The header of a NASA-Ames format file must contain the following descriptors in the order listed below:
The first line of a NASA-Ames file always contains two integers (e.g. 20 1001). The first integer represents the number of header lines in the file, including the first line itself. The second integer is a four-digit number called the file format index (FFI). The first digit of the FFI indicates the number of independent variables (dimensions). The next digit specifies the number of necessary auxiliary variables. The remaining differences cover the way the independent variables increment and the nature (numeric or character) of the variables.
The data source description requires six lines:
The independent variable(s) are used to define the dimensions (or 'axes') of the data, and must be monotonic meaning increasing but never decreasing or decreasing but never increasing. The description of the independent variable(s) changes based on whether they are what is called bounded or unbounded and whether they are numeric or character variables. With bounded variables, the range of values in known in advance, and the range and interval must be specified either within the NASA-Ames file header or, as is the case with some formats, as auxiliary variables. Thus if an independent variable (e.g. time) interval is always the same, you can specify the interval between different points in the header. If the spacing between points can change then this interval should be set to zero. With an unbounded variable this range is unknown until after the file has been completely read. It is meaningless to have more than one unbounded variable in a NASA-Ames format file.
Primary variables are the main variables that are in the file: they are the quantities that have been observed or derived e.g. temperature, winds or ozone. The primary variables are considered as functions of the independent variables. Three quantities are needed to describe each primary variable: the name, the value which has been substituted for missing data and the scaling factor by which one multiples recorded values to produce the required values. The variable name should be chosen from the Climate and Forecast (CF) Metadata Conventions standard names list and must include the units.
The format stipulates that all scale factors for the different primary variables should be held on the same line, all missing data values should be held in the next line and each primary variable name should be written in separate subsequent lines.
The auxiliary variables are used for ancillary information about the observations, if this is needed. Three quantities are needed to describe each auxiliary variable although these vary depending on whether the variable is numeric or a character string. Numeric auxiliary variables require a scaling factor, a default value to be substituted when data are missing and the CF name of the variable. Character auxiliary variables require the length of the string, a default string for missing data and the CF name of the variable(s).
The format makes provision for two comment fields structured in identical ways. The first line is the number of comment lines and subsequent lines are the actual comments. These can include community preferred names for variables and/or a column headings line. Either one or both of these can be missing.
The data section consists of columns of data with the structure of independent, auxiliary and primary variables set for each file format index.
For simplified examples see
Dummies Guide to NASA Ames format with 1D data and
Dummies Guide to NASA Ames format with 2D data
Alternatively see The complete BADC documentation on NASA Ames with detailed examples