The GPS chartplotter is the most transformational navigation tool since the compass. It has also made a generation of sailors dangerously overconfident in their position. The chartplotter gives you a coordinate. The chart gives you a picture of where that coordinate sits relative to the bottom, the rocks, and the hazards. The accuracy of both components matters, and both have real limits. Understanding them is not an academic exercise — it's how you avoid putting the boat on the rocks in what appears to be clear water.
How GPS Works: Enough to Know the Limits
Your GPS receiver calculates position by measuring the time delay between signals from multiple satellites. Four satellites are the minimum for a three-dimensional position fix. With a clear sky view and 8–12 satellites in view, modern civilian receivers achieve horizontal accuracy of roughly 10 meters (about 33 feet) under normal conditions. This is the baseline without augmentation.
WAAS (Wide Area Augmentation System) in North America, and the broader category of SBAS (Satellite-Based Augmentation System) worldwide, use a network of precisely surveyed ground stations to calculate and broadcast corrections to GPS errors. A WAAS-enabled receiver in open sky can achieve accuracy of 3 meters or better. Most modern chartplotters and handheld GPS units support WAAS. Look for the "D" (differential) indicator on your GPS to confirm it's receiving the correction signal.
The practical limits: GPS accuracy degrades under heavy tree canopy (not usually relevant on a boat), near tall buildings or cliffs, during ionospheric events, and when the satellite geometry is poor — when all visible satellites are clustered in one part of the sky rather than distributed. In a high-sided marina surrounded by buildings, your GPS position can drift by 20–30 feet over a few minutes. Offshore with a clear sky, the WAAS-corrected position is close enough to the actual bottom that chart datum, not GPS error, is usually the dominant source of discrepancy.
Chart Datum: The Problem That Catches Sailors Off Guard
GPS coordinates are expressed in WGS84 — the World Geodetic System 1984 datum, a global standard maintained by the US Department of Defense. Most modern nautical charts are referenced to WGS84, and when your chartplotter overlays your GPS position on a digital chart, the chart is designed to match. This is the good scenario.
The problem arises with older charts — particularly those digitized from paper charts that were originally surveyed using a local datum. The US East Coast used NAD27 (North American Datum 1927) for decades; many charts weren't updated to NAD83 or WGS84 until the 1990s or 2000s. Charts for some international waters still reference local datums: the British use OSGB36, parts of the Caribbean use older local systems. When a WGS84 GPS coordinate is plotted on a chart referenced to a different datum, the resulting offset can range from a few feet to hundreds of feet, depending on location.
The headline example: certain charts for the Pacific Islands have datum shifts of 200–400 meters relative to WGS84. The GPS says the pass is ahead; the actual pass is somewhere to the left. Sailors have gone aground on clearly charted reefs because the GPS position was accurate and the chart datum was not WGS84. Most chartplotters allow you to set the chart datum in the configuration; if you're using raster charts in unfamiliar waters, check what datum the chart was published on and whether your chartplotter is correcting for it.
For US coastal and offshore waters with current NOAA charts, datum is generally not the dominant issue — NOAA has converted its charts to WGS84/NAD83. The caution applies most strongly in developing-world waters, older paper charts that have been scanned, and any chart where the publication date is more than a decade old.
Raster vs. Vector Charts: What the Difference Means Practically
Raster charts are scanned images of paper charts. They look exactly like the paper original — the same symbology, the same typeface, the same depth soundings in the same positions. When you zoom in past the intended display scale, the image pixelates and you lose resolution. Zooming out is fine; zooming in beyond 1:20,000 on a chart designed for 1:80,000 gives you a blurry image that's been magnified beyond its survey resolution. The danger: it looks like a detailed chart, but the underlying data doesn't support that detail.
Vector charts store the underlying chart data as individual objects — a depth sounding is a point with an attached value, a shoal is a polygon, a lighthouse is a symbol at a precise coordinate. This allows smooth zooming in either direction and enables the chartplotter to perform safety functions like automatic depth alarms and hazard queries. Navionics and C-MAP are the dominant vector chart providers for recreational sailors. They differ in their source data, compilation methods, and update cycles, but both are vector format.
The practical difference: vector charts are generally better for navigation planning and active piloting because the software can process the data. Raster charts are often more familiar to experienced navigators and may be easier to interpret when they match the paper charts you're accustomed to. Neither is definitively superior in all situations. Many chartplotters support both, and some navigators use raster for overall situational awareness and vector for detailed piloting in tight quarters.
