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Technical Article   FAQ:  Why does the data bit depth increase when a raster is projected, rotated, or clipped?

Article ID: 28470
Software:  ArcGIS - ArcEditor 8.3, 8.1, 8.1.2, 8.2, 9.0, 9.1, 9.2, 9.3, 9.3.1, 10 ArcGIS - ArcInfo 8.2, 9.0, 8.0.1, 8.0.2, 8.1, 8.1.2, 8.3, 9.1, 9.2, 9.3, 9.3.1, 10 ArcGIS - ArcView 8.3, 8.1, 8.1.2, 8.2, 9.0, 9.1, 9.2
Platforms: N/A

Question

Why does the data bit depth increase when a raster is projected, rotated, or clipped?

Answer

Pixel depth is increased to provide the space to store 'nodata' values. This allows changes, such as a shift or a mask, to be applied to the raster. Pixels within the new raster's bounding extent may need to be assigned as nodata to preserve data values and also keep a proper extent.

For example, values range from 0-255 in an 8-bit raster. If a nodata value is introduced, the bit depth must increased to 16-bit to store the nodata value, which would be the 257th value.

Related Information


Created: 1/5/2005
Last Modified: 10/6/2015

Article Rating: (2)
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Comments

By FionaG - 12/03/2015 4:05 PM

The article needs to be updated.

I'd like to revise my previous comment. The missing piece of information is that ArcGIS will assign a default nodata value to a raster that does not currently have any "nodata". The default value assigned by ArcGIS is outside the raster's current bit depth. When the raster is clipped or projected, suddenly that nodata value is required, and so the bit depth is increased by default. To prevent this, the user must change the default nodata value of the raster in ArcCatalog to something that is within the current bit depth's range before clipping or projecting.

By FionaG - 12/02/2015 9:39 AM

The article is incorrect or the solution didn’t work.

The bit depth of a clipped raster increases even when a nodata value has already been specified, both in the raster's properties and in the Clip dialog box. In this case the reason specified above does not apply. This suggests raster Clip is defaulting to increasing bit depth in all situations, which is undesirable behaviour, causing an unnecessary and unwanted file size increase. (ArcGIS 10.3)

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By CodyBenkelman - 10/06/2015 1:14 PM

The article needs to be updated.

As written, this does a disservice to users who do not want the bit depth to be increased - it does not explain that promoting bit depth to allow a unique NoData value to be defined is the DEFAULT, but not REQUIRED. The most common case is 8 bit data, and the user can enter "0" for NoData value, and avoid the changed bit depth. As long as they understand their output *MIGHT* have zero values that should be considered NoData, it is a perfectly acceptable outcome, and it is what most users *want*.

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