On-the-Fly Mosaicking Continued from page 11 such as brightness, contrast, and gamma, through multiple stretch types.  Convolution function—Performs filtering on the pixel values in a raster, which is primarily for sharpening an image. It can also be used for blurring an image, detecting edges within an image, or other kernel-based enhancements. The mosaic dataset also includes an advanced color correction option to match the color of different images and remove Mosaic dataset Mosaicked image—stretch function applied fx Mosaic dataset Mosaicked image—convolution applied fx Pixel being recalculated Pixels included in the calculation, using a 3x3 kernel kernel path Stretches are generally used to enhance imagery as well as to convert imagery with high bit depths to the 8-bit depth required for display. lighting trends in the imagery to improve its visual quality. Color correction is defined using the color correction tool and determines for each raster a set of color corrections to be applied on the fly, enabling mosaic datasets to return imagery that is visually pleasing as well as scientifically correct from the same source without data duplication or extensive processing. Serving Mosaic Datasets Image services are a service provided by ArcGIS Server for the optimized serving of imagery. Any raster dataset or raster layer can be served as an image service so that it is accessible to a wide range of desktop and Web applications. These image services are dynamic. Client applications define properties such as the extent of the request, projection, and sampling method. When the server receives a request for imagery, it accesses and processes the imagery as required. Client requests can include information on the compression to be used for transmission. Setting a lower compression quality enables users to quickly access imagery over low-bandwidth networks (for example, for navigation purposes) and then set high quality to get imagery for analysis purposes. Image services can return imagery as a picture that can be used as a background for applications or as data values that can be used in analysis. Image services provide a one-to-many relationship between source raster datasets and image services. A single raster dataset can be served in multiple forms. The functionality, if further extended in client applications, can also define additional processing that is performed on the server. The Image extension extends ArcGIS Server to serve mosaic datasets. This enables a many-to-many relationship so that a large collection of images can be served as a single virtual image in multiple forms. Using the Image extension, large collections of imagery become accessible. These image services can be accessed not only as an image but also as a catalog that enables applications to access the rich metadata or define selection queries to search or refine the imagery to be displayed. If a user requires imagery locally, either it can be exported to a specified extent and format or the original pixels can be downloaded. Conclusion Mosaic datasets resolve many of the traditional raster management issues by cataloging large collections of imagery and performing on-the-fly processing and dynamic mosaicking. The benefits of using mosaic datasets include  Reducing processing time because imagery can be directly used without extensive preprocessing  Maintaining information on overlapping imagery that is traditionally lost when creating a static mosaicked image  Easily handling large disparate datasets such as imagery along pipelines or transportation corridors  Handling datasets with different resolutions without the need to sample the imagery up or down  Reducing resampling of source rasters thereby retaining image quality and integrity  Reducing storage by removing redundancy traditionally created with multiple image products  Easing maintenance because imagery can be added or changed as required  Retaining valuable information by maintaining metadata For more information, visit help.arcgis.com and search on “Essential raster data vocabulary.” 12 ArcUser Summer 2010 www.esri.com