An overview of raster storage settings


The raster storage settings section of the Environment Settings dialog box provides settings for raster dataset results. Included are the following settings that can be changed: the default compression type, the default settings for pyramid creation and calculating statistics, and the tile size used when loading raster data to an existing raster dataset.

Not all settings apply to all storage types. Refer to the Raster storage matrix (below) for more details. File Group 2 consist of the following file formats: BSB, BMP, DEM, ECW, GIF, JPEG, JPEG2000, MAP, PIX, PNG, and XPM. All remaining ArcGIS supported raster file formats fall into File Group 1.


Raster storage matrix
Storage settings File Group 1 File Group 2 personal GDB File GDB ArcSDE
Pyramids yes yes yes yes yes
    Resampling
automatic automatic automatic yes yes
    Levels
automatic automatic automatic yes yes
Raster statistics automatic yes automatic yes yes
    skip factor
yes automatic yes yes yes
    ignore value
yes automatic yes yes yes
Compression yes* yes* yes yes yes
    LZ77
yes* no yes yes yes
    JPEG
no yes* yes yes yes
    JPEG 2000
no yes* yes yes yes
Tile size no no no yes yes

NOTE: Compression is dependant on the type of file format. Please refer to the Technical Specifications to see which file formats are able to support compression.

The following subtopics supplement the reference topics found in Environment settings.

Compression type

The compression type setting is used by any tool whose output is a raster dataset. Three types of compression are supported: LZ77, JPEG, and JPEG2000.

LZ77 (the default) is a lossless compression that preserves all raster cell values. It uses the same compression algorithm as the PNG image format and one similar to ZIP compression. As you can rely on the pixels not changing their values after you compress them, use LZ77 for performing visual or algorithmic analysis.

JPEG is a lossy compression, because raster cell values may not be preserved after compression and decompression. It uses the public domain JPEG (JFIF) compression algorithm and only works for unsigned 8-bit raster data (single-band grayscale or three-band raster data).

JPEG2000 uses wavelet technology to compress rasters, so they visually appear lossless, meaning that although the cell values do get manipulated, the differences between the original and the same raster with compression are not easily distinguishable. Use JPEG or JPEG2000 for rasters that are meant as pictures or backdrop imagery.

If JPEG or JPEG2000 is selected, you can also set the compression quality to control how much loss the image will be subjected to by the compression algorithm. The values of the pixels of an image compressed with a higher compression quality will be closer to those of the original image. Valid value ranges of compression quality for JPEG are from five to 95. Valid value ranges for JPEG 2000 are from 1 to 100. The default compression quality is 75. The amount of compression will depend on the data and compression quality. The more homogeneous the data, the higher the compression ratio. The lower the compression quality, the higher the compression ratio. Lossy compression normally results in higher compression ratios when compared to lossless compression.

The primary benefits of compressing data are that compressed data requires less storage space and data display times will be quicker, as there is less information to transmit.

Pyramids

pyramid are reduced-resolution representations of your dataset. They can speed up display of raster datasets by retrieving only the data that is necessary at a specified resolution.

By default, pyramids are created for raster datasets by resampling the original data. There are three resampling methods available: nearest neighbor, bilinear, and cubic.

The default is nearest neighbor. It works for any type of raster dataset. Use nearest neighbor for nominal data or raster datasets with colormaps, such as land use data, scanned maps, and pseudocolor images.

Use bilinear interpolation or cubic convolution for continuous data, such as satellite imagery or aerial photography.

If you uncheck Build pyramids, pyramids will not be created with the output raster. Not building pyramids saves storage space but will lead to slower display speeds, especially for larger raster datasets.

Statistics

The Statistics option enables you to build statistics for output raster datasets. Statistics are required for your raster dataset to perform certain tasks in ArcMap or ArcCatalog, such as applying a contrast stretch or classifying your data. It is not essential to build statistics if they have not already been calculated, since they are calculated the first time they are needed. However, it is recommended that you calculate statistics for your raster datasets before using them if you want to use certain features that require statistics. The default display of your raster will be improved in most cases if statistics have already been calculated, because a standard deviation stretch is applied if statistics are present.

Setting a Skip factor allows you to speed up the process of calculating statistics by skipping pixels. The Skip factor does not apply for GRID datasets.

Values you set to ignore will not participate in the statistics calculation. Normally, you may want to ignore the values of the background. This only applies to personal geodatabase raster datasets.

Tile size

The tile size setting is used by any tools that create raster datasets and are stored in blocks.

The default tile size is 128 by 128, which is good for most cases. However, if the tile size is too big, you will end up bringing up more data than is needed each time you access the data. For example, you want to display a window of 100 by 100 and it only covers one tile. If you set the tile size to 512, you need to get the tile of 512 by 512 pixels. If your tile size is set to 128 by 128, you'll bring up less extra data if the display window is 100 by 100.

See Also