A subtlety here, obstruction identification surfaces are designed from 35' above the departure end of runway (DER)
One of the problems with Type A charts.
The sensible ops engineer then has to look at that area either side of the Type A splay to make sure whatever the relevant splays are might be observed for the shorter TODR. Usually that is not a major problem .. but, if there be significant obstacles upon which the splay is overlaid .. a different matter altogether.
The problem with using just the declared data is tracking over the DER .. not so much these days as we get better gee whizz gadgetry to help. Years ago, it was a pipe dream to argue along the lines that the pilot, say, could track visually. About the only use of visual reference was for those departures where a DER turn was required to turn away from tiger country some distance beyond and head off down a benign track, eg HBA 30 in Oz.