If your dataset does not have statistics, no stretch will be applied and the minimum and maximum values of the data will be scaled to 0 and 255, respectively.
Rendering alpha bands
An alpha band acts as a transparency mask, providing a transparency value for each pixel. An alpha band can be toggled on or off for multiple-band raster datasets rendered with the RGB Composite renderer.
If you want to toggle the Alpha channel on or off, you will need to check the appropriate check box to turn it on or uncheck it to turn it off within the Symbology tab of the raster layer Properties dialog box.
Rendering raster catalogs
A raster catalog is a collection of raster datasets defined in table format in which the records specify the individual rasters in the catalog. A raster catalog can be used to display a collection of adjacent rasters without mosaicking them into a single large file. Raster catalogs can also be used to hold disparate, semioverlapping, or fully overlapping raster datasets.
By default, raster catalogs are displayed as a wireframe if more than nine images are in the current display extent; otherwise, the actual raster data will be displayed. Using the wireframe speeds up the display of raster catalogs. The default of nine images can be changed in the display properties of your raster catalog or on the Raster tab on the Options dialog box.
ArcMap can render each raster dataset member of a geodatabase raster catalog with its most appropriate renderer. The Symbology tab of a raster catalog's Layer Properties dialog box lists the renderers available for the catalog. This list can be edited by adding or removing various renderers. Only the renderers in the list can be used to render the catalog. In the available renderers list, ArcMap places an asterisk next to each of the currently active renderers that are applied to one or more raster dataset members of the raster catalog. However, the active list can only be triggered when an image is displayed on-screen. This list will not be complete until the entire catalog has been viewed. The active renderers persist even after you change the display to another area, to the full extent, or back to a wireframe display.
As mentioned, each raster catalog item is rendered with an appropriate renderer in the list, following similar rules as a raster dataset. Therefore, the classified renderer will never be a default renderer, unless it is the only renderer in the available renderer list.
Rendering mosaic datasets
A mosaic dataset is a data model within the geodatabase that is a hybrid of a raster catalog and raster dataset—it represents an on-the-fly mosaic view of a raster catalog. When you add a mosaic dataset to ArcMap, ArcScene, and ArcGlobe, it will appear in the table of contents as a special group layer with a minimum of three layers: Boundary, Footprint, and Image. The Image layer is used to control the rendering of the mosaicked image of the mosaic dataset. You can right-click the Image layer to open the Properties dialog box, and similarly to the raster dataset, you can change the renderer on the Symbology tab. Modifications you make to the Image layer properties do not affect the mosaic dataset--just how it is rendered at the time you are viewing it.
Hillshading raster datasets
Hillshading a raster dataset allows you to visualize the terrain of the surfaces represented by your raster dataset. There are several ways to display hillshading with your raster dataset. One way uses the Hillshade effect renderer, which is available within the Stretched renderer. The Hillshade renderer predicts where shadows exist in the DEM, depending on the origin (direction) of the light source and the elevations that exist. You can adjust the hillshade lighting by doing the following:
- Clicking the Options button on the Image Analysis window and modifying the parameters on the Hillshade tab
- Opening the Data Frame Properties dialog box that contains the raster dataset layer and adjusting the controls on the Illumination tab
With the Image Analysis window you can create both a hillshade and a shaded relief from your DEM. A shaded relief combines a hillshaded version of the DEM and the same DEM with a color ramp applied.
You can also use the Hillshade tool in either the Spatial Analyst or 3D Analyst extension of ArcGIS to create a raster dataset with the hillshaded values recorded. Additionally, you can apply hillshading to a TIN.