This is part 1 in a 2-part blog. Part 1 provides an overview of pyramids and overviews. Part 2 will provide you with some guidance on generating them when creating a mosaic dataset.
Basically—overviews are not pyramids and pyramids are not overviews. But pyramids generated by ArcGIS have an .ovr extension (short for overview)…Wait, did I just write that?
Yes, the storage format for a pyramid is an .ovr file. But please don’t confuse this with overviews. Fortunately, overviews are organized in a folder named *.overviews. Both are similar but pyramids are created for raster datasets and overviews are created for mosaic datasets.
Pyramids |
Overviews |
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Description |
Lower resolution (downsampled) images of the original data. |
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Purpose |
Improve display speed and performance. |
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Created for |
Raster datasets |
Mosaic datasets |
Format |
Writes .ovr files—with a few exceptions. |
Writes as .tif files. |
Reads pyramids stored externally as *.ovr or *.rrd or internally (e.g. MrSID) |
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Storage |
In a single file that generally resides next to the source raster dataset and using the same name. |
By default, in a folder next to the geodatabase with a *.overviews extension, or internally for ArcSDE.Storage location is customizable. |
Storage size |
2 to 10% (compared to original raster datasets) |
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Downsampling factor |
2 |
3 (default) |
Extent |
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Options when building |
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Why are you generating them?
Pyramids aren’t mandatory—but without them, the display speed of your raster dataset can be prohibitively slow, especially if the datasets are very large.
Overviews aren’t mandatory—but they are highly recommended. You do not have to create them or you can generate them in only particular parts of the mosaic dataset, such as a highly visited part of the imagery. However, if you don’t create them you may not see any imagery (you may see a wireframe instead or gray images), since there is a limit to the number of rasters that will be processed at one time (which you can change). Without them, the mosaic dataset may display slowly because of all the processing.
How to create pyramids
If a raster dataset doesn’t have pyramids, then you will often be prompted to create them when you display the data in an ArcGIS application, such as ArcMap. But it’s better to create them before you use the data. Pyramids can be created using geoprocessing tools. There are a few tool choices, since it depends on whether you have one or many datasets to process. To learn about these, see Building pyramids using geoprocessing tools. You can change the properties of the pyramids, such as the resampling method and compression via the geoprocessing environments.
How to create overviews
To create overviews, first they are defined, then generated. When they are defined, the application analyzes the mosaic dataset and using the parameters set for the overviews it defines how many are needed, at what levels, and where. Then they are added as items in the mosaic dataset which appear as new rows in the attribute table. At this point, only the rows have been created to identify the properties and number of the overviews. Next, the overview files are generated. Both defining and generating can be done with one tool—Build Overviews. However, if you need to modify any properties, such as defining a new output location or tile size, then you must run Define Overvews first (to define the properties and add the items to the attribute table), then run Build Overviews to generate the overview files.
When you do generate them, the mosaic dataset keeps track of any changes made to it, such as updating an image, adding or removing images, or altering the footprints. By running the Build Overviews tool or Synchronize Mosaic Dataset tool, with the appropriate options, the overviews will be updated.
Associated blogs
- Oh where oh where have my pyramid files gone?
- Pyramids and overviews, or pyramids or overviews?
- Take a look at raster compression, pyramids, time, and space
Submitted by: Melanie Harlow
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