Nowadays, acquiring imagery has become increasingly convenient and efficient, and the need for managing and publishing large volumes of imagery has also become more common. To facilitate this process, a solution based on mosaic datasets is provided.

A mosaic dataset manages data using a metadata + raw imagery file approach. When adding imagery data to a mosaic dataset, only the metadata such as image file paths, footprints, and resolutions are recorded in the mosaic dataset, and the required imagery files are loaded based on the metadata when needed. Compared to traditional database storage methods, this approach significantly improves data import speed while reducing disk space usage.

Features of Mosaic Dataset

• Supports Different Raster Formats: A mosaic dataset is used to manage raster-format data, allowing different formats (*.tif, *.tiff, *.img, *.ecw, *.pix, *.sid) to be added to the same mosaic dataset. It also supports bulk addition of large volumes of imagery.
• Consistent Coordinate System for Subdatasets: The imagery added to a mosaic dataset may have different coordinate systems. For example, in the global imagery mosaic dataset shown below, the original imagery files use a UTM projection based on WGS1984. To ensure seamless dynamic mosaicking, the projection must be unified into a common geographic coordinate system, WGS1984. Therefore, when creating a mosaic dataset, its coordinate system should be set to WGS1984.

  Figure: Managing Global Imagery Data Using a Mosaic Dataset

• High Import Efficiency, Low Disk Usage: Managing large volumes of imagery with a mosaic dataset not only ensures high import efficiency but also minimizes disk space usage. Test results show that a global 30-meter resolution DEM dataset containing 22,667 images (0.98TB) was entirely imported into a UDB datasource in 14 minutes. After import, the UDB file size was 16MB, and the UDD file size was 22MB.

  Figure: Managing Global DEM Data Using a Mosaic Dataset

Preparation for Use

Before adding imagery to a mosaic dataset, you need to understand the basic details of the imagery being imported, including file size, number of files, and whether pyramids have been built to ensure adequate preparation.

• Estimating Import Time and Disk Usage: By assessing the file size and number of files, you can estimate the required time and disk space for the import process.
• Pyramid Construction and Block Storage: Both of these factors significantly improve display efficiency. Building image pyramids and converting images to block storage can be done either before or after importing the imagery.
• Performance Optimization: Image pyramid generation, block storage conversion, and later map tile generation support multi-threading. To achieve higher efficiency, using a high-performance machine for these operations is recommended.

The above considerations focus on the resources required for image import. Additionally, understanding the imagery content is essential—whether it represents land surface reflectance or elevation data, as this influences how the imagery is visualized in maps. For example, DEM data can be displayed using color ramps and 3D hill shading effects.

Checking Projection Information

When creating a mosaic dataset, it is crucial to ensure that the imagery displays efficiently and correctly in the target coordinate system. You need to check the coordinate system of the imagery files (by opening them as a datasource and checking dataset properties) and determine the coordinate system required for displaying the mosaic dataset in the map.

For example, if the original imagery files use a China2000 zone projection, and the map display requires China2000, the mosaic dataset should be set to China2000. Since the map internally converts the imagery from the China2000 zone projection to China2000, no additional user processing is needed. Setting the mosaic dataset coordinate system to the map's required projection results in higher display efficiency than setting it to the original image projection and enabling dynamic projection on the map. Therefore, dynamic projection is not recommended when using mosaic datasets.

Handling Different Projection Scenarios

1. If the original imagery files have a coordinate system different from the required map display coordinate system, it is recommended to set the mosaic dataset's coordinate system to match the required map display coordinate system.

   • Example: If the original imagery uses China2000 zone projection, but the map requires China2000, then set the mosaic dataset's coordinate system to China2000.

2. If the original imagery files already have the same coordinate system as the required map display coordinate system, set the mosaic dataset's coordinate system to the same coordinate system.

3. If the original imagery files lack a coordinate system (e.g., use a planar coordinate system) but the user knows the correct coordinate system, there are two possible handling methods:

   • (1) Do not modify the original imagery files and set the mosaic dataset coordinate system to the known data coordinate system. In this case, the map display coordinate system must match the data coordinate system.
     • Example: If the original imagery has no coordinate system but the data is known to use China2000 zone projection, then set the mosaic dataset to China2000 zone projection. The map display must also use China2000 zone projection, and dynamic projection cannot be used, as the system cannot perform automatic reprojection on images without coordinate system metadata.
   • (2) Assign the known coordinate system to the original imagery files, effectively converting them into georeferenced images. The mosaic dataset's coordinate system can then be set according to scenarios (1) and (2) above.

Related Topics

Mosaic Dataset Management

Display Mosaic Datasets

Build Mosaic Datasets