GeoTIFF Web Page Table of Contents Top of Section 2

2.2 GeoTIFF Design Considerations

Every effort has been made to adhere to the philosophy of TIFF data abstraction. The GeoTIFF tags conform to a hierarchical data structure of tags and keys, similar to the tags which have been implemented in the "basic" and "extended" TIFF tags already supported in TIFF Version 6 specification. The following are some points considered in the design of GeoTIFF:

o Private binary structures, while permitted under the TIFF spec, are in general difficult to maintain, and are intrinsically platform- dependent. Whenever possible, information should be sorted into their intrinsic data-types, and placed into appropriately named tags. Also, implementors of TIFF readers would be more willing to honor a new tag specification if it does not require parsing novel binary structures.

o Any Tag value which is to be used as a "keyword" switch or modifier should be a SHORT type, rather than an ASCII string. This avoids common mistakes of mis-spelling a keyword, as well as facilitating an implementation in code using the "switch/case" features of most languages. In general, scanning ASCII strings for keywords (CaseINSensitiVE?) is a hazardous (not to mention slower and more complex) operation.

o True "Extensibility" strongly suggests that the Tags defined have a sufficiently abstract definition so that the same tag and its values may be used and interpreted in different ways as more complex information spaces are developed. For example, the old SubFileType tag (255) had to be obsoleted and replaced with a NewSubFileType tag, because images began appearing which could not fit into the narrowly defined classes for that Tag. Conversely, the YCbCrSubsampling Tag has taken on new meaning and importance as the JPEG compression standard for TIFF becomes finalized.