The nodes of one map's interpolation cell that carry weight in a query and that the product gives as non-available.

A node carries weight when its bilinear weight is nonzero, so a query sitting on a node, or on the edge between two, uses only the nodes it lies on.

map_number counts from 1 as a file numbers its maps. lat_index, lon_index and lon_index_next are positions in the node axes, which count from 0. Within the grid lon_index_next is lon_index + 1; on the cell that closes the longitude circle it is 0, because an axis such as 0 to 355 by 5 covers every longitude without naming the seam twice. The latitude axis does not wrap, so the cell's other row is always lat_index + 1.

missing is the four corners in the order [lat_index][lon_index], [lat_index][lon_index_next], [lat_index + 1][lon_index], [lat_index + 1][lon_index_next].

This is the payload of both the degraded status field and the {:nodes_not_available, node_gap} refusal, on the IONEX slant surface and on Sidereon.GNSS.Ionosphere.TecGrid, carried inside a Sidereon.GNSS.Ionosphere.NodeGap.