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What does the pixel depth mean?
The pixel depth, or bit depth, determines the range of values that a particular raster file can store, which is based on the formula 2^n where n is the pixel depth. For example, an 8-bit raster can have 256 unique values, which range from 0 to 255.
The following indicates the range of possible stored value for each type of bit depth.
unsigned 1 bit = 0 to 1
unsigned 2 bit = 0 to 4
unsigned 4 bit = 0 to 16
unsigned 8 bit = 0 to 255
signed 8 bit = -128 to 127
unsigned 16 bit = 0 to 65535
signed 16 bit = -32768 to 32767
unsigned 32 bit = 0 to 4294967295
signed 32 bit = -2147483648 to 2147483647
floating point 32 bit = -3.402823466e+38 to 3.402823466e+38
There are exceptions to this that are hidden from the user. A GRID dataset is always 32 bit (either signed, unsigned, or float), but ArcGIS shows them as being the most appropriate bit depth above; ArcInfo Workstation always indicates the bit depth of the GRID.
When NoData is added to a raster that already has a full bit range, that means that there is at least one cell in the raster extent occupying all of the values in the bit range. If for example, 0 thru 255 are all represented, it will be promoted to the next higher bit depth. Another example, a hillshade grid with cell values of 0 - 255, which then has NoData added to it, will be represented as unsigned 16 bit.