Data support in ArcGIS > Raster data > Building and managing a raster database
How raster data is stored in a geodatabase |
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Release 9.2
Last modified September 22, 2008 |
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This is an overview of how raster data is stored as a raster dataset, raster catalog, and as an attribute in a geodatabase. There are three main types of geodatabases: ArcSDE geodatabases, personal geodatabases, and file geodatabases.
Storing raster data in an ArcSDE geodatabase
When storing a raster dataset in ArcSDE, there are seven constituent tables: one is a business table, two are feature tables (however, in Informix and Oracle Spatial, when an RTREE index is employed, the S table is not present), and the other four are for raster storage. There are also many indexes on those tables.
The feature tables hold the footprint for the raster dataset, which is the same as when you have a feature class—one table stores the geometry and the other stores the spatial index information. For each raster dataset, there is one row in the feature table that stores the envelope.
The raster storage tables include (This schema is not valid for the Oracle GeoRaster.):
- Business table—Stores the references to the information stored in the other tables
- Raster auxiliary table (AUX)—Stores optional metadata, such as raster statistics, a color map, or transformation information if it exists
- Raster block table (BLK)—Stores the pixels of each block in a raster band
The block table is the largest table and the one that stores the actual pixel information and pyramids. - Raster band table (BND)—Stores band information
- Raster table (RAS)—Stores a record for each raster dataset
ArcSDE evenly tiles the bands into blocks of pixels according to a user-defined dimension (the default is 128 by 128). Tiling the raster band data enables efficient storage and retrieval of the raster data. The pyramid information is stored according to a declining resolution. The height of the pyramid is determined by the number of levels specified by the application or user.
The BLK table stores one row per block (tile) per band in a raster dataset and per pyramid level. For example, a three-band raster divided into 12 blocks with no pyramids built will have 36 rows in the BLK table—12 separate blocks for each of the bands. The column containing the pixel data for the block is a binary large object (BLOB).
A raster catalog is stored in ArcSDE as multiple rows in the business table, whereas a raster dataset is only a single row in the business table. The table schema is the same as that of a raster dataset. The only difference is that the feature table will have many rows, whereas each row represents the extent of a raster dataset in the raster catalog.
When storing a raster dataset as an attribute, the storage architecture for the raster dataset is the same as for raster catalogs. Each record in the business table will have an attribute value for the column of type RASTER. This attribute will be used to relate the business table to the supporting raster tables (RAS, AUX, BND, BLK). When you load the image, it will be converted to an ArcSDE raster format and the pixels will be stored in the BLK table.
Learn about storing raster datasets and raster catalogs in a geodatabase in DB2
Learn about storing raster datasets and raster catalogs in a geodatabase in Informix
Learn about storing raster datasets and raster catalogs in a geodatabase stored in Oracle
Learn about storing raster datasets and raster catalogs in a geodatabase in SQL Server
Storing raster data in a personal geodatabase
In a personal geodatabase, the raster dataset is converted to an IMG file and stored inside an image database (IDB) folder. The IDB folder is located in the directory next to the personal geodatabase. When you delete a raster dataset, the raster in the IDB folder is permanently deleted.
When storing a raster catalog in a personal geodatabase, the raster catalog is a table that points to the stored raster datasets it contains. These raster datasets can either be stored as managed or unmanaged. If it is managed, then the entry in the raster catalog table points to the location in the IDB file where the raster dataset is stored. The IDB folder is organized so that it can be referenced to a row in a raster catalog. In an unmanaged case, the raster catalog contains the path location where the raster datasets are stored. Each row in the raster catalog business table points to the stored raster dataset. The operations on an unmanaged raster catalog do not affect the stored raster files; therefore, if you delete the raster datasets in a raster catalog, they will only be deleted from the raster catalog and not from the disk.
When storing a raster dataset as an attribute, the raster is stored as an IMG file in the system-defined location or as it is in the file system; this depends on whether it is managed or not. The storage is similar to a raster catalog.
Storing raster data in a file geodatabase
The file geodatabase is new for ArcGIS 9.2. This method of storage shares features from both the ArcSDE geodatabase and the personal geodatabase. Managed raster data is stored in the same way as an ArcSDE geodatabase, and unmanaged raster data is stored in the same way as a personal geodatabase.
When storing managed raster datasets, there are five constituent tables: one is a business table and the other four are for raster storage. There are also many indexes on those tables. One of the other tables is the feature, where only one row in the feature table stores the envelope of that raster. There is one main table that is the largest table and stores the actual pixel information and pyramids. All these tables are stored in a native file format that is hidden from the user; they are not directly accessible. The main table storing the actual pixel information stores one row per block per band in a raster dataset, and per pyramid level. The column containing the pixel data for the block is a BLOB.
The bands are tiled into blocks of pixels according to a user-defined dimension (the default is 128 by 128). Tiling the raster band data enables efficient storage and retrieval of the raster data. The pyramid information is stored according to a declining resolution. The height of the pyramid is determined by the number of levels specified by application.
A raster catalog is stored as multiple rows in the business table, whereas a raster dataset is only a single row in the business table. The table schema is the same as that of a raster dataset. The only difference is that the feature table will have many rows, whereas each row represents the extent of a raster dataset in the raster catalog.
When storing a raster dataset as an attribute, the storage architecture for the raster dataset is the same as for raster catalogs. Each record in the business table will have an attribute value for the column of type RASTER. This attribute will be used to relate the business table to the supporting raster tables.
When storing unmanaged raster datasets or raster catalogs, each are represented by tables that contain the path location where the raster datasets are stored. In the case of a raster dataset, the table only has one row, and for raster catalogs, it has a row for each raster dataset it contains. If you delete a raster dataset, its reference is only deleted from the file geodatabase, not from its stored location on disk. Likewise, if you delete a raster dataset from a raster catalog, it will only be deleted from the raster catalog and not deleted from the disk.
When storing a raster dataset as an attribute, the raster is stored as it is in the file system and only the path location on disk where the raster datasets are stored exists in the file geodatabase.