Creating raster overviews

You can create raster overviews of various dimensions. It is recommended that you create all of the powers-of-two overviews that your file requires; however, you can also create powers-of-three overviews or, if necessary, custom overviews.

That is, a power-of-two decimation creates overviews in powers of two, provided the smaller dimension of the overview is greater than 64 pixels. For example, with an input file of 8000 x 600 pixels, the overview levels created would be 2, 4, and 8, which would correspond to overviews of 4000 x 300, 2000 x 150, and 1000 x 75 pixels, respectively.

A power-of-three decimation creates overviews in powers of three, provided the smaller dimension of the overview is greater than 64 pixels. For example, with an input file of 8000 x 600 pixels, the overview levels created would be 3 and 9, which would correspond to overviews of 2667 x 200 and 889 x 67 pixels, respectively.

With a custom decimation, the levels are created based on the levels you specify. For example, entering values 2,3,8 on a file of 8000 x 600 pixels creates overviews of 4000 x 300, 2667 x 200, and 1000 x 75 pixels, respectively.

You can also create overviews for file formats that, by default, do not support overviews.

To create raster overviews

In the Overview Manager window, under General Information, do the following:
- In the File box, type or select the full path and file name of the file for which you want to create overviews, or click Browse to open a folder containing the file you want.
- In the Layer box, type or select the layer you want to use to create the overview.
  
  You can build an overview for all layers or for a single layer; that is, you can create different overviews for different layers.
Notice that the values for Raster Size and Type are updated with information on the selected layer.
Under New Overviews, in the Downsampling method list, click one of the following:
- Nearest neighbor: Each pixel in the resampled raster acquires the same value as its nearest neighbor in the original raster. Although this method computes quickly, it can introduce small errors in the resampled raster, which may cause it to have a "jagged" appearance.
- Block average: Calculates the average value of a group of image pixels and uses that value for the screen pixel. This method often provides a more meaningful representation of the image in the viewer. This method is well suited to continuous-tone images, such as radar imagery.
- Block mode: Finds the most common value in a block of the image pixels and assigns that value to the output pixels. This method is well suited to thematic data, where any mathematical operations on pixel values (class numbers) have no meaning.
In the Decimation rule list, click one of the following:
- Powers of 2: overview levels are created automatically in powers of two (that is, 2, 4, 8, 16...) based on the dimensions of the raster.
- Powers of 3: overview levels are created automatically powers of three (that is, 3, 9, 27, 81...) based on the dimensions of the raster.
- <Custom>: overview levels are created based on the values you enter in the Decimation Levels box.
Click Run.

Under Existing Overviews, information on any existing overviews associated with the active layer is displayed, such as the type of sampling, the associated number of levels, and the overview decimation levels.