60

SECTION 7. STANDARD EXTENSIONS

Table 7.7.

Usage of TZEROn to represent non-default integer data types.

Native

Physical

TZEROn

TFORMn

Data Type

Data Type

(-27)

unsigned

signed byte

'B'

-128

(215)

signed

unsigned 16-bit

'I'

32768

(231)

signed

unsigned 32-bit

'J'

2147483648

(263)

signed

unsigned 64-bit

'K'

9223372036854775808

of the FITS file, or, in the case of the complex data types C and M, in the real part of

the field, with the imaginary part set to zero. The default value for this keyword is 0.0.

Equation 7.1 is used to compute a true physical value from the quantity in field n. For

fields of type P or Q, the values of TSCALn and TZEROn are to be applied to the values in

the data array in the heap area, not the values of the array descriptor (see §7.3.5).

In addition to its use in representing floating point values as scaled integers, the

TZEROn keyword is also used when storing unsigned integer values in the field. In this

special case the TSCALn keyword shall have the default value of 1.0 and the TZEROn

keyword shall have one of the integer values shown in Table 7.7

Since the binary table format does not support a native unsigned integer data type

(except for the unsigned 8-bit 'B' column type), the unsigned values are stored in the

field as native signed integers with the appropriate integer offset specified by the TZEROn

keyword value shown in the table. For the byte column type, the converse technique can

be used to store signed byte values as native unsigned values with the negative TZEROn

offset. In each case, the physical value is computed by adding the offset specified by the

TZEROn keyword to the native data type value that is stored in the table field.

TNULLn Keywords  The value field for this indexed keyword shall contain the integer

that represents an undefined value for field n of data type B, I, J or K, or P or Q array

descriptor fields (§7.3.5) that point to B, I, J or K integer arrays. The keyword must

not be used if field n is of any other data type. The value of this keyword corresponds

to the table column values before applying any transformation indicated by the TSCALn

and TZEROn keywords.

If the TSCALn and TZEROn keywords do not have the default values of 1.0 and 0.0,

respectively, then the value of the TNULLn keyword must equal the actual value in the

FITS file that is used to represent an undefined element and not the corresponding

physical value (computed from Eq. 7.1). To cite a specific, common example, unsigned

16-bit integers are represented in a signed integer column (with TFORMn = 'I') by

FITS Standard