About serving raster and image data
Raster and image data can be published as an image service using ArcGIS Server. You can also serve the data as part of a document, such as a map or globe document, or as part of other services, such as a geodata service. An image service provides access to raster data through a Web service. An image service is always served with Image Service capabilities; however, you can also choose to serve it with a Web Map Service (WMS) or Web Coverage Service (WCS) capabilities. Once an image service is published, users can connect to it mainly through the ArcGIS Server or the Open Geospatial Consortium, Inc. (OGC), WMS or WCS service connections.
The source of the raster data can be a raster dataset (from a geodatabase or file on disk), a mosaic dataset, or a layer file referencing a raster dataset or mosaic dataset. Serving raster datasets or raster layers that define on-the-fly processing, such as symbology or raster functions, is a core capability of image services and does not require an extension. The ArcGIS Server Image extension is required to serve a mosaic dataset or a raster layer containing a mosaic function. This does not only affect the image service. For example, if you have a map document containing a mosaic dataset, you require the Image extension.
If you have a compiled image service definition file (.ISCDef) created using ArcGIS Image Server, you must first register ArcGIS Image Server with ArcGIS Server to serve the file.
Image service capabilities
When you publish an image service, the default Image Service capability is always enabled. You can also choose to enable it with Open Geospatial Consortium, Inc., WMS or WCS capabilities. The different sources for an image service result in slightly different capabilities and affect the layer properties.
An image service served with Image Service capabilities is designed for the GIS Web server architecture and is similar to the mapping capabilities of a map service. It can host any type of raster dataset or mosaic dataset or their layers. You can connect to these image services via the Add ArcGIS Server connection by entering a Universal Resource Locator (URL, such as an HTTP address) or by providing a LAN address, such as the name or IP address of a computer.
The Image Service capabilities can be broken into two main categories of operations: image and catalog. All inputs are served with an image operation, allowing you to export an image or view the on-the-fly processing and dynamic mosaicking or performing an identify function that uses metadata. Mosaic datasets allow you to serve enhanced image services that allow the operations on the catalog, such as querying, making a selection, viewing footprints, previewing each input raster, and downloading source data.
When an image service is opened within ArcGIS, it is treated like any other raster dataset, for example, there are renderers that are used to display the data, and the data can be used as input for a geoprocessing tool.
Additional layer properties will be available if a mosaic dataset is published, including allowing users to change the mosaicking options and providing specific attribute information for each raster dataset within the mosaic.
An image service served with the OGC WCS capabilities has many of the Image Service capabilities. You can enable the WCS capability on any type of image service, regardless of the raster data source type. You can connect to a WCS service via ArcGIS Desktop or third-party applications that support WCS. The layer properties of the WCS image services are the same as the default Image Service, but the raster is rendered on the client side, unlike the default Image Service, where the server renders the raster data. Therefore, the values transmitted are the raw data values; thus, the image service can be used as input for analysis or display. For example, a WCS image service layer can be used in a geoprocessing model or used to generate the surface in the ArcGlobe application.
An image service served with the OGC WMS capabilities has functionality similar to other WMS services. You can serve a raster dataset directly with WMS capabilities; you do not need to create a map document containing a raster dataset layer. An image service served as a WMS service contains only one layer, since it represents one input. You can connect to a WMS service in ArcGIS Desktop, through Web mapping applications, or using other applications that support WMS. When the raster data is served using WMS, it is rendered by the server and delivered to the consumer as a picture with coordinates. Users cannot change any of the properties of the data delivered via the WMS service. When publishing raster data using WMS, the image service is limited to grayscale or RGB color images, provided by rendering a single band (grayscale), a single band with a color map, or a three-band combination. By default, a raster dataset is served with the default layer settings (and band combinations). Additionally, the data is resampled to an eight-bit image.
Preparing image services
There are many things to consider when you are preparing to create an image service, because not all raster data is served in an equal manner; it depends on the capabilities and the operations within them you choose.
Is there one raster dataset or many raster datasets?
