Almost everything you do on this coast happens behind the islands. The sounds open to the ocean and the rivers run in behind them, with creeks tying one river to the next, so a day's run is a sequence of bends with marsh on both sides. The bottom is sand and mud, and it moves. Across the whole area the pack counts 51 underwater rocks against 574 charted obstructions, which tells you what you are reading the chart for here. You are not reading it for rock. You are reading it for the bar across a sound entrance and for how much water the falling tide has taken out from under you since you left the dock.
The pack for this coast is built from 91 official NOAA ENC cells and downloads at 9 MB. Inside it are 44,839 charted soundings, 11,806 depth areas, 11,874 depth contours, 1,023 lateral beacons, 438 lateral buoys, 1,146 charted lights, 627 dredged areas, 262 bridges and 179 overhead cables. The same box holds 115 NOAA tide prediction stations. It runs from the St. Marys River entrance at the south end up past Savannah to Hilton Head and Daufuskie.
The pack carries 1,023 lateral beacons and 438 lateral buoys, more than two beacons for every buoy. That ratio is the local chart in one number. A buoy floats over the channel it marks and can be shifted when the channel shifts. A beacon is driven into the bank or the shoal and stays where it was put. Most of this water is creek and river bend rather than an approach that gets re-marked, so most of what you steer by is fixed.
There are 13 safe-water marks in the entire area, which says the same thing from the other side. A mid-channel mark belongs at the mouth of something wide enough to have a middle. Inside, you are almost always reading a red or a green against a bank, and the question is which side to favor rather than where the fairway starts. After dark there are 1,146 charted lights in the pack, and they are in it with or without a signal on the phone.
| In the chart pack | Count |
|---|---|
| NOAA ENC cells | 91 |
| Charted soundings | 44,839 |
| Depth areas | 11,806 |
| Depth contours | 11,874 |
| Lateral buoys | 438 |
| Lateral beacons | 1,023 |
| Charted lights | 1,146 |
| Anchorage areas | 6 |
| Bridges | 262 |
| Overhead cables | 179 |
| Dredged areas | 627 |
| Charted wrecks | 227 |
| Charted obstructions | 574 |
| Underwater rocks | 51 |
| Restricted areas | 296 |
| NOAA tide prediction stations | 115 |
Counted from the Helm chart pack for Georgia & Savannah, built from official NOAA electronic navigational charts. The pack downloads at about 9 MB.
627 areas in this pack are marked as dredged, and the charted depth on them runs from 0.6 meters at the shallow end to 15.7 meters at the deep end, with the median sitting at 11.7. In feet that is a spread from about 2 to about 51, with the middle of the set near 38. The deep end is commercial ship water. The shallow end is a cut through a creek or a basin behind a dock. Both carry the same boundary on the chart, so in this area the label is worth very little and the number attached to it is worth a great deal.
Helm reports the shallowest point on a planned route up front, and the measure tool gives a depth profile along any line you draw, including where the low spot falls. Where one dredged area is at 0.6 meters and another is at 15.7, the profile along the line tells you more than the depth at either end of it. Each depth comes with the date it was surveyed, and in a maintained channel an Army Corps survey is often a good deal fresher than the chart.
The box holds 6 charted anchorage areas. Where you stop for the night is therefore a decision you make out of the depth areas and contours rather than a box someone else drew, and it is a question of water at low tide plus room to swing. Drop a marker on the spot and it stays, and it is on the other device too.
There are 227 charted wrecks in this area and 574 charted obstructions, so obstructions outnumber wrecks by more than two to one, and behind both of them sit those 51 underwater rocks. A hazard here is usually something that was built, lost or grown rather than something geological. The practical difference is that it can be recent. A rock has been on the chart for as long as there has been a chart of this coast. A piling stub or a hull that settled into the mud is on the chart because somebody reported it after the fact.
That is why Helm puts CATZOC, the chart's own confidence grade, and the survey date next to what it is telling you. Both matter more in water charted like this than in water where the bottom has not moved since the day it was measured. On a route, a leg that runs close to charted hazards gets a sentence saying how far it runs near them and that it does not enter them.
The pack also carries 296 restricted areas and 15 marine farming areas. When a plan steers around a restricted area, the explanation names it and says how much of it the straight line would have crossed, so you can see what the detour bought and overrule it if you disagree.
NOAA keeps 115 tide prediction stations inside the area this pack covers. Several of them are nothing more than a position along one river: "2.5 miles above mouth", "8 miles above mouth", "Bailey Cut, 0.8 mile west of". A river gets split up that way when high water at the entrance does not arrive at the same time or the same height as high water upstream, which is the normal condition on this coast and the reason an entrance station is a poor stand-in for the creek you are actually in.
The named ones read like a list of where people go:
Helm takes the tide from the station nearest the spot in question and labels the figure with where it came from. The strongest form is a NOAA prediction corrected by the residual at the nearest live gauge, so an afternoon when the real water is sitting under prediction shows up as less water rather than as the prediction. With no signal, the curve is computed on the device from that station's harmonic constants. None of those numbers is allowed to decide whether a route is passable. Routing is computed at chart datum, MLLW, with nothing added, so the line you were given is one you can still run when you leave an hour later than you meant to.
Running inside means passing under things all day. The pack counts 262 bridges and 179 overhead cables, and the cables are the ones that get forgotten, because a cable strung across a creek does not announce itself from a mile off the way a bridge does.
Helm reads the charted vertical clearance against the air draft you entered. Fixed spans 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. A charted clearance is the figure at mean high water, which means that at most states of tide here you have more room than the number rather than less. Where the chart carries no clearance figure at all for a bridge, Helm reports it as unknown and does not print a zero.
These counts describe what the chart pack carries, not the state of the water today. Charted data ages, channels shoal and marks move. Keep a lookout and verify locally.
There are 115 NOAA tide prediction stations inside the area the pack covers, from entrance stations like Bear River Entrance and Beach Creek at Cumberland Island to upriver ones like Bellville Point on the Sapelo and Champney Island on the South Altamaha. Helm uses the station nearest the spot you are asking about, and it labels the figure with its source: a NOAA prediction corrected by the nearest live gauge, the published prediction on its own, or a curve computed on the device from harmonic constants when there is no signal.
From the St. Marys River entrance at the south end up past Savannah to Hilton Head, Daufuskie Island and Calibogue Cay at the north end, taking in the Sapelo, the South Altamaha, the Bear and the Belfast rivers along with the creeks behind Cumberland and Blackbeard islands. It is built from 91 official NOAA ENC cells and downloads at 9 MB.
The pack carries 627 dredged areas, and the charted depth on them runs from 0.6 meters to 15.7 meters with a median of 11.7, which is roughly 2 feet to 51 feet with the middle of the set near 38. The deep end is commercial ship water and the shallow end is a creek cut, so the dredged boundary by itself tells you almost nothing here. Helm shows the depth with the date it was surveyed, and in a maintained channel an Army Corps survey is often fresher than the chart.
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