A topographic map describes the shape of ground you can see. A nautical chart describes the shape of ground you cannot, plus everything on it that will damage your boat, plus how to find your way among it in the dark.
It is a denser document than a map and it rewards study in a way few things do. It is also, for anyone who fishes salt water, the single best structure-finding tool available, because every feature that holds fish is drawn on it and most people never look.
Depths, and what they are measured from
The numbers scattered across the water are soundings, and like everything else in American tidal work they are referenced to a datum rather than to the actual water.
In United States waters, charted depths are referenced to mean lower low water, the same datum used for tide predictions. That is deliberate and it makes the two documents work together: the charted sounding is the depth you can expect at an average lower low tide, and the tide prediction is what to add or subtract for the moment you are actually there.
So a sounding of 6 at a moment when the tide table predicts plus 2.5 means roughly eight and a half feet of water. At minus 1.0 it means five. That single piece of arithmetic is the core skill of chart reading and it is why reading a tide table and reading a chart are really one subject.
Check the units. Charts are published in feet, fathoms, or meters depending on the chart, and the unit is stated prominently in the title block because getting it wrong is the classic and occasionally fatal error. A fathom is six feet.
Areas that dry out at low water get their own treatment, usually a green tint with the drying height underlined or otherwise distinguished. Those numbers mean the ground rises above the datum rather than below it, which is the opposite sign from everything around them.
Understanding Nautical Charts, with NOAA's Marine Chart DivisionThe colors are depth bands
Chart shading is a depth code and it is worth learning because it lets you read a chart at a glance instead of number by number.
Deep water is white. Shallower water carries a blue tint, sometimes in more than one shade for successive depth bands. Ground that dries at low water is green. Land is buff or gray. The exact depths those bands correspond to are chosen for the chart and printed on it.
Between the bands run depth contours, which do exactly what contours do on a topographic map: connect points of equal depth. Everything true about reading a topographic map applies here. Contours close together are a steep drop-off. Contours far apart are a flat. A closed contour with shallower numbers inside it is a hump or a bank, which is the underwater equivalent of a hill and is very frequently the best spot on the chart.
The abbreviations that anglers should read first
Scattered among the soundings are small italic letters, and they describe the bottom.
S is sand. M is mud. Rk or R is rock. Co is coral. G is gravel, Sh is shells, Wd is weed, Cl is clay. There are many more and they combine, so fS is fine sand and hrd is hard.
This is the part of the chart most recreational users never notice and it is the part that decides where fish are. A transition from mud to hard bottom is an edge. A patch of shell or rock in the middle of a sand flat is structure. Weed beds hold bait. Anyone who has spent money on side-scanning sonar to find hard bottom should know that a hundred-year-old lead-line survey wrote a good deal of it down already.
The same goes for wrecks and obstructions, which get their own symbols and are among the most reliable fish-holding features in any coastal area. A charted wreck is a permanent, published, publicly known piece of structure, which means it is fished, and it also means it is findable without any electronics at all.
Aids to navigation
Buoys, beacons and lights are the chart's other main content and they follow a system.
In United States waters, the lateral convention is red on the right when entering from seaward, which is where the phrase red right returning comes from. Red marks are usually even numbered and often nun shaped; green marks are odd numbered and often can shaped. Worth being clear that this is the American convention. Much of the world uses the opposite, so the habit does not travel and neither should the mnemonic.
Lights carry a description in abbreviated form: the character of the flash, the color, the period in seconds, the height and the nominal range. Fl is flashing, Oc is occulting, Iso is isophase, Q is quick. A light noted as Fl R 4s is a red light flashing once every four seconds, and at night that description is how you tell it from the three other lights you can see.
Daymarks, ranges, radio beacons and sound signals all get symbols. Ranges in particular are underused by recreational boaters: two marks that line up to define a safe line through a channel, which is a piece of pre-electronic engineering that still works perfectly.
How to Read a Marine Chart, and Chartplotters TooThe compass rose, and why there are two
Somewhere on the chart is a compass rose, and on paper charts it is usually drawn as two concentric circles.
The outer ring is true north. The inner ring is magnetic north, offset by the local variation, with the amount and the annual rate of change printed in the middle. Your compass reads the inner one. Anything you plot on the chart with a parallel rule and anything you read off the outer ring needs the correction applied.
This is the same idea as declination on a topographic map, under a different name, and it drifts for the same reason. On an old chart the printed variation is stale.
How old is this data
Here is the part that surprises people who assume charts are surveyed continuously.
A great deal of the depth information on American charts derives from surveys conducted a very long time ago, some of it by lead line from small boats. Modern charts carry source information describing where the data for each area came from and how reliable it is, and electronic charts carry a similar quality indicator. That information exists precisely because the answer is not uniform.
