Unsigned Integer vs Signed Integer for Raster Data

Unsigned Integer vs Signed Integer

Last Updated: Jan 24, 2017

Briefly:

An unsigned integer is always non-negative numbers.

A signed integer can store negative values.

But how are they used for raster data?

Unsigned Integer Raster

With an 8-bit unsigned raster, valid values are from 0 to 255. This means that an 8-bit raster has 256 values in total.




When you talk about valid ranges (or pixel depth), this is the radiometric resolution of a raster image.

Radiometric resolution is the amount of detail in each pixel expressed in units of bits. A wide range of values gives the ability for pixel values to discriminate very slight differences in energy.

All things being equal, an 8-bit raster produces significantly greater detail than a 4-bit raster.

Radiometric Resolution
Radiometric Resolution

The valid range for a 8-bit signed is -128 to 127. If you have values larger than 127 or less than -128 in the input, than you will need to use 16-bit signed.

Some raster formats do not support signed images:

Signed Integer vs Unsigned Raster Formats

Unsigned Integer vs Signed Integer

When you convert an image in ArcGIS ( Data Management Tools > Raster > Raster Dataset > Copy Raster  or another program, specify the pixel depth and file type.

When exporting as a signed raster, make note of the formats that support it.

For example:

  • JPEG, JP2, BMP, GIF and PNG do not support 8-bit signed, 16-bit signed or beyond. This means that you won’t be able to store negative values with these raster formats.
  • IMG, TIFF (GeoTIFF), GRID, JPEG, JP2, BMP, GIF, PNG, BIL/BIP/BSQ and DAT raster formats can store negative values.
  • IMG, TIFF an GRID are the most versatile formats. These formats accept 8-bit unsigned, 8-bit signed, 16-bit unsigned, 16-bit signed, 32-bit signed and 32-bit floating.

In Conclusion

Next time you’re selecting a raster output, carefully select whether or not it’s an unsigned integer or signed integer.

Unsigned numbers are always non-negative numbers.

Signed numbers can store negative values.

IMG, TIFF and GRID are acceptable formats for 8-bit, 16-bit and 32-bit signed integers.

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