Cell size ranges are used to determine which rasters are processed to create the dynamically mosaicked image from the mosaic dataset. You can view the cell sizes of the raster data in the mosaic dataset by viewing the attribute table. The LowPS and HighPS columns define the actual range of cell (pixel) sizes the mosaic dataset will read from the raster datasets. The two values are calculated based on the source raster resolution, the pyramid levels, and also the settings in Raster Pyramid Options when using the Add Rasters To Mosaic Dataset tool. For example, a raster dataset that contains cells of 1 meter and a pyramid with cell sizes of 2, 4, 8, and 16 meters would have a LowPS of 1 and a HighPS of 16 if all the pyramid levels are used. If you set the Maximum Cell Size to 8 in the Raster Pyramid Options, the HighPS will be 8 instead of 16.
The cell size values are populated when the raster datasets are added to the mosaic dataset and their units are defined by the spatial reference system of the mosaic dataset. These cell sizes are only used to identify the range of cell sizes and are not used when georeferencing the rasters; thus, the accuracy of the values has little importance.
The MinPS and MaxPS columns define the range of cell size requests for which the raster dataset should be used. These values are populated when you check the compute cell size ranges option on the Add Rasters to Mosaic Dataset dialog box or use the Calculate Cell Size Ranges tool. MinPS and MaxPS define the range of scales at which the raster should be displayed. The raster is only displayed if the size of the request is between these ranges.
- MinPS is equal to the minimum visibility of the raster. If a raster is to be visible to all large scales, the value would be 0. If the raster should turn off when the request pixel size is less than 2 meters, the value would be 2.
- MaxPS defines the smallest scale at which the raster should be displayed. For example, if the raster should turn off when the screen pixel size is larger than 30 meters, this value would be set to 30. When the pixel size is recomputed, this value is determined by the resolution of the overlapping imagery.
You can take advantage of the cell size ranges when using multiple resolutions of raster datasets for a single mosaic dataset. For example, at a countrywide level, you could use Landsat satellite images with approximately a 30-meter cell size to display when viewing the mosaic dataset at a particular scale and area of interest. However, as you zoom in, requesting a larger scale, the mosaic dataset can provide a SPOT image with a 5-meter resolution, and as you zoom closer, you will eventually see aerial orthophotography, which has a much smaller cell size (higher resolution).
You can also take advantage of the cell size ranges when generating overviews. Overviews can be generated for use when the mosaicked image is created at particular scales; therefore, an overview can be created for all the images displayed when viewing an area of interest covering the entire mosaicked area, and other overviews can be generated for different scale ranges—with the scale range being specified using cell size ranges.
The LowPS and HighPS values are determined when the raster data is added to the mosaic dataset and should not be changed:
- LoPS is equal to the source pixel size of the raster.
- HiPS is equal to the maximum pixel size of the pyramids that are used.
Since there is a direct relationship between them, the formulas for calculating the scale or the cell size are as follows:
- Cell Size = Scale * 0.0254 / 96
- Scale = Cell Size * 96 / 0.0254
For example, if the scale is 1:20,000, the cell size would work out as 20,000 * 0.0254 / 96, or roughly 5.29 meters. If the cell size is 5 meters, the scale would be determined to be 5 * 96 / 0.0254, or about 1:18,898.
The cell size is used in a mosaic dataset, instead of scale, because a cell size is an inherent value of a raster dataset.
The minimum cell size defines the lower limit of the image; therefore, any requests for imagery below this value will not be handled. The default is 0, but it can be set to a larger number to limit the resolution at which the imagery can be accessed. If a request is made below this resolution, the request fails and no image is returned.
The maximum cell size defines the upper limit of the image; therefore, any requests for imagery above this value will not be handled. The default value is a large number, unlike the minimum cell size default. If a request is made above this resolution, the request fails and a black-and-white checkerboard pattern is returned. There is a maximum cell size range factor that is used to define the maximum cell size. This value defines the maximum factor by which a cell will be subsampled. By default, this is 10.