Displaying rasters
Release 9.1 Last modified December 7, 2006	Print all topics in : "Working with rasters in ArcMap"

About displaying rasters

The way in which you display a raster depends on what type of data it contains and what you want to show. Some rasters have a predefined color scheme–a colormap–that ArcMap automatically uses to display them. For those that don't, ArcMap chooses an appropriate display method that you can adjust as needed.

The map gallery contains an example of a raster map.

Display options

If you want, 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. This drawing method often improves your ability to distinguish features in the raster.

Rendering raster datasets

Raster datasets can be displayed, or rendered, in your map in many different ways. Rendering involves the process of displaying your data visually.
When a raster dataset layer is added to ArcMap, it will be displayed with a default renderer, which is usually the most appropriate for the particular raster dataset layer. Generally, there are particular ways to display a raster dataset to take advantage of all its data. ArcMap allows you to choose from different drawing methods based on your display and analysis needs. Both individual raster datasets and raster catalogs provide similar display methods.

Methods of rendering raster data

There are several methods of displaying raster data. You can choose which renderer you want to use for your data on the Symbology tab of the layer’s properties.

RGB Composite

Use RGB Composite for a multiband raster layer. You can draw a three-band composite of raster data, with the option of displaying fewer than three bands or changing the band combination.

Unique Values

Use Unique Values when you want each value in the raster layer to be displayed individually. For instance, you may 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.
The Unique Values renderer displays each value as a random color. If your data has a colormap, you can use the Colormap renderer to display your data with assigned colors.

Stretched

The Stretched renderer displays continuous raster cell values across a gradual ramp of colors. Use the Stretched renderer when you want to draw a single band of continuous data. The Stretched renderer works well when you have a large range of values that you would like to display, such as in spectral imagery, aerial photographs, or elevation models.
You can choose from several different automatic stretches and a manual option when deciding exactly how to stretch the values.

Classified

The Classified renderer is used with a single-band raster layer. The Classified method displays thematic rasters by grouping cell values into classes. It is typical to 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.
You can choose a manual classification method or one of these standard methods:

• Equal Interval—The range of cell values is divided into equally sized classes, where you specify the number of classes.
• 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.
• Natural Breaks (Jenks)—The classes are based on natural groupings of values.
• Standard Deviation—Shows you the amount a cell’s value varies from the mean.

Colormap

You use the Colormap renderer much as you would the Unique Values renderer. However, you use the Colormap renderer when you choose to have the values in the raster layer represented by a prespecified color.
If your data has a colormap, you can use the Colormap renderer to display your data with assigned colors. The Colormap renderer displays automatically in the list of available renderers on the Symbology tab.

Rendering raster catalogs

A raster catalog is a collection of raster datasets defined in table format, in which the records define the individual rasters that are included in the catalog. A raster catalog can be used to display a collection of adjacent rasters without having to mosaic them into a larger 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. The use of 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 in the Options dialog box.

ArcMap has the ability to 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 that are available for the catalog. This list of renderers can be edited to add or remove various renderers. Only the renderers in the list can be used in the rendering of the catalog. In the available renderers list, ArcMap places an asterisk next to each of the renderers that are currently active and 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 the 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.

Hillshading raster datasets

A raster dataset can be displayed as a hillshade, which a shaded relief view that considers the illumination angle and shadows. To render a raster dataset, such as digital elevation data so that it is drawn using hillshading, you have to create a new hillshaded raster dataset from your original raster dataset. This requires the use of either the Spatial Analyst extension or the 3D Analyst extension. If you have one of these extension, you can either use the Hillshade command in the pulldown menu in that extension's toolbar in ArcMap, or the Hillshade Tool in that extension's toolbox in ArcToolbox.

If you have installed either 3D Analyst or Spatial Analyst, there is more information available about creating and displaying hillshades.

You may also be interested in learning how to show hillshading by drawing TINs as surfaces.

How to display rasters

Drawing a multiband raster dataset as an RGB composite

1. In the table of contents, right-click the raster layer that you want to draw as an RGB Composite and click Properties.
2. Click the Symbology tab.
3. Click RGB Composite.
4. Click the Bands dropdown arrow next to each color and click the band you want to display for that color.
5. Optionally, click the Stretch Type dropdown arrow and click the stretch you want to apply.
6. Optionally, click Histogram to modify the stretch settings.
7. If the raster contains a background or border around the data that you want to hide, check Display Background Value and set the color to No Color. The cells will display transparently.
8. Click OK.

Tips

To see the RGB value for a given cell, turn on MapTips for the layer. To do this, right-click the layer in the table of contents and click Properties. Check the Show MapTips box on the Display tab. If the raster draws too dark, you can alter the overall brightness and contrast of a raster through the Effects toolbar. Alternatively, some raster drawing methods will allow you to stretch the data values to take advantage of the available colors. You can quickly change the colors applied to a raster by clicking the color square beside the band you want to turn off and unchecking Visible to turn it off. Use the Effects toolbar to draw raster layers transparently over other layers on your map. Having current statistics will improve the ability of the renderer to display the raster accurately. Statistics can be calculated for each raster in ArcCatalog.

