Every depth, drying rock and bridge clearance on a nautical chart is measured from one invisible line. On the Great Lakes that line is a fixed low lake level for each lake. On the coast it is the lowest normal low tide. They behave differently, and if you mix them up you will read the chart backwards. Here is the whole thing, with the math and the gotchas.
The short version
It is set low on purpose, so the water is almost always deeper than the chart says and the chart never promises water that isn't there.
Charted depth plus how far the water sits above datum right now equals the water you have today. Where that number comes from depends on where you are: the lake level on the Great Lakes, the tide table on the coast.
Great Lakes heights and clearances start at chart datum. Coastal heights and clearances start at high water (HHWLT), not chart datum. Same symbol, opposite behaviour.
What chart datum actually is
A chart can only print one number for each spot, but the water never holds still. Lakes rise and fall with the seasons and the decade. The sea goes up and down twice a day. So the hydrographer picks a reference surface and measures everything from it. That surface is chart datum.
The choice is deliberately conservative. On Canadian charts of non-tidal water like the Great Lakes, chart datum is set so the water is above it roughly 95 percent of the time. On the coast, Canadian charts use Lower Low Water, Large Tide (LLWLT): about as low as the tide normally goes. The logic is the same in both places. If the chart says there is 0.4 m over a rock, you will almost always have at least that much, usually more. Set the datum any higher and the chart would sometimes promise water that isn't there.
That's why a chart reads as a worst case for depth. It is also why "the chart said two metres and I hit a rock" usually means the paddler read a drying height as a depth, or used a coastal habit on a lake.
Chart datum vs IGLD
IGLD 1985, the International Great Lakes Datum, is the measuring stick. It is a height system Canada and the US share for the whole Great Lakes and St. Lawrence, with its zero at the long-term average water level at Rimouski, Quebec, near where the St. Lawrence opens to the sea. Every lake level you look up is a height on that stick. Chart datum is one mark on it for each lake: 176.0 m for Huron, 74.2 m for Ontario. The numbers are big because they are heights above that sea-level zero, which is why Lake Ontario's is 74 m and Superior's is 183 m. To find your extra water, subtract the lake's mark from today's reading.
Datum Lab: Great Lakes
The scenario: your route on a Great Lakes chart passes three things. A shoal charted 04 (0.4 m below chart datum), a rock with an underlined drying height of 03 (0.3 m above chart datum), and a creek-mouth bridge with a charted clearance of 2.5 m. The picture is a side view of that shoreline. Your job is to work out what you will actually find.
- Pick a lake. Each lake button loads an example level: how far that lake might sit above chart datum, the number you would work out from the CHS monthly bulletin. The line under the buttons shows the arithmetic.
- Or drag Lake level to set your own.
- Add Wind setup to simulate a storm surge. Plus piles water up at your end of the lake. Minus blows it away from your end.
- Read the three cards under the sliders: water over the shoal, the rock, and your headroom under the bridge.
Great Lakes: will you clear it?
metres above chart datumCharted values in this sketch: shoal 04, drying rock 03, bridge clearance 2.5 m. On Great Lakes charts all three are measured from chart datum. Lake presets are typical example levels, not today's.
Each lake has its own datum
Each lake has its own chart datum, written as an elevation on the International Great Lakes Datum (IGLD 1985). Subtract it from the lake level and you have the number to add to every depth on the chart. The example levels below show the arithmetic. They are the kind of number the CHS monthly bulletin gives, not today's.
| Lake | Chart datum | Example level (IGLD 1985) | Above datum |
|---|
Chart datum elevations from the Canadian Hydrographic Service. Example levels are typical fall values. For the real numbers, use the CHS monthly bulletin and CHS water levels.
The zero is in Quebec
Every Great Lakes water level is measured from one point: the long-term average water level at Rimouski, Quebec, on the lower St. Lawrence. All the water in the system ends up there. Superior drains through Huron, Erie and Ontario and down the St. Lawrence to the sea, so a zero at the outlet puts every lake on one scale. That is why the numbers in the table step down toward the ocean: Superior 183 m, Huron 176 m, Erie 173.5 m, Ontario 74 m. Most of the 99 m drop between Erie and Ontario is the Niagara River, falls included.
