Mosaicdatasets are the basis of image management. This guideassumes that you have a basic knowledge of creatingmosaic datasets from different types of imagery.
Forsimple collections of imagery, the general guidelines outlined here can be followedto create mosaic datasets (using default values) that can be useddirectly, served as image services, used for caching, or used as input to the raster analytics capability of ArcGIS Image Server.
Though not addressed in the Standard Workflow, raster functions can be applied to mosaic datasets, allowing you to define processing operations that will be applied to one or more rasters. Details and instructions for each of the availablefunctions can be found in the ArcGIS Help.
Mosaicdatasets can be used to manage a wide range of imagery types,from preprocessed orthophotos, to elevation data, to unprocessedsatellite imagery. The volume and number of images can quicklybecome massive. To enable flexibility and scalability in managingsuch large volumes of imagery, there are many different tools towork with mosaic datasets, and many parameters that can be set tooptimize them for different types of imagery. The imagerymanagement workflows on the ArcGIS Imagery Workflows site outlinebest practices and provide scripts and sample data for working withdifferent types of imagery.
These workflows assume that largercollections of imagery are to be managed. In such cases, it isoften impractical to work with a single mosaic dataset for allimagery, so the workflows follow a pattern of using source,derived, and referenced mosaic datasets. These are described in Imagery Data ManagementPatterns and Recommendations. This pattern breaks apotentially complex task into smaller tasks, and makes it easier tomanage multiple sources, perform quality assurance of the mosaicdatasets, and maintain the services.
Although it is possible to create a single mosaic dataset from many collections of imagery, the best practice is to use a combination of different mosaic datasets, as is summarized in the following sections and diagrammed in the image below.
Source mosaic datasets
Foreach collection of similar images, a source mosaic dataset iscreated. A source mosaic datasetrepresents a single manageable unit typically used for checking that metadata is defined correctly, defining specificprocesses to be applied, or doing quality assurance. Each record inthe source mosaic dataset defines a dataset with specificmetadata. A source mosaic dataset could represent all imagery from aspecific type of sensor, or represent imagery that was acquired as apart of a discrete project that covers a known extent or period in time. The number ofimages in each source mosaic dataset typically ranges from tens tohundreds of thousands of images. Source mosaic datasets aregenerally not made accessible to the end users or served as imageservices.
All imagery in a source mosaicdataset should have the following attributes:
- Similar in terms of the number of bands, bitdepth, and type of metadata
- Require a single raster type when added to the source mosaic
- Have similar scales or pixel size (though possibly in different projections)
Typically, if modifications to the raster item (within the mosaicdataset) are required—clipping images to a footprint for example, or applying astretch or orthorectification—they are defined andrefined in the source mosaic dataset.
The spatial reference of asource mosaic dataset should be the best choice to encompass allimagery. For example, do not use a state plane projection tocontain data across an entire country. Instead, use a projectionsuitable to contain the entire country's data. The imagery to beadded to the source mosaic should within the extent horizon of the selected spatialreference system.
The number of bands and bit depth of the sourcemosaic dataset are set to be suitable to contain all the data. Forexample, a source mosaic dataset with high-resolution satelliteimagery, such as GeoEye-1, IKONOS, or QuickBird, would be definedas 4-band, 16-bit (not 1-band 8-bit).
In some but not all cases,overviews may be generated for a source mosaic dataset to enablevisualization at smaller scales.
Sourcemosaic datasets do not have to be static; over time additionalrasters can be added. In some workflows, source mosaic datasets arecreated manually, while for others the creation of source mosaicdatasets may be fully automated.
Derived mosaic datasets
Derived mosaic datasets are created from multiple source mosaic datasets. The derived mosaic dataset typically defines an imagery product that is to be served for a specific purpose—for example, natural color imagery for visual interpretation, multispectral imagery for analysis, or digital surface models that best represent the terrain.
Imagery is added to the derived mosaic dataset primarily by using the Table raster type. This enables all records from one or more source mosaic datasets to be added. In some cases, only a subset of the source mosaic dataset(s) will be added to a derived mosaic dataset. For example, images with too much cloud cover may be excluded based on metadata provided in the source mosaic dataset. All properties and metadata from the source records are copied to the derived mosaic dataset. The spatial reference of the derived mosaic dataset is set to encompass all the imagery, and the number of bands and bit depth is set to be appropriate for all the data sources.
Optionally, functions can be applied to transform the data. Forexample, the Extract Bands function may be used to convert imageryfrom 4-band to 3-band, or a stretch might be applied to convertfrom 16-bit to 8-bit. In most cases, each derived mosaic datasetwill have a range of functions added to define different products.For example, a mosaic dataset that provides elevation data may havea set of functions added to provide hillshade, slope, and aspectrepresentations.
Multiple derived mosaic datasets may use the same source mosaicdatasets. For example, a derived mosaic dataset for natural colorimagery and one for enabling multispectral analysis may use thesame source mosaic dataset from a high-resolution satellite.
Insome cases, imagery is directly added to a derived mosaic dataset, rather than organized into a source mosaic dataset first.For example, an image source such as World Imagery or NaturalVue (available onArcGIS Online as an image service or cached map serviceproviding global 15-meter resolution imagery) may be added toprovide a background image for natural color imagery, or anoverview image from some other source may be added to providecontext at small scales. If no suitable overview exists for thederived mosaic dataset, then overviews may be built.
Derived mosaicdatasets do not need to be static, and over time, the source mosaicdatasets from which they are derived may change or new sourcemosaic datasets may be added. To update the derived mosaicdatasets, two different approaches can be used. The Synchronize MosaicDataset tool can be used, which checks for changes in allsources and updates any changes. Alternatively, if the process of creating the derivedmosaic dataset is automated, the derived mosaic dataset can bere-created, as the process is generally very fast.
Derived mosaicdatasets may be directly served, but since serving a mosaic datasetcan lock tables, often referenced mosaic datasets are usedinstead.
Referenced mosaic datasets
Referenced mosaic datasets are sometimes created by referencing derived mosaic datasets.
They define parameters that are either defaults or enforce specific rules to be applied when the imagery is accessed. For example, from a derived mosaic representing elevation data for the whole world, a referenced mosaic dataset may be created to define a hillshade or slope map product for a selected area.
Referenced mosaic datasets are also often created to define different restrictions. For example, downloading may be restricted in one service, but enabled in another that is used for geoprocessing.
Referenced mosaic datasets are also used to create subsets. For example, a referenced mosaic dataset may be defined with a limited boundary or query to limit access to a specific area or type of imagery.
Next: Overview of serving imagery
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