Drawing thematic raster datasets representing unique categories such as land use

1. In the table of contents, right-click the raster layer that you want to draw showing unique values and click Properties.
2. Click the Symbology tab.
3. Click Unique Values.
4. Click the Value Field dropdown arrow and click the field you want to map.
5. Click the Color Scheme dropdown arrow and click a color scheme.
6. Optionally, click a label and type in a more descriptive one.
7. Optionally, select a color or no color to display any NoData values.
8. Click OK.

Tips

If the raster draws too dark, you can alter the overall brightness and contrast of a raster through the Effects toolbar. Alternatively, some raster drawing methods will allow you to stretch the data values to take advantage of the available colors. You can quickly change the colors applied to a raster by clicking on the color in the table of contents. Use the Effects toolbar to draw raster layers transparently over other layers on your map. If your raster is just a picture that doesn't align to any other geographic data, you can simply place it on a layout as a map element. Alternatively, create a hyperlink and associate it with a geographic feature on your map. Having current statistics will improve the ability of the renderer to display the raster accurately. Statistics can be calculated for each raster in ArcCatalog.

Drawing a continuous raster dataset such as an orthophoto

1. In the table of contents, right-click the raster layer that you want to display across a color ramp and click Properties.
2. Click the Symbology tab.
3. Click Stretched.
4. If your raster dataset has multiple bands, optionally, choose the band you want to stretch.
5. Optionally, click the Label boxes and type labels for the table of contents.
6. Click the Color Ramp dropdown arrow and click a color ramp.
7. Optionally, choose a color or no color to display any NoData values.
8. Optionally, click the Stretch dropdown arrow and click the type of Stretch, if any, you would like to perform.
9. Optionally, scroll down and click Statistics to change the statistics.
10. Click OK.

Tips

You can quickly change the colors applied to a raster dataset by clicking the color ramp in the table of contents. Use the Effects toolbar to draw raster layers transparently over other layers on your map. You can use the Statistics portion of the Symbology tab to examine and modify how your data displays. Having current statistics will improve the ability of the renderer to display the raster accurately. Statistics can be calculated for each raster in ArcCatalog.

Drawing thematic raster datasets representing continuous data such as elevation

1. In the table of contents, right-click the single band raster dataset layer that you want to display by grouping values into classes and click Properties.
2. Click the Symbology tab.
3. Click Classified.
4. Click the Value dropdown arrow and click the field you want to map. If your raster dataset does not have a table, the Value dropdown arrow is unavailable.
5. Optionally, click the Normalization dropdown arrow and click a field to normalize your data.
6. Click the Classes dropdown arrow and click the number of classes you want.
7. Click Classify and choose the classification method you want to use. You can choose a standard or a manual method of classification. Click OK.
8. Click OK.
9. Click the Color Ramp dropdown arrow and click a color ramp.
10. Optionally, choose a color or no color to display any NoData values.
11. Click OK.

Tips

If your raster is just a picture that doesn't align to any other geographic data, you can simply place it on a layout as a map element. Alternatively, create a hyperlink and associate it with a geographic feature on your map. If the raster draws too dark, you can alter the overall brightness and contrast of a raster through the Effects toolbar. Alternatively, some raster drawing methods will allow you to stretch the data values to take advantage of the available colors. You can quickly change the colors applied to a raster by clicking on the color in the table of contents. Use the Effects toolbar to draw raster layers transparently over other layers on your map. Having current statistics will improve the ability of the renderer to display the raster accurately. Statistics can be calculated for each raster in ArcCatalog.

Drawing thematic raster datasets that represent categories using a colormap

1. In the table of contents, right-click the raster layer that you want to draw with a colormap and click Properties.
2. Click the Symbology tab.
3. Click Colormap.
4. Click OK.

Tips

You can check to see if your raster dataset has a colormap in the Properties dialog box. Click the Source tab and see whether or not a colormap is present. For GRID files, the colormap information is stored in a .clr file with the same name as the GRID. For most other formats, the colormap is embedded within the raster dataset. Use the Effects toolbar to draw raster layers transparently over other layers on your map.

Displaying a time series raster catalog

1. Right-click the raster catalog layer in the table of contents and click Properties.
2. Click the Display tab.
3. Click Never show wireframe.
4. Check Redraw whole display after each redraw.
5. Type a Delay draw value in miliseconds, in the text box.
6. Optionally, choose a field in the Order By dropdown box. If an order field is chosen, choose whether to have the raster items in ascending or descending order.
7. Click OK.

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