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I am estimating water depth (bathymetry) using satellite data over a US water. I have obtained tidal values (datum is MLLW) as at the time of satellite overpass to correct my satellite-derived bathymetry for tide. I was wondering if the value obtained (e.g. 0.68m) will be subtracted or added to my SDB estimates - to account for tide.

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Good question as sign conventions can often trip up when dealing with water depths.

  • Water depths are generally considered as positive down.
  • Tide heights are generally considered as positive up.

Given your instantaneous water depth from SDB (positive down), and the tide height above MLLW datum (positive up), then to perform the tidal correction to attain depth w.r.t. MLLW chart datum:

charted depth = SDB_depth - tide_height
              = SDB_depth - 0.68 

Assuming that at the time of the SDB depth observation the tide is 0.68m above MLLW.

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