Halliday & Matthiessen (1999: 535):
… the semantic system that is construed in (say) Australian or British Sign is not the semantic system of English, despite its being constantly permeated by English in the ways referred to above. It is a system needing to be described in its own terms, based on detailed studies of its grammar and discourse such as now being carried out for Auslan by Trevor Johnston. Such a description is designed first and foremost for the needs of the deaf community; but it will have general significance as a source of further insight into the semantics of spoken languages, making it possible to view them comparatively in the light of an alternative construction of reality.
From Sign, we can get further insight into the construal of experience as ideational meaning because of its greater potential for iconicity in the expression. Semantic space can be construed iconically in signing within a continuous space-time, constituted as bodily experience for the signer and as part of shared visual experience for signer and addressee.
From a comparative standpoint, spoken languages and deaf sign languages stand to each other in a metaphoric relationship, as alternative construals of a (largely, though not totally) shared experience. What appear at first simply as different modes of expression turn out to have, associated with them, somewhat different constructions of meaning.