ESP applications can process NDFs stored in any of the following HDS data types. The correspondence between Fortran types and HDS data types is as follows:
| HDS Type | Number of bytes | VAX FORTRAN Type |
| _DOUBLE | 8 | DOUBLE PRECISION |
| _INTEGER | 4 | INTEGER |
| _REAL | 8 | REAL |
| _UBYTE | 1 | BYTE |
| _BYTE | 1 | BYTE |
| _UWORD | 2 | INTEGER*2 |
| _WORD | 2 | INTEGER*2 |
(_UBYTE) and (_UWORD) types are unsigned and so permit data ranges of 0–255 and 0–65535 respectively.
Those applications that output NDF files propagate images containing only a DATA component stored using the _REAL (8 byte) data type. Other information (such as variance) is not propagated to reduce the storage space required and because these other components are not used by any of the ESP applications.
In most of the ESP applications all processing is carried out using REAL values. However, ELLFOU, ELLPRO, HISTPEAK, HSUB and LOBACK all occasionally employ double precision variables when that increased degree of accuracy is required.