Physical Blocking

In 1979, when FITS was originally developed, the dominant medium for data storage and transport was 1/2-inch nine-track magnetic tape. In FITS Paper I, the physical block size was set equal to the logical record size. As time passed, it became clear that many of the major data producers regarded this block size as inefficient, in terms of both tape length used and the number of I/O operations required to write data. The new generation of computers, with Megabyte size memory, could easily read much larger blocks. As a consequence, FITS Paper III included a provision that there could be up to 10 logical records per physical block on 1/2-inch nine-track magnetic tape. New storage media, such as cartridge tapes and optical disks were replacing magnetic tape. Many of the new media could access data only in blocks of fixed length, typically $2^{n}$ bytes, and the FITS 23040-bit logical record length would not correspond to an integral number of these blocks. Whereas FITS had been discussed in FITS Paper I in the context of files on magnetic tape, the increasing use of electronic transport for files was leading to the concept of a FITS file as a pure bit stream, without special ties to any particular medium. However, a set of prescriptions for the physical expression of FITS files on different media was still needed. General rules for all media, and in particular for how to write FITS logical records to the $2^{n}$-byte physical blocks, were proposed by Wells and P. Grosbøl (ESO) in 1991. With minor changes, they were approved by the IAUFWG in the spring of 1994. They appear in section 3.8 of this Guide.