The practical consequence: in well-traveled channels and harbors the data is modern and excellent. Off the beaten track, over a shifting sand bottom, or in a place with no commercial traffic, the soundings may predate anyone alive and the bottom may have moved since. Sandbars migrate. Channels shoal. Storms rearrange inlets in a single night.
So read the source information if the chart has it, treat unsurveyed and sparsely surveyed areas with respect, and do not assume that a blank patch means deep water. It may mean nobody has measured it.
Finding fish on a chart
Worth setting out explicitly, because this is the argument for a chart as a fishing tool rather than a safety document.
Every feature that concentrates fish is a feature that concentrates water, and a chart draws the shape of the water better than anything else you own.
Look for edges first. A channel edge, where contours crowd along the side of a dredged or natural cut, is a highway with a wall beside it, and predators use walls. The steeper the contour crowding, the harder the edge.
Look for humps and banks: closed contours with shallower numbers inside, sitting in deeper water. These are the classic offshore fishing marks and they work because current has to go around and over them, which concentrates bait on the up-current side and creates an eddy on the down-current side.
Look for constrictions. Anywhere the chart narrows, between a point and a shoal, through an inlet, between two islands, the current accelerates. Acceleration means bait cannot hold position, which means something is waiting where it slows down again.
Look for bottom transitions, which is what those italic letters are for. The line where sand meets rock, or mud meets shell, is an edge in a different dimension and it holds fish that a depth contour would never reveal.
And look at the same spot at different tide stages, because a hump that is in fifteen feet of water at high tide and eight at low is a different place in each case, and the stage at which it becomes interesting is something you can predict from the chart plus the table rather than discover by accident.
None of this requires electronics. All of it is on a document you can buy for the price of a couple of lures and read at your kitchen table in winter.
Charts versus fishing charts
A navigational chart is drawn for keeping boats off things. A fishing chart is drawn from the same data with a different priority, and the differences are worth understanding.
Fishing charts typically add depth contours at much finer intervals, highlight structure, mark hard bottom and known reefs, and often overlay proprietary survey data. What they usually are not is the authoritative document for navigation, and the good ones say so.
Use the navigational chart for the question of whether you will hit something and the fishing chart for the question of where to stop. The habit of keeping those two questions separate is the one that keeps people off rocks.
Everything else on the water
Charts show a great deal that is neither depth nor navigation aid, and some of it is regulatory.
Restricted and prohibited areas, security zones, naval operating areas, and anchorage grounds are all charted. So are cable and pipeline routes, which matter because anchoring on one is both dangerous and expensive. Dumping grounds, spoil areas and fish haven or artificial reef sites are marked, and the last of those is a charted invitation.
Marine protected areas and marine reserves may or may not appear depending on the chart and the area, and their boundaries are set by whichever agency established them rather than by the chart maker. That is a place to check the source rather than trust the rendering, for the same reasons laid out in why map boundaries are wrong.
The shore side of the chart is thinner than a land map, showing mostly what is visible from the water and what helps you fix a position. It will not answer questions about who owns the beach, which is the subject of public tidelands versus private, and it will not tell you where you may legally launch, which is how to find public fishing access.
Paper and electronic
The national chart production system has been transitioning from traditional raster charts, which are essentially pictures of a paper chart, toward electronic navigational charts, which are databases of individual objects that a plotter draws at whatever scale you ask for.
Vector charts are genuinely better in several ways. They scale cleanly, they carry attributes you can query, and they can be updated in pieces. They also introduce a failure mode paper never had: at a given zoom the display may not be drawing everything the database contains, so a hazard can be present in the data and absent from your screen. Knowing that your plotter has a display setting which hides objects is a specifically modern piece of chart literacy.
The unglamorous advice remains what it has always been. Carry a paper backup for the water you actually use, because the plotter is a single point of failure and the chart in the drawer is not.
The gear
Paper charts for your home water, current edition, folded to the area you use. They are the backup, they show a large area at once, and they let you plan at a table.
A chart symbol reference, which is published free by the chart agency and answers the question you will actually have, which is what that particular squiggle means.
Dividers and a parallel rule or plotter, for measuring distance and transferring bearings. Learning to plot a position by hand takes an afternoon and it is the skill that makes the chart legible even if you never use it in anger.
A tide table for the same water, because a sounding without a tide height is only half a number.
A depth sounder, which is the instrument that tells you when the chart and reality have parted company.
A handheld backup for position, kept separate from the boat's electrical system, because the failure that takes out your plotter usually takes out everything wired to the same bus.
And a habit of updating. Charts are corrected continuously through published notices, and hazards move. An uncorrected chart from a few seasons ago is a good planning document and a poor navigational one.
The short version
Soundings are depths at mean lower low water, so add the tide. Colors are depth bands and contours are drop-offs. The little italic letters are the bottom, and they are where the fish are. Red on the right coming in, in this country only. And the survey behind any given patch of water may be a great deal older than you assume.
Learn those and a nautical chart stops being a wall of numbers and becomes the most information-dense document in the outdoors.
