Nullkohad

Nullkohad is a term used in geographic information science to describe locations that are absent from a given spatial dataset. The word is formed from Estonian roots meaning zero and places, reflecting the idea of data absence rather than physical emptiness. In practice, a nullkohad may refer to coordinates, polygons, or areas for which no attribute value is recorded in a dataset such as a census, land-use map, or transportation network.

A nullkohad can arise at any scale and from various causes, including privacy-protecting data aggregation, sample nonresponse, measurement limitations, sensor failure, or deliberate redaction. In raster data, cells with null values are treated as nullkohad; in vector data, entire features may be designated as such when data are missing or withheld.

Implications include biased estimates, distorted spatial patterns, and challenges for interpolation and modeling. Analysts commonly document the location and extent of nullkohad, use imputation or data fusion to fill gaps, or apply methods that are robust to missing data.

Nullkohad as a concept varies by project and discipline; it is not a universally standardized term but a descriptive label used to draw attention to data gaps in spatial analyses.

See also: missing data, data gaps, imputation, data quality, spatial analysis.