Surface pressure already contains everything a decompression algorithm needs to know about altitude - if it's built to use it directly.
Diving at altitude - a mountain lake, a high-elevation quarry - changes decompression math because the surface you're diving from isn't at sea-level pressure. A modern, pressure-based algorithm accounts for this automatically from the surface pressure reading itself; it's older, table-derived approaches that needed a separate manual altitude correction bolted on.
Three things shift when the surface you dive from is at reduced pressure:
Barometric-sensor dive computers measure current ambient pressure directly, which already reflects wherever you are - but a sensor only knows where you are now, not where you traveled from or how long ago. That gap is why some systems layer a manual altitude setting on top of a pressure sensor: it's a holdover from table-based decompression systems built around sea-level assumptions, patched to approximate altitude conditions rather than computing them directly.
The scenario a pure pressure reading isn't enough for on its own: you drive up to altitude, and your tissues haven't yet equilibrated to the new, lower ambient pressure before you start diving. Immediately after ascending to altitude, your tissues are still carrying gas loading appropriate to the pressure you came from, not the pressure you've just arrived at - which is physiologically similar to having just finished a dive.
Equilibration isn't instant, but it isn't slow either:
For a modern implementation, the cleanest model initializes tissue compartments using the actual current surface pressure as ambient, with no separate altitude correction layered on top - altitude is already fully represented in that pressure reading. The assumption that matters is equilibration: the model assumes your tissues have already adjusted to the atmospheric pressure at your point of entry. That assumption is only true if you've actually given your body time to acclimatize - not if you've just driven up (or down) the mountain and gone straight to the water.
DiveCast takes into account the atmospheric pressure at your point of entry rather than a separate "altitude" setting, on the assumption that your tissues have adjusted to it. Give yourself real acclimatization time before you dive on that assumption.
See Total Control on the homepage →No - DiveCast takes into account the atmospheric pressure at your point of entry rather than a manual altitude setting, on the assumption that your tissues have already adjusted to it.
Commonly cited guidance ranges from about 6 hours (PADI) to 12 hours (US Navy) before a first dive, allowing tissues to equilibrate to the new ambient pressure.
Not inherently - but it does carry a smaller margin for error because the pressure gradients involved are less forgiving, which is why accounting for actual surface pressure (and equilibration time) matters more than it would at sea level.