How to get the real number
- Find your lake in the CHS monthly water level bulletin, or open the nearest real-time water level station on tides.gc.ca.
- Read the level in metres. It is an elevation on IGLD 1985.
- Subtract your lake's chart datum: it is in the table above and in the notes under your chart's title.
- Add the result to every depth on the chart. A negative result means less water than charted.
Worked example: Tobermory
Say the bulletin has Lake Huron (with Georgian Bay) at 176.39 m. Chart datum is 176.0 m, so the lake is 0.39 m above it.
- A charted depth of 10 in the Fathom Five channels is about 1.4 m.
- A drying rock charted 01 off the west end of Cove Island is covered by about 0.3 m. That's invisible in a chop and exactly where a wave will stand up and break.
- A green shelf with a 10 drying height is 0.6 m out of the water: a landing this fall, maybe a reef after a wet spring.
Plan with three numbers
- The month's average. Great Lakes charts carry a small water level graph (a hydrograph) showing the typical level for each month, measured from chart datum. Find the month you are paddling, read the typical level off the graph, and add it to charted depths for a first estimate.
- This year's level. The CHS monthly bulletin. Subtract chart datum and you have your correction.
- Today's wind. A strong steady wind along the lake can add or remove tens of centimetres at your end. Real-time gauges on tides.gc.ca show it as it happens.
The Lake Ontario high-water line
Lake Ontario and Lake Erie charts draw the shoreline at a level 1.3 m above chart datum. When the lake rose past that in the high-water years, docks, beaches and launch spots went under. A shoreline on the chart is a level, not a promise.
Reading the numbers on the chart
Canadian charts print soundings in metres with the decimetres as a small trailing digit, so 24 is 2.4 m. Tap each card.
The misread that puts people on rocks
An underlined 12 and a plain 12 look almost the same at a glance. One is 1.2 m of water at datum. The other is a rock poking 1.2 m out of the water at datum. On paper charts in a wet map case, with spray on the plastic, the underline is easy to lose.
Datum Lab: the coast
The scenario: the same three features on a coastal chart. A shoal charted 04, a rock with an underlined drying height of 18, and a bridge charted at 2.0 m clearance. On the coast, depths and drying heights start at chart datum (the lowest normal tide). The bridge clearance starts at high water (the dashed line).
- Pick a tide stage or drag the Tide slider. Each button sets the height of tide above chart datum, from the lowest normal tide (0 m) to high water (3.0 m in this sample harbour). On a real trip that number comes from the tide table.
- Watch the bridge card. On the lake the clearance shrank as the water rose. Here you only gain clearance as the tide drops.
Coast: will you clear it?
LLWLT · HHWLT 3.0 mCharted values in this sketch: shoal 04, drying rock 18, bridge clearance 2.0 m above high water. Sample harbour with a 3.0 m high water; real ports vary a lot.
Lake versus coast, side by side
Great Lakes
- Chart datum
- A fixed low lake level for each lake, on IGLD 1985
- How often you're below it
- About 5 percent of the time
- Heights and clearances from
- Chart datum
- When the water rises
- More depth, less clearance, islands look smaller
- Timescale
- Seasons and years, plus wind setup during storms
Tidal coast
- Chart datum
- Lower Low Water, Large Tide (LLWLT)
- How often you're below it
- Rarely, mostly with strong offshore wind or high pressure on a big tide
- Heights and clearances from
- Higher High Water, Large Tide (HHWLT)
- When the water rises
- More depth, clearance shrinks toward the printed minimum
- Timescale
- Hours, twice a day
The practical upshot: on the coast, a printed clearance is the worst case you will see. On the Great Lakes it is the best case, and the lake is almost always eating into it.
Datum math
Example, already filled in: your chart shows a rock with an underlined 05 on your route, and the bulletin puts Lake Erie 0.71 m above chart datum. The answer: the rock is covered by 0.21 m. That is enough to hide it and shallow enough to break in any swell. Change the numbers to match your own chart.