Both are only as good as their source surveys. A beautifully rendered vector chart of an area last surveyed in 1960 is still a chart based on 1960 data. The rendering quality tells you nothing about the survey quality.
Chart Accuracy by Region: The 1850s Problem
NOAA publishes the survey vintage for its charts, and the variation is dramatic. The Gulf Coast and Northeast US have been resurveyed relatively recently with modern multibeam echo sounders. Significant portions of Florida Bay — the shallow, reef-studded body of water between the Florida mainland and the Keys — were last surveyed in the 1800s. So were parts of the Bahamas that are covered by older NOAA or British Admiralty charts.
What does this mean? The bottom in these areas may have shifted. Shoals may have migrated, new grass beds may have built up sediment, channels may have deepened or shoaled since the original lead-line surveys. The chart may show 6 feet where there are now 4. It may show a clear passage that has since built up. This is not a theoretical concern — boats go aground in the Bahamas and Florida Keys every season in areas where the chart appeared to show adequate depth.
The response is not to avoid these areas but to use the chart as a general guide while navigating on the water: posting a bow watch, following the color of the water (light green over sand, dark blue over deep water, brown over grass, white over coral), and reducing speed to where you can stop before hitting a bottom that appears without warning. The cruising guide often knows more about the actual conditions than the official chart.
The Squiggly Line at the Edge of the Chart
On paper charts and some raster renderings, survey limits are shown as irregular lines — often called caution notes or survey limit lines — that mark the boundary between areas with actual soundings and areas where the chart has been extrapolated or left blank. Beyond the squiggly line, the chart is speculation. Soundings may be absent or widely spaced. Hazards may exist that were never recorded.
In vector charts, the equivalent is an absence of depth data or a sudden shift to a default depth contour where soundings give out. The chartplotter may still display a smooth chart surface, but the underlying data is gone. The seamanlike response is to notice where your surveys end and treat the region beyond as unsurveyed water — slow down, post a watch, and navigate on what you can observe rather than what the chart implies.
Parallel Indexing: Radar and Chart Together
Parallel indexing is a technique that combines radar ranges and bearings with chart work to produce a position that's independent of GPS. The method: identify a prominent radar target (a headland, a buoy, an isolated rock) that's clearly visible on both radar and chart. Draw a line on the chart at a known range from that target, parallel to your intended track. When the radar target appears at that range on the appropriate bearing, you're on the line. As you proceed, the target range changes in a predictable way; deviation from the expected rate of change tells you whether you're inside or outside your intended track.
Parallel indexing is most useful in restricted visibility — fog, rain, night approaches to unfamiliar coastline. It requires a radar that shows accurate ranges, a chart with well-surveyed targets, and practice before the conditions demand it. It's the kind of technique that feels academic until the chartplotter fails in the entrance to a fog-bound harbor, at which point it's the difference between making the approach and anchoring outside.
When Not to Trust the Chartplotter
The chartplotter is a confidence machine. The displayed position looks definitive, the chart looks current, the boat icon is pleasingly precise. The trap is assuming that the displayed accuracy reflects actual accuracy — that a position shown to tenths of an arc-minute is necessarily accurate to within a few meters on a chart that may have 30-meter datum offsets and 150-year-old surveys.
The specific situations where skepticism is warranted: entering unfamiliar harbors with scattered rocks and complex bottom topography; night approaches to anchorages you've never entered in daylight; approaches to low-lying atolls or barrier islands where the chart was last surveyed before GPS existed; any situation where the chartplotter shows you 50 feet of water but the color of the water says otherwise. In these situations, the chartplotter tells you where you are relative to the chart. The chart tells you where you are relative to the chart's survey data. Neither tells you what's actually on the bottom right now.
Keeping Paper On Board
Paper charts are not romantic relics. They are a different information source with different characteristics: they don't require power, they can't crash, they display a larger geographic context at a glance, and they're harder to misread at unfamiliar scales. A navigator with a paper chart and a pencil can work out a dead reckoning position, plot a range and bearing to a known landmark, and lay off a course correction without any electronics involved.
The paper chart also forces a different mode of engagement with the passage. Plotting waypoints by hand on a paper chart requires you to examine each segment, consider each hazard, and consciously decide on each course. The chartplotter makes it easy to drag a route around obstacles without really thinking about what the obstacles are. Both modes have value. The practical recommendation: carry the relevant paper charts for the waters you're in, keep them current enough to trust the major hazards, and use them alongside the chartplotter rather than as a backup you hope never to use.