Good point Tony, I suppose it would make sense to introduce IHS as well, for those who are wondering what we are talking about.
Back in the mists of the 1990s it was percieved that the relationship between the habitat data being collected (mainly Phase 1 and NVC) and the way we were increasingly being asked to report on that data (BAP Broad/Priority habitats and EC Habitats Directive Annexe 1) had not been fully investigated. It was clear however that there were mismatches, such as the Phase 1 category Marshy Grassland which could be used to record habitats which belonged in up to 4 broad habitats and might include any of a number of priority habitats. The Phase 1 handbook pre-dated the BAP and the definitions it contained were not adequate to resolve such issues.
Another problem presented by Phase 1 stemmed from the fact that it had its origin in a pre-GIS world. The solution it provided to the complexity of recording habitat information was basically to colour in a shape on the paper map determined by the habitat present and then record anything else which the surveyor deemed relevant as a target note in the margin. The vagaries of allowing the recorder to decide what to record and how, was later compounded when we started to digitise our maps as there was no rule about what should be recorded as an attribute, nor in what format it should be.
The design brief for IHS then was based on a number of simple principles (this is off the top of my head, there was more to it than this)
1. A hierarchical habitat classification, based on the BAP broad & priority habitats, with Annexe 1 habitats inserted in appropriate locations and augmented by additional categories to allow reporting of the full range of habitats. All habitat categories to be backed by a published definition (analagous to the authorities used in species taxonomy).
2. That at each level of the hierarchy, the options available should be exhaustive of all possible habitats. So the broad habitats cover every possible UK habitat, within the Neutral grassland broad habitat we have 2 priorities, Lowland meadows and Upland meadows plus an additional 'inverse' category, Other neutral grassland.
3. That the classification should be useable for all kinds of data gathering, from field survey to remote sensing. The more information that is available, the further it is possible to progress through the hierarchy. The limits of uncertainty of each level being defined, the data should be compatible whatever the source.
4. The primary habitat categories being supplemented by a rigid structure of secondary categories, enabling the recording of subordinate habitat features (such as woodland formation), management practises and land uses. This provided an alternative to Phase 1 target notes whereby useful information could be contained with the habitat data in a single rigorously analysable GIS attribute field. So we have e.g. GC1.SC21Z.GM11 = Lowland calcareous grassland with scattered native species scrub, grazed by cattle.
After more than 10 years of more-or-less continuous development we have a system including over 400 defined codes (habitats and secondaries), which we hope provides a pragmatic solution to the needs of the recorder, the data collator (LRC) and the end user alike. We also have a large and growing number of users, almost every one of whom probably has a pet gripe about the system as is, or a suggestion of something they would like to be able to record.
We are fully open to suggestions but be aware that the coherence of the system now is due in no small measure to our adherence to the rules as they were originally concieved. If we hadn't done that, we would probably be looking at a system with 2000 categrories to choose from.
You will note the number of times I have used the word we in the above text. What am I like? I'm not even on the payroll any more!