Lake Michigan looks like one sheet of water and charts as 201 separate cells. A day on it tends to split in two. The middle of the lake asks almost nothing of the chart, and then the whole passage compresses into a dredged cut with piers either side and a river full of bridges behind it. Those two halves want different things from a chart app. Out in the open you are reading distance and weather. Coming in you are reading individual charted objects, and on this lake there are a lot of them.
Helm builds the Lake Michigan pack from 201 official NOAA ENC cells, which is the tenth largest cell count of the 18 US regions in the app, and the download comes to 41 MB for the region entire. Inside it are 75,323 individual charted soundings, 10,277 charted depth areas and 10,840 depth contours drawn between them. The hazard layers add up to 4,443 charted obstructions, 1,735 underwater rocks and 477 wrecks. Aids to navigation run to 2,028 charted lights, 871 lateral buoys and 457 beacons. Once the pack is downloaded, none of it needs a signal.
Helm divides US waters into 18 regions and builds each one from the ENC cells that cover it, so the counts are directly comparable between regions. Lake Michigan is mid table for the raw depth data and noticeably higher for charted objects. The full picture, with the region rank beside each figure:
Read down that list and a pattern shows up. Soundings, depth areas and contours sit in the middle of the table, while obstructions, rocks, bridges and overhead cables sit eighth or ninth. That describes water where the open passage is straightforward and the individual charted objects are the thing you plan around.
| In the chart pack | Count |
|---|---|
| NOAA ENC cells | 201 |
| Charted soundings | 75,323 |
| Depth areas | 10,277 |
| Depth contours | 10,840 |
| Lateral buoys | 871 |
| Lateral beacons | 457 |
| Charted lights | 2,028 |
| Anchorage areas | 26 |
| Bridges | 1,150 |
| Overhead cables | 477 |
| Dredged areas | 393 |
| Charted wrecks | 477 |
| Charted obstructions | 4,443 |
| Underwater rocks | 1,735 |
| Restricted areas | 429 |
Counted from the Helm chart pack for Lake Michigan, built from official NOAA electronic navigational charts. The pack downloads at about 41 MB.
Add the obstruction, rock and wreck layers together and the region carries 6,655 individual charted hazards. Most of them sit nowhere near any line you would draw. The ones that do sit near your line are the reason Helm puts the shallowest point on a leg in front of you instead of the average depth along it, because an average over a mile of water with four charted obstructions in it tells you nothing useful.
1,735 of those objects are charted underwater rocks. A rock is a different sort of contact from a sand shoal and it does not shift between seasons, so an old survey over rock ages better than an old survey over sand. That is worth knowing when you are deciding how much room to give a charted object whose survey is decades behind you. Helm shades water your draft cannot take and prints the chart's own confidence grade next to the sounding, so a sparsely surveyed area looks different from a measured one rather than both rendering as plain blue.
The 477 charted wrecks are the layer some boats are looking for rather than avoiding. They hold structure, they are fixed, and they are marked on the chart with a position you can run to. If you are working the lake rather than crossing it, that layer earns its place on screen.
The chart carries 393 dredged areas for the region and the ENC records a depth for each one. Across all 393, the median is 5.7 m, about 18.7 ft. The deepest is 18.2 m, close to 59.7 ft. The shallowest recorded value is 0.1 m, four inches, and that figure is the argument against reading a chart by color alone, because at a glance it renders as water like anything else.
A median near 18.7 ft means most dredged water on this lake is comfortable for most of what floats on it. The spread between four inches and sixty feet means the number for the specific cut you are entering is worth reading rather than carrying a rule of thumb for the whole lake. Helm reports the shallowest point along any line you draw, and it shows the date the depth was measured, because a dredged figure is a statement about the day a survey boat went over it.
Lake Michigan carries more charted bridges than all but seven of the 18 regions, and its overhead cable count ranks ninth. A bridge crosses narrow water by definition, so those 1,150 bridges and 477 cables describe the rivers and cuts behind the entrances rather than the lake itself. For a boat with a rig or a tower, that is where the day slows down and the looking up starts.
Helm reads charted vertical clearance against the air draft you entered, and takes both the fixed spans and the down position of an opening bridge. Where a bridge carries no clearance figure in the chart, it reports the clearance as unknown rather than printing zero. Across 1,150 of them, that distinction decides whether a detour was your call or an empty chart field's.
2,028 charted lights, 871 lateral buoys and 457 beacons is a lot of marked water for one region. Against that, the whole of Lake Michigan carries four safe-water marks. A skipper who likes to pick up a mid-channel mark and run in from it is not going to find that habit supported here. The approach is lights and lateral pairs, and after dark it is the light characteristics that sort one entrance from the next, which is why Helm carries all 2,028 of them rather than a filtered selection.
Two other counts belong in the same thought. There are 26 charted anchorage areas for a body of water this size, which puts most nights inside a harbor. And there are 429 restricted areas, a number large enough that a straight line drawn across the chart will sometimes clip one. Helm treats those as water to plan around and tells you in plain words when a leg steers clear of one.
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.
One pack for the region. It is built from the 201 official NOAA ENC cells that cover the lake, it downloads at 41 MB, and it spans 41.5 N at the southern end up to 46.2 N, and 88.1 W across to 84.7 W, so both shores and the harbors along them are in the same file. Once it is on the device it works with no signal.
The chart carries 393 dredged areas for the region, each with a depth recorded in the ENC. The median across all of them is 5.7 m, about 18.7 ft. The deepest is 18.2 m, near 59.7 ft, and the shallowest recorded value is 0.1 m. That spread is the reason to read the value for the cut you are actually entering, and to check the date beside it, rather than assuming the lake's channels are uniform.
477 charted wrecks, 1,735 charted underwater rocks and 4,443 charted obstructions, 6,655 objects in total. The rock and obstruction counts both rank ninth of the 18 US regions in Helm, higher in the table than the region's sounding count at twelfth, so on this lake the charted objects rather than the depth grid are what most planning turns on.
Helm Boating