Many pilots seem to think that they can rely on the obstructions shown on the aerodrome chart and just add 300' to the highest one.
Brave (and, I think, a little foolish) pilots, indeed -
(a) not ALL relevant heights will be shown on such charts, necessarily. I can recall one or two celebrated examples where an aid box detail then overlaid the critical obstacle so it disappeared from the chart .. go figure.
(b) do consider the accuracy of any heights so identified .. if you are relying on such heights to be accurate to a foot or two, you might be in for a surprise
(c) terrain spot heights are just that and don't include surrounding vegetation .. some of those trees can be both tall and solid ... they'll win out every time over an aircraft ..
(d) if the chart design gets it right for cultural obstructions, well and good .. but the number of obstacle detail screwups we've seen in Australian charts over the years is a concern
(e) the guys who run up the designs start with available data and end up doing a series of flight checks at the relevant procedure heights. The terrain data nominated may not be a guarantee but, if you stay at circling height within the radii prescribed, you have a VERY good chance of NOT hitting anything hard, midairs notwithstanding.
(f) me, come below circling height ? Not on your Nellie... but, that's just my view. Getting in just isn't that important .. we can always come back for another looksee a bit later on. Accidents at Young come the mind ?
(g) if someone wants to use 300ft, fine, but do your OWN homework on the REAL obstacles as a due diligence exercise.
Unless like John above you have access to the airport survey documents
Getting copies of published data is not a difficult exercise and is open to any operator to do so. What I did, when considered necessary or appropriate, was actually speak with the specific procedure designer to find out what obstacle details he/she used in the design of the chart.