A nautical chart shows you a sounding. It does not show you whether your boat clears it today. Helm closes that gap by merging four independent sources and reporting the result against your own draft, with the date the measurement was taken.
| Source | What it contributes |
|---|---|
| NOAA ENC | The official electronic navigational chart. Authoritative, but a sounding on it can be decades old. |
| USACE eHydro | Recent dredging and channel surveys from the Army Corps of Engineers, often far fresher than the chart in maintained channels. |
| State lake elevation data | Inland lakes are largely blank on NOAA charts. State surveys fill them in, which is why Helm is usable on a lake and not just at sea. |
| Live tide | The water level now, from NOAA CO-OPS. A chart datum number is a floor, not the current depth. |
Two soundings that read the same can mean very different things. One measured last season in a dredged channel is a different promise from one measured in the 1970s over a shifting bar. Helm shows you which one you are looking at rather than flattening them into a single number.
Electronic navigational charts carry a quality field that most apps never surface. It records how the survey was done and how much the depth can be trusted. Helm exposes it, so a low-confidence zone looks different from a surveyed channel instead of both rendering as plain blue.
This is assistance, not a guarantee. No chart app, Helm included, replaces a depth sounder, local knowledge or prudent seamanship. Use it to plan and to know where to slow down.
CATZOC is a field inside the electronic navigational chart itself. It is the surveyor’s own statement about how the measurement was made and how far off it might be. Helm prints the grade next to the sounding in plain words.
| CATZOC | What Helm shows | What it means for you |
|---|---|---|
| A1 | A1, best | Modern full-coverage survey. The depth is as good as charted depth gets. |
| A2 | A2, good | Full coverage, slightly looser tolerances than A1. Normal for maintained channels. |
| B | B, fair | Uncharted hazards are possible between sounding lines. Give it more room than the number suggests. |
| C | C, poor | Sparse or old survey. Treat the number as an indication and run slow. |
| D | D, low | Worse than C. Local knowledge matters more than the chart here. |
| U | Unsurveyed | Nobody has measured it. Helm says so rather than drawing plain blue water. |
Helm labels the tide figure with its own source, because the three are not equally trustworthy. The strongest is a NOAA CO-OPS prediction corrected by the residual from the nearest live gauge, so a day when the real water is sitting eight inches under prediction shows up as eight inches less water. Next is the published NOAA prediction on its own. Offline, with no signal, Helm falls back to computing the curve on the device from the station’s harmonic constants. The sentence on screen tells you which of the three you are reading.
What Helm will not do is let any of those numbers decide whether you can go. Routing is computed at chart datum, MLLW, with no tide added at all. A tide that would carry you over a bar is reported to you as information for that hour, and the route still refuses to plan across water your boat cannot clear at datum. The reason is the failure mode: a route that was only passable because the tide cooperated is a route that traps you when you are an hour late leaving.
Helm also stays quiet about precision it does not have. Datum uncertainty from NOAA VDatum is shown when the figure exists for your area, and when it does not, Helm says nothing about it rather than implying a confidence nobody gave it.
Under your keel, Helm treats three feet as the floor for calling water clear. Anything below that reads as tight, because a foot or two of margin is smaller than the things that move the real number around on any given afternoon: the error band on a tide prediction, the trough of a passing wave, and the uncertainty CATZOC itself declares.
If you set your own safety margin larger than three feet, yours wins. A skipper who enters a metre of margin sees tight water all the way up to a metre, which is the point of entering it.
Overhead clearance comes from the charted vertical clearance in the chart, measured above mean high water, checked against the air draft you entered. Fixed bridges and the down position of an opening bridge are both read, so a bascule you would have to wait for does not look like open sky.
When a bridge carries no clearance figure in the chart, Helm reports it as unknown. It does not print zero. A zero on a chart field is the absence of data, and rendering that as a hard stop would send you the long way around a bridge you could have cleared.
Enter your draft once and Helm stops showing you abstract numbers. Water you cannot cross is shaded, routes plan around it, and the measure tool reports the shallowest point along any line you draw rather than the average.
Draw a line anywhere on the chart and Helm gives you distance, bearing, and a depth profile along it — including where the shallowest point falls. Useful before you cut a corner or take an unfamiliar cut on a falling tide.
Yes. Lake depth comes from state elevation surveys rather than NOAA charts, which is why inland water is not a blank space in Helm.
No. Once the chart pack for your area is downloaded, depth works offline like the rest of the app.
No. A sounder tells you what is under the hull this second. Helm tells you what to expect ahead and how much to trust it.
Helm Boating