What is a "Nodata value"

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A nodata value simply is a pixel value in a vector or raster map representing no information. It is not to be confused with a pixel value of 0 (zero).

E.g. in a raster-map containing temperatures of a surface in °C, a pixel value of 0 represents a temperature of 0 °C, while a nodata-pixel simply represents a part of the surface where no information on the temperature is available.

Accumulations of neighboring pixels result in so called “nodata-areas”.

The way nodata values are saved depends on the respective data type, but may also vary within in the latter. They may be saved as numbers or strings. Examples are shown in the table below.

Data type Nodata value (string) Nodata value (number)
GRASS raster null
GeoTIFF nan
Arc/Info ASCII Grid -9999
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