GPS vs Barometric Altimeter: Which Is More Accurate?

For most outdoor and aviation uses, a barometric altimeter is more accurate for measuring changes in elevation over short periods, while a GPS altimeter is often more accurate for absolute altitude above sea level in stable conditions. However, neither is universally superior—accuracy depends on the device, the environment, and how the altitude is being measured. This article explains how each system works, where they excel, and how to choose the right one for your needs.

Formula

The calculator above computes your average pace and speed from a finish time and distance. The formulas are:

• Pace (min/km) = Finish time (min) ÷ Distance (km)
• Speed (km/h) = Distance (km) ÷ (Finish time (min) ÷ 60)

Where:

• t = finish time in minutes
• d = distance in kilometers

Worked Example

Using the default values: t = 50 minutes, d = 10 km.

1. Pace = 50 ÷ 10 = 5.00 min/km
2. Speed = 10 ÷ (50 ÷ 60) = 10 ÷ 0.8333 = 12.00 km/h

The calculator shows an average pace of 5:00 min/km and an average speed of 12.00 km/h.

How Barometric Altimeters Work

A barometric altimeter measures altitude by sensing air pressure. Atmospheric pressure decreases predictably as you gain elevation, so the device converts pressure changes into altitude changes. Most barometric altimeters use a reference pressure (often set at sea level) and apply the hypsometric formula to estimate height.

Key characteristics:

• Excellent short-term relative accuracy: They detect small elevation changes (as little as 1–3 meters) quickly, making them ideal for tracking climbing or descending.
• Drift over time: Weather changes cause air pressure to fluctuate, so a barometric altimeter can drift by tens of meters over hours unless recalibrated.
• Requires calibration: You must set a known reference altitude (e.g., a trailhead marker or airport elevation) for best accuracy.

How GPS Altimeters Work

GPS receivers calculate altitude by triangulating signals from satellites. They use the same timing data that gives horizontal position, but altitude is computed from the geometry of the satellite constellation. This is inherently less precise than horizontal positioning because satellites are mostly above the receiver, not spread around it.

Key characteristics:

• Absolute altitude without calibration: GPS provides an altitude above the WGS84 ellipsoid (or geoid if corrected) without needing a reference point.
• Error sources: Multipath (signal reflection), atmospheric delays, and poor satellite geometry can cause vertical errors of 5–50 meters, often worse than horizontal errors.
• Stable over long periods: GPS altitude doesn’t drift with weather, so it can be more consistent for long trips if the satellite geometry remains good.

Accuracy Comparison in Practice

Real-world tests and user reports consistently show:

• Barometric: When calibrated and used over a few hours, barometric altimeters typically achieve accuracy within 3–10 meters. They excel at showing whether you’re gaining or losing elevation.
• GPS: Consumer GPS devices often show vertical errors of 10–30 meters, but with WAAS or multi-band GNSS, errors can drop to 5–10 meters in open sky.
• Combined devices: Many modern smartwatches and handheld GPS units fuse both sensors, using barometric data for short-term changes and GPS for absolute reference, yielding the best overall accuracy.

For example, a hiker climbing a steep ridge would see the barometric altimeter respond almost instantly to each 5-meter gain, while a GPS might lag or jump by 10 meters. Conversely, after a weather front moves in, the barometric reading could be off by 30 meters, while GPS remains unchanged.

Common Problems and Misconceptions

Misconception: GPS altitude is always wrong. While GPS vertical accuracy is generally worse than horizontal, modern receivers with good satellite coverage can be quite accurate. The bigger issue is that GPS altitude is referenced to an ellipsoid, not mean sea level, so it can be offset by dozens of meters unless a geoid model corrects it.

Problem: Barometric drift from weather. A sudden drop in atmospheric pressure (e.g., approaching storm) makes the altimeter think you’ve gained altitude. This is why pilots get updated pressure settings from weather stations.

Problem: Temperature effects. Barometric altimeters assume a standard temperature lapse rate. In unusually cold or hot conditions, the reading can be off by several percent. Some advanced units compensate with temperature sensors.

How to Choose: GPS vs Barometric

Consider your primary use:

• Hiking and mountaineering: A barometric altimeter (or a watch with both) is preferred for tracking ascent/descent and detecting small elevation changes. Calibrate at known points.
• Aviation: Barometric altimeters are standard in aircraft, set to local pressure (QNH) for accurate altitude above sea level. GPS is used as a backup but not for primary separation.
• Running and cycling: GPS is often sufficient for elevation profiles, but barometric sensors provide smoother data in urban canyons or under tree cover where GPS signals degrade.
• Long expeditions: If you can’t recalibrate frequently, GPS may be more reliable for absolute altitude, but barometric data is better for day-to-day elevation gain.

In practice, most modern devices combine both. If you must choose, ask: do you need quick, precise changes (barometric) or a stable absolute number without calibration (GPS)?

Frequently Asked Questions

Why does my GPS altitude show a different elevation than a topographic map?

Maps use mean sea level, while GPS often uses an ellipsoid model. The difference (geoid undulation) can be 20–50 meters in some regions. Many GPS devices apply a geoid correction to display sea-level altitude, but not all do. Check your device settings.

Can I use a barometric altimeter on an airplane?

Yes, but you must set the local pressure (QNH) from air traffic control or an airport weather report. Without that, the altitude will be off by the difference between current pressure and standard pressure (1013.25 hPa).

How often should I calibrate a barometric altimeter?

Calibrate at least once per trip at a known elevation (e.g., trailhead sign, summit marker). If weather changes significantly, recalibrate. Some devices auto-calibrate using GPS when available.

Is a smartphone’s altitude accurate?

Smartphones typically use a barometric sensor (if present) combined with GPS. Accuracy varies by model and conditions. In open sky, expect 5–15 meters; indoors, barometric data may be used but can drift.

Which is better for tracking vertical gain on a run?

A barometric altimeter is better because it detects small changes quickly and isn’t affected by GPS signal loss under trees or near buildings. Many running watches use barometric data for elevation gain.

Does weather affect GPS altitude?

Weather can affect GPS signals slightly due to atmospheric refraction, but it’s not a major factor. Barometric altimeters are far more sensitive to weather changes.

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in t | Finish time | min | 50 | 0
in d | Distance | km | 10 | 0
out pace | Average pace | min/km | t / d | 2
out speed | Average speed | km/h | d / (t / 60) | 2