Why Do Two Tide Stations Show Different Times?

"Ok I just didn't understand how 2 site can show almost completely different tide estimates. I use tide alert but it's slowly went bad. Never accurate now."

— r/boating

"Different stations show different times."

— r/saltwaterfishing

If you've ever opened two tide apps for the same fishing spot and watched them disagree by 30 minutes — or by a foot of water — you're not seeing a bug. You're seeing how tide tables actually work. Most tide apps do not explain this, because the explanation requires admitting that "the tide at your location" is a much softer concept than the chartlets pretend.

What "the tide at this spot" actually means

There is no sensor on every beach. Every tide prediction you look at — on a phone app, a paper table, a coastal website — is a prediction at a specific station, plus some adjustment to your spot. The station has a tide gauge that has been recording water levels for years. The numbers you see are computed from that station's data, not from your water.

This is not a guess. The 1999 documentation for the academic WWW Tide/Current Predictor at the University of South Carolina put it bluntly:

"The harmonic constituents must be based on actual observations at the site — unlike sunrise and moon phases, they are not pure math."

That single sentence is the answer to the r/boating complaint. A tide prediction is only as good as the observations at the specific station it derives from. Different stations have different observation histories, different equipment, different locations in the tidal system. Of course they give different numbers.

Reference stations vs subordinate stations

NOAA organizes its tide stations into two tiers, and most tide apps do not tell you which tier you're looking at:

  1. Reference stations — long-term monitoring sites with their own independently-derived harmonic constituents. NOAA recomputes these periodically as sea level and coastal conditions change.
  2. Subordinate stations — shorter-record sites whose predictions are calculated as an offset from a nearby reference station. They inherit the reference station's tidal regime plus a local time/height correction.

This is from the CO-OPS data API documentation, which I am not paraphrasing so much as pointing you at: subordinate tide prediction stations can only provide tide predictions on a high/low interval, and the harmonic station is the one that has the full constituent set. The subordinate station's prediction is mathematically tethered to its reference.

The catch that nobody flags: when a subordinate station inherits its reference's regime, it also inherits its reference's error band and any regime shift the reference has been re-tuned for. If the reference station has been adjusted for a sea-level trend in the last ten years, the subordinate station inherits that adjustment automatically. If the reference station is in a different tidal sub-basin than your actual spot, the subordinate offset can be off by tens of minutes and fractions of a foot.

So when two sites disagree, they may not be disagreeing about the tide. They may be disagreeing about which station is "the tide".

What fishermen actually need to know

If you fish a particular spot consistently, you already know which chart works for you. The information most apps leave out is why:

  1. The closest station on the chart is not always the best station. Two stations can be 3 miles apart on a map and in completely different tidal sub-basins — one inside a bay, one at the bay mouth, with phase differences that look like a tide app bug.
  1. Reference stations are more stable over years than subordinate stations, but a subordinate can be more local. A subordinate station might match your spot better if its reference is well-tuned and the offset is recent. A reference station's broader regime might be off for your specific cove.
  1. The "wrong" app is sometimes right for a different station. When the r/boating user says "I use tide alert but it's slowly went bad. Never accurate now," the most likely diagnosis is that the app is using a subordinate station whose reference has been re-tuned by NOAA, and the re-tuning has not propagated cleanly to the app's stored offsets. This is a known failure mode for apps that cache constituents locally rather than re-querying NOAA on every load.
  1. Your GPS-based app can be more accurate — and more wrong. The Reddit reply in the same thread recommended an app that "uses the gps to pinpoint your location to know the tide at that spot and not the nearest buoy." That works when the GPS-tagged spot is close enough to a well-tuned station for the offset to be meaningful. It does not work when the app silently picks a station 15 miles away because that's the nearest active one.

What to actually do with two disagreeing tide sites

Three rules that hold up regardless of which app you use:

  1. Pick a station, not an app. Identify the NOAA station ID behind the chart (each per-station page on Coastal Tide Guide names its station explicitly). The station is the real input. The app is a renderer.
  2. Verify against a reference station you can sanity-check. If a chart claims a 6-foot high tide at 09:00 and the closest NOAA reference station shows 4 feet at 09:30, the chart is probably using a subordinate with an outdated offset. Trust the reference.
  3. Re-check after NOAA's monthly verification. NOAA verifies and re-tunes subordinate offsets on a monthly cycle. An app that cached old constituents will drift. A fresh pull from NOAA will track the latest.

What this means for your tide planning

The two-site disagreement is not a flaw you should paper over. It is information. Once you see the underlying mechanics — per-station observations, reference/subordinate tier, inherited offsets, monthly re-tuning — the disagreement tells you which station is doing the work for each app, and whether that station is a good fit for your spot.

The honest framing: there is no single "tide" for a fishing spot. There is a tide for a specific station, plus an offset to your spot, plus a monthly verification cycle. Two apps are showing you two stations' interpretation of your spot. When they disagree by a lot, one of them is using the wrong reference. When they agree to within a few minutes and a few tenths of a foot, both are probably tracking the same reference through different renderers.

For current station-by-station predictions, see the Maine locations directory or the Charleston, SC reference page. Each Coastal Tide Guide page names its NOAA station ID explicitly so you can verify the input yourself.

References

  1. Dean Pentcheff, WWW Tide Prediction FAQ, 1999. Recovered via Wayback Machine, 1999-11-28 snapshot: https://web.archive.org/web/19991128144929id_/http://tbone.biol.sc.edu/tide/faq.cgi
  2. NOAA CO-OPS, Data Retrieval API — Station / Datum / Interval reference, current docs: https://api.tidesandcurrents.noaa.gov/api/prod/
  3. r/saltwaterfishing and r/boating threads (linked at the top of this article), September 2026.