Opening DEMs

Digital Elevation Models (DEM) and Digital Terrain Models (DTM) provide an effective method of storing information for continuous surfaces. Measured elevation values of the earth’s surface is the most common application of DEMs, however, other applications include measurable phenomena such as rainfall, temperature and density. DEM data needs to be rendered so it can be displayed visually. Depending on what type of DEM data it is and how it should look, the type of rendering schema needs to be customized or automatically assigned.

Requirements

The image must be a supported form of elevation or terrain data to invoke the Import DEM file dialog box:

•ArcInfo ASCII Grid	•BIL	•USGS DEM
•ArcInfo Binary Grid	•ERDAS IMAGINE	•USGS SDTS
•ArcInfo Binary Floating Grid	•Military Elevation Data / DTED	•SRTM

See detailed information about each supported DEM format

Import DEM File Dialog Box

Open a supported DEM format using one of the methods described here. A detected DEM will prompt the Import DEM File dialog box.

The Import DEM file dialog box provides options to apply a schema based on the statistics of the raster data set. Use a custom DEM schema, an auto-stretched schema or a raw data schema. A DEM schema increases the visual contrast of the raster display. Depending on the required appearance of the DEM, apply a schema suitable for the range of data within it. For example, DEMs may not contain the entire range of values that can be displayed; therefore, applying an appropriate schema may result in a sharper image and certain physical features may become easier to see.

Assign a DEM Schema

When a supported DEM is opened, the Import DEM file dialog box appears. To prevent it from appearing every time, click the Don’t show again check box. This forces all DEMs to use the last used schema every time. This is only recommended if you're using Auto-stretched schema all the time. To reset when the Import DEM file dialog box appears, click the Reset 'Do not show again' preferences link in the Geographic Imager Preferences dialog box.

In the Select Schema drop-down list, several schema are available and are discussed below. To assign a DEM schema, click the OK button to accept the settings of the Import DEM file dialog box.

	Auto-stretched The Auto-stretched schema displays continuous raster cell values across a gradual ramp of Adobe Photoshop grayscale colour space. This schema displays a single band of continuous data and works well when a large range of values need to be displayed. Auto-stretched allows for the best precision preservation and automatically generates high-contrast images.  You can’t mosaic files unless they have the same schema. Use Advanced Import to import DEMs with different schemas and mosaic them using a combined value range in a new schema.
	Raw Data and Negative Raw Data These schemas assign elevation data directly to the Adobe Photoshop grayscale colour space. The Raw Data schema facilitates mosaicking with other raw imported images. Since it does not produce a high-contrast image, use Adobe Photoshop adjustment tools such as Levels and Curves. Use the Negative Raw Data schema with negative elevation values. The image preview in the Georeference dialog box or the Survey tab of the Geographic Imager panel can be used to view elevation data values. More information on this in the Important Notes section at the end of this topic.
	Custom Schema Custom schema are designated by blue text (or in its own list group) in the Select Schema drop-down list. When a custom schema is chosen, the Remove Schema and Edit Schema buttons are enabled.

 

DEM units and Precision

The default DEM units are displayed during import. To change the default units, create or edit a DEM schema and click the Change Schema Units button. The precision of the DEM is displayed but cannot be edited. The precision value is also later displayed in the HTML Panel, once it has been imported.

 Importing DTM data using Advanced Import will not display precision on the Geographic Imager panel if Custom Schema is chosen or if DTM is not the selected Type. If appropriate, use Auto-stretched as the schema and DTM as the type.

Delete a DEM Schema

To remove a DEM schema from the Select Schema drop-down list, click the Remove Schema button.

Edit a DEM Schema

To edit a DEM schema, click the Edit Schema button. Geographic Imager analyzes the DEM to be edited and opens the Edit DEM Schema dialog box.

Creating a DEM Schema

To create a DEM schema, click the Create Schema button. Geographic Imager analyzes the DEM to be imported and opens the Edit DEM Schema dialog box. The Edit DEM Schema is populated with information available from the DEM. Any of the boxes can be edited to customize the schema.

The Lowest Elevation and Highest Elevation values represent the lowest and highest data values to be mapped in the Adobe Photoshop grayscale colour space. Editing these values will change how the image is displayed. The File Data Range shows the lowest and highest data values in the current DEM. Use it as a guide when entering custom elevation values.

The Restore Default Data Range Values button is only enabled when either the Lowest Elevation or Highest Elevation box has a custom value entered. Click the button to restore the data range as specified in the File Data Range display.

Vertical units are only used for mosaicking. The meter unit is displayed by default, which is not dependent on the reference file of the current document. To change the Vertical Unit, click the Change Schema Units button and choose a new unit in the Select a unit for the DEM file dialog box.

Advanced DEM Schema Settings

Advanced DEM schema settings must be used in conjunction with a custom schema. To access the advanced DEM schema settings, click the Advanced button in the Import DEM file dialog box. These advanced schema settings affect how specific values in the DEM are displayed.

Display as transparent pixels

For DEM files where a No Data value can not be detected, specify any value to be displayed as transparent. The unique values list displays any data values specified to be a transparent pixel. The pixels are not removed or deleted from the DEM when specified as transparent. A No Data value (-32767.00) is always displayed as transparency and cannot be removed from this list.

Add and Remove values

Click the Add New Value button to add a new value to the list (range values are not supported). Double-click the <new value> item and type in a new value (displayed with blue color). To remove a value from the list, select value(s) and click the Remove Selected Value(s) button.

DEM elevation range

When the Always scan the dataset option is enabled, the whole DEM dataset is scanned for values when first opened instead of reading a data range from the DEM header (if it exists). By default, this option is disabled, which improves performance. However, sometimes the values stored in the DEM header may be incorrect, so this option to scan the entire file will attempt to obtain the correct DEM elevation range.

Some values in a DEM can be interpreted a positive or a negative integer. If a DEM dataset doesn't specify whether it is signed or unsigned, the Use signed data range option can be enabled to specify it as signed. Essentially this option allows you to change the data type (singed/unsigned) interpreted from DEM.

Important DEM Schema Notes

Mosaic DEM files

When mosaicking DEM files together, each file must have the same DEM schema and an appropriate Vertical Unit specified. It is important to know the data range of all the DEM files that will be mosaicked so that a DEM schema can be created that encompasses the highest and lowest elevation of all the DEM files. It is recommended to use Advanced Import to mosaic multiple files that have different DEM schemas.

Esri BIL files

When opening BIL files without a colour table, the Import DEM file dialog box will show the options to Open as raster image or Open as elevation data. When a colour table (.clr file) is present in the same folder as the BIL file, it is automatically opened as a raster image.

View Elevation Values of a DEM

There are two ways to view DEM elevation values. Open the Georeference dialog box, the actual elevation value is displayed below the image preview. Move the mouse cursor to any position within the image view to see the values (updated in real time).

 

The other way is to plot points with the Color Sampler Tool and view DEM elevation values in the Survey tab of the Geographic Imager panel. Up to four different points can be viewed at a time.

DEM Schema and Scripting

The creation of DEM schema cannot be recorded by Adobe ScriptListener or by using example scripts. Any DEM schema will have to be created prior to using automation.

Out of Range DEM Values

A warning is displayed when DEM files that contain low or high elevation values out of the schema range are imported. These out-of-range images may import incorrectly.

Negative Raw Data

The Negative Raw Negative schema only imports negative elevation values. To work with both positive and negative values,  duplicate the DEM and re-open each with the appropriate DEM schema.

 

Related topics

Opening Images with Advanced Import

Terrain Shading DEMs