With ArcGIS Server, you can serve single raster datasets, such as a mosaicked raster dataset in ArcSDE or a large DEM. If you need to serve many raster datasets that compose a single image service, or you want to serve up many raster datasets that overlap completely and were captured at various dates or times and that compose a single image service, you have to create and serve a mosaic dataset.
A raster catalog cannot be served directly. You need to create a mosaic dataset from the raster catalog or add it to a mosaic dataset and serve the mosaic dataset. You can create a mosaic dataset directly from the raster catalog using the Create Referenced Mosaic Dataset tool to serve it without building any overviews or editing any of the properties of the input raster dataset, . If you need to build overviews or edit the properties of the raster datasets, or even want to combine multiple raster catalogs within one mosaic dataset, then you need to create a new mosaic dataset using the Create Mosaic Dataset tool and add the raster catalog using the Add Rasters To Mosaic Dataset tool.
Is the raster data for viewing as an image or as input for analysis?
The raster data is available for both viewing as an image and as input for analysis, depending on which capabilities you enable when you publish the service. If the users of the image service do not make adjustments to the appearance of the image, other than adjusting things like transparency, brightness, and contrast, and do not use the raster data as input for any analysis, then this data can be served with any of the capabilities. For the user to access the data in the image service to do further analysis, such as spatial analysis with the geoprocessing tools or generating a surface from elevation data, enable the additional WCS capability for the image service.
When working within ArcGIS, the Image Service capability provides the pixel information required to do analysis using the image service. However, if the user's application cannot connect to ArcGIS Server, then the WCS service is an ideal alternative.
Does the data have multiple bands or need to be enhanced?
The layer properties of an image service with Image Service or WCS capabilities allow the user to change the band combination or apply a stretch to the histogram to enhance the appearance of the image. If serving raster data using WMS, the user is not able to change the rendering or the band combination; therefore, a raster dataset layer, mosaic dataset, or mosaic dataset layer are the preferred inputs for a WMS service so the display properties can be set correctly. If a raster dataset is served using the WMS capabilities, the default raster display settings are applied. If a mosaic dataset is used, and the source images contain more than three bands, then the mosaic dataset needs to have its number of bands defined, so it serves either a three-band color (RGB) or one-band grayscale image service.
Is any processing required?
When you serve a raster dataset or raster dataset layer, the raster dataset must be in its final state and ready for the user to use. However, when you publish a mosaic dataset, processing can be defined that is applied by the server on the fly, for example, orthorectification, enhancements, band combinations, band algebra, pan-sharpening, and filtering.
Publishing image services
Image services are published similarly to all other services using ArcGIS Server. This includes right-clicking the data file in ArcCatalog and choosing Publish to ArcGIS Server or using the Server Manager Web application to publish the image services. By default, the image services are always published with Image Service capabilities, and you can optionally choose the WMS and WCS capabilities. Users can then connect to these services as they would any other published ArcGIS Server service.
You can only publish mosaic datasets if you have the ArcGIS Server Image extension.
Caching image data
Caching is generally only required when you must create the fastest possible service containing image data. Generally, the pyramids generated for raster datasets or the overviews generated for mosaic datasets result in image data being served at an acceptable rate. However, if you know that a particular image or area of interest will be repeatedly visited, you may want to generate a cache.
You cannot generate a cache on an image service, but you can generate a cache on a map service or globe service. Therefore, you need to add your image data (either the source dataset, such as the mosaic dataset or raster dataset, or an image service) to the map or globe document and serve the document. You are then able to use the caching tools to generate a cache.
There may be times when you serve a map service containing an image service. Unless there is a need to cache all the contents, it is generally recommended that you cache the vector data and not the image data.
Using an image service
You can access an image service the same way you would any other service—by first connecting to the GIS server, then choosing the image service that is available. As mentioned above, how you use the image service depends on the source data. Therefore, a served raster dataset can be used like a raster; however, a served mosaic dataset can be used like a single raster (image) or catalog. See the following to learn about using an image service: