Fundamentals about derived images

Legacy:

ArcGIS 10 is the last release of the stand-alone ArcGIS Image Server product. The image service definition (.ISDef) has been replaced by an improved geodatabase data model—the mosaic dataset—which can be published as an image service using the ArcGIS Server Image extension.

To optimize access to published image service definitions, different levels of derived images can be generated. There are three types of derived images: service overviews, derived tiles, and derived rasters. You can equate these derived images to caching or raster pyramids; however, they are not identical.

Low-resolution raster datasets can be created to view the imagery contained in the entire image service definition; this is often done using service overviews. High-resolution rasters can also be pregenerated to reduce on-the-fly processing; this is done using derived tiles or derived rasters. Creating derived images enhances the loading speed and reduces the processing time of image service definitions, because it can generate several different preprocessed images that can be served.

The parameters used to create overviews, tiles, or rasters for optimization can be set on the Image Service Properties dialog box under the Service Defaults tab. The derived images are processed raster datasets that need to be stored on disk; therefore, you will need to be sure you have the space to generate them.

The regeneration of derived images in an image service definition is managed by the Image Service Editor component of the ArcGIS Image Server. After making any modification to the derived images (such as overviews, tiles, or rasters), the derived images are automatically regenerated when running a build operation. These modifications may affect the radiometry, geometry, or visual appearance of the derived images.

The Image Service Properties dialog box includes the Skip Existing Images check box in the Service Defaults node. The Skip Existing Images check box remains unchecked by default; therefore, partial pyramiding is turned on and derived images are regenerated if higher-resolution raster datasets are modified. When you check this option, only derived images that have not yet been created are generated while optimizing the image service definition.

Image Service Editor creates a .DIInfo file of the same name as the derived image the first time it is generated. The information stored in this file is used by the program to determine if that particular derived image needs to be regenerated. It is recommended that you do not delete this file.

When you try to close a map while generating derived images, the Image Server displays an error message and prompts you to stop the build process before closing the map. After the build process is stopped, you have to close the map again. Also, when you remove the layer directly while generating derived images, the build process stops without displaying any message, and the layer is removed.

Service overviews

Service overviews are preprocessed, low-resolution raster dataset tiles covering the entire service. These overviews speed up the display of imagery at lower resolutions, because they are used when clients are viewing large portions of the image service definition at a low resolution. With service overviews, a lower-resolution copy of the data appears quickly while viewing entire datasets. When you zoom in, levels of finer resolutions are drawn and the performance is maintained because the service transmits successively smaller areas. The server chooses the most appropriate service overview based on the display scale. Without service overviews, the entire dataset would have to be processed on the fly. Service overviews enable you to create an overview of the complete image service definition for viewing.

To generate service overviews, you first need to define service overviews. This adds entries for the overviews into the service table. Now you can edit and merge the defined service overviews. Next, you need to build the service, using the Generate Derived Images option, to generate the service overview. Building the image service definition fills in the empty columns in the service table, as well as generates raster datasets that are stored next to the .ISDef folder by default. You can edit the properties used to define the service overviews on the Image Service Properties dialog box under Service Defaults. The greater the number of rasters in a service, the longer it takes to generate service overviews. The existence or pyramids (.rrd) or internal overview within the raster datasets reduces the number of service overviews that are required.

Service overviews are created using the default mosaic method; if a client defines a mosaic method that is different from the default, the output overview image is still displayed using the default mosaic method, which may be different than what the client is expecting.

Derived tiles

Derived tiles are preprocessed, tiled raster datasets, generated at a high resolution for an area defined by a polygon. These derived tiles can be generated for the areas of interest that are most often requested by clients. They can require a lot of additional disk space. They are used instead of on-the-fly, computed rasters when the client uses the default mosaic method. Derived tiles are generally not created unless there is a requirement to speed up a service that includes extensive raster dataset processing and seamline mosaicking, such as a service that includes orthorectification with pansharpening and feathered seamline mosaicking. Derived tiles can also be created to optimize image services created from formats that are slow to read.

To generate derived tiles, you first need to draw or import a polygon graphic. You can draw a polygon graphic using tools on the Draw toolbar. Next, create the derived tile using the selected graphic. You are prompted to select parameters for the pixel size to create the derived tile as well as for parameters specifying how to break the area into tiles. The created, derived tiles are added as entries in the service table. To generate the derived tiles, you need to build the service using the Generate Derived Images option.

Derived rasters

Derived rasters are preprocessed raster datasets created at the pixel size of the highest-resolution (smallest pixel size) raster in a raster dataset, and cover the complete footprint. They can be generated to reduce the time to serve data of a highly visited area of interest.

You can derive preprocessed rasters so that there is no need to perform computation on the fly. In this case, mosaicking is not performed, so tile ordering is not affected. Derived rasters are generally not created unless there is a requirement to speed up a service that includes extensive raster dataset processing, such as orthorectification and pan sharpening.

The derived rasters are added as rasters within the Raster Process Definition. To generate them, you need to build the service using the Generate Derived Images option. You can build them for every raster dataset within the image service definition, or for a selection.

Image service processes are not applied to derived tiles, since they are applied to all the raster datasets. The derived tiles are created with the same number of bands and same pixel type as the image service definition.