Why the lakes won't sit still
Chart datum is fixed. The water isn't. Three things move a Great Lake, on three timescales:
-
HOURS
Wind setup and seiche. A strong wind along a shallow lake piles water at the downwind end and drains the upwind end. Lake Erie is the worst for it because it is shallow and lines up with the prevailing southwesterlies. The level at your launch can swing by a metre or more during a big blow, then slosh back after. -
MONTHS
The seasonal cycle. Lakes are generally lowest in winter, rise with snowmelt and spring rain, and peak in summer before falling away in fall. A launch that works in October can be underwater in June. -
YEARS
Long swings with precipitation and evaporation. Michigan–Huron hit its lowest level on record in January 2013, about 0.4 m below chart datum, then rose nearly a metre in two years and kept climbing to roughly 1.5 m above datum by 2020. Almost 2 m of swing in seven years. Lake Ontario set records high in 2017 and again in 2019.
Check a water level gauge, even on a lake
There is no tide, but CHS runs real-time water level stations around the lakes. For a crossing to a shoaly island or a rock-garden play session, check the nearest gauge that morning, not the monthly average.
The datum is about to move
The land around the Great Lakes is still rebounding from the weight of the last ice sheet. The north is rising and the south is sinking, by centimetres per decade in places. That slowly tilts the whole reference system, so IGLD gets redone every 25 to 35 years.
IGLD 1985 is due. Its replacement, IGLD (2020), is expected to be adopted in the US by 2027, and the low-water datums used as chart datum are being recalculated alongside it. Preliminary US work suggests the upper lakes' datum may be set too high and the lower lakes' too low, with adjustments anywhere from about 16 cm down to 48 cm up depending on the lake. Canada and the US maintain IGLD jointly, but we have not seen a published Canadian chart changeover date yet.
What it means for you: when your charts change datum, the printed soundings may shift by a few decimetres even though the water didn't. Make sure your chart and your water-level source are on the same datum, and don't mix them.
Finding the datum on your chart
It's in the notes under the title. A Great Lakes chart will say depths are in metres, reduced to chart datum, with the lake's elevation on IGLD 1985 (176.0 m for Huron). A coastal chart will say depths are reduced to chart datum (Lowest Normal Tide or LLWLT), and that elevations and clearances are above Higher High Water, Large Tide. Older charts for neighbouring areas can differ, so check every sheet you plan across.
Where this matters in a sea kayak
A loaded sea kayak draws roughly 15 to 25 cm, so you can float over almost anything. Datum still matters because it tells you what the water is going to do.
- Shoals that break. A 06 shoal with the lake 0.2 m up has 0.8 m over it. Add a 1 m swell and the trough exposes it. That's a breaking reef, not a depth.
- Drying rocks on a crossing. When the lake is low, rocks that were awash become hard obstacles, and a gap you used last year can close.
- Landing beaches. At high levels, a charted beach may be a strip of boulders against a bluff. Plan a backup landing.
- Low bridges and culverts. Creek mouths and marsh channels on the Great Lakes lose headroom as the lake rises. Check clearance with the paddle shaft vertical, not just your head.
- Coastal tide windows. On the coast, an underlined 18 bar between you and camp means you need more than 1.8 m of tide to cross it. That's a time window, so plan the crossing around it.
Check yourself
Want the full chart work?
Datum is a few minutes on our Level 2. Our navigation courses go further: bearings, deduced reckoning, tides and tidal streams, plotted on real Canadian charts.
Sources
- Canadian Hydrographic Service, Vertical datum chart references
- Canadian Hydrographic Service, Great Lakes–St. Lawrence River system
- NOAA, IGLD (2020) FAQ and Low Water Datum
- Great Lakes Commission, Resolution on the IGLD update, March 2025
- Eos, Water levels surge on Great Lakes
- Environment and Climate Change Canada, LEVELnews, March 2020
1 comment
That’s pretty cool Aaron. I’m going to have to read it again ( and again)