# Auxiliary coordinate construct

An auxiliary coordinate construct must contain

A list of some (at least one) of the dimensions of the field construct in any order. A coordinate array with dimension sizes corresponding to the list of dimensions of the auxiliary coordinate construct. If there is a dimension with size greater than one, the elements of the coordinate array must all be of the same data type (numeric, character or string), but they do not have to be distinct or monotonic. Missing values are not allowed (in CF version 1.5).

and may also contain

A boundary coordinate array with all the dimensions, in the same order, as the coordinate array, and a fastest-varying dimension (first dimension in Fortran) equal to the number of vertices of each cell. Properties serving to describe the coordinates.

Auxiliary coordinate constructs correspond to auxiliary coordinate variables named by the coordinates attribute of a data variable in a CF-netCDF file. CF recommends there to be auxiliary coordinate constructs of latitude and longitude if there is two-dimensional horizontal variation but the horizontal coordinates are not latitude and longitude. As for dimension constructs, auxiliary coordinate constructs of different field constructs are independent in the data model.