The NBN Atlas stores records based on a pair of coordinates and a measure of the uncertainty of the coordinates. For records supplied with a national grid reference – which is the majority of the records – the size of the national grid reference square was previously being used as the coordinate uncertainty (e.g. 1000 m uncertainty for a record in a 1 km grid square such as SK5740). However, the Darwin Core standard states that the ‘coordinateUncertaintyInMeters’ should be the radius around the point which contains the entire location.
We have now corrected our calculation for all records supplied with grid references, so that the coordinate uncertainty now reflects the radius of the circle touching the corners of the grid square, i.e. approximately 0.70 km for a record in a 1 km grid cell.
We have added a ‘gridSizeInMeters’ attribute to all occurrence records which reflects the size of the national grid square of the occurrence and it is possible to filter records by this attribute.
When supplying records in the data file, it is not necessary to include a coordinate uncertainty or grid size for records with grid references, as these can be derived from the number of digits in the grid reference. Records supplied with coordinates must either have a coordinate uncertainty or a grid size column. The grid size should indicate the size of the grid square that should be assigned to the record.
More details can be found here: https://docs.nbnatlas.org/grid-and-coordinate-based-records-on-the-nbn-atlas/.