A road map shows you what sits on the land. A topographic map shows you how the land rises. A bathymetric map shows you the part nobody can see from the dock: how the water deepens, where the bottom shelves out, where it drops away, and where the old channel runs.
If you own a depth map of your lake, or are thinking about one, here is what all those lines and numbers actually mean.
The short answer
NOAA describes bathymetry as the study of the beds, or floors, of bodies of water: oceans, rivers, streams and lakes. A bathymetric map draws that floor with depth contours, lines that connect every point at the same depth. Mapmakers also call them isobaths.
Follow one contour around a lake and the depth under your boat never changes. Cross it and you are one step deeper.
Where the depths come from
Every depth on a chart started as a sounding, a single measurement of how far it is to the bottom. For most of history that meant a lead line: a rope with a lead weight on the end, let down by hand and read one depth at a time, with nothing known about the bottom between one sounding and the next.
Sound changed that. The U.S. Coast and Geodetic Survey adopted acoustic sounding in 1923, timing an echo off the bottom instead of waiting for a weight to land, and by about 1940 the echo sounder had taken over. Multibeam sonar came next. It was developed for the Navy in 1964, first used by NOAA's Coast Survey in 1977 and standard for its surveys since 2000, and it maps a wide strip of the bottom on every pass instead of a single point.
A lead line gave you one depth at a time. Multibeam sonar sweeps a whole strip of the bottom on every pass.
The numbers on a chart
On a nautical chart, the small numbers scattered across the water are soundings. They are given in feet or fathoms, and the chart says which. A fathom is six feet.
The depths are measured from a fixed low-water level, not from wherever the water happens to sit today. On the Great Lakes that level is called Low Water Datum, a surface so low the lake will seldom fall below it. So the water under your hull is usually at least as deep as the chart says.
How to read the lines
- Lines close together mean a steep drop. When contours bunch up, the bottom is falling away fast. When they spread out, you are over a long, shallow flat.
- A ring with deeper water all around it is a hump. A shoal, a reef or a sunken island. Anyone who fishes the lake usually knows exactly where these are.
- A ring with shallower water all around it is a hole. The deepest spot in a lake often shows up as a small closed ring inside the last layer.
- The spacing of the lines is a choice. A map drawn every 5 feet looks far busier than one drawn every 20 feet, even of the same water.
Every lake has its own fingerprint
Once you can read the lines, you start to see that no two bottoms look alike, and the shape usually tells you how the water got there.
Crater Lake: a collapsed volcano
Crater Lake in Oregon fills the caldera of Mount Mazama, a volcano that collapsed about 7,700 years ago. The walls drop so steeply that the layers bunch tight against the rim, then open onto a floor 1,943 feet down, the deepest in the United States.
Houghton Lake: wide and shallow
Houghton Lake is the largest inland lake in Michigan, and it averages only about 9 feet deep. On a map it is a few broad steps from shore to middle and very little else.
Lake Champlain: mostly shelf, then a plunge
Lake Champlain runs 120 miles long but averages just 64 feet deep. Then, between Charlotte, Vermont and Essex, New York, it drops to 400 feet. Most of the lake is shelf, and the middle falls away hard. More in Lake Champlain by the Numbers.
The Great Lakes: neighbors that could not be more different
Lake Superior reaches 1,332 feet. Lake Erie, further down the chain, bottoms out at 210. Side by side on a Great Lakes map, one keeps stacking layers long after the other has run out.
From the chart to the wall
A depth map in wood turns each contour into something you can touch. On the laser-engraved maps, every depth band is its own laser-cut sheet of Baltic birch, stacked in order up to 12 layers deep, with names and soundings engraved into the wood behind an acrylic front. The UV-printed maps use up to 5 layers, with shorelines and place names printed sharp on the top layer and nothing over the wood.
Either way, run a finger from the shore toward the middle and you can feel the bottom fall away.
Know a lake by heart? Find it on the map or browse the best sellers.
Keep reading: The 4 Deepest Lakes in the United States · Laser-Engraved or UV-Printed? · Lake Champlain by the Numbers
Sources: NOAA Ocean Service (bathymetry and soundings); NOAA Office of Coast Survey (history of hydrographic surveys); NOAA Tides and Currents (Low Water Datum); National Park Service (Crater Lake); NOAA Great Lakes Environmental Research Laboratory (Great Lakes depths); Lake Champlain Basin Program (Lake Champlain); Wikipedia (Houghton Lake).