# Grade, Elevation, and GPS Accuracy FAQ

• Updated

Read on to learn about climb data between routes, rides, and different recording platforms, and set expectations for a margin of difference between examples.

Use the elevation profile on web and mobile to estimate the climbing involved in a particular route or ride, and leverage the Ride Center to compare across rides.

## Elevation on Routes and Rides

Rides and routes are not directly comparable, because they are slightly different files.

• A ride is a GPS recording of an activity, which calculates elevation gain and loss from the barometric altimeter of the device (phone or GPS computer) used to record it. These recordings are more accurate than the available estimated data sets on routes.
• A route is a planned activity for GPS navigation. These include elevation gain and loss estimates calculated with an elevation dataset built into our Route Planner. Because of how this data was collected, routes may have less accurate elevation data.

### Elevation on Planned Routes

Estimated elevation data on route comes from a data set collected via radar by a shuttle mission in the early 2000's (SRTM). The resulting elevation data is a grid made up of points every 90 square meters (or about 1/3 the size of a tennis court). For each point in a route planned using Ride with GPS, we calculate estimated elevation using this dataset, with some added math to interpolate between points to increase accuracy (i.e., when a route point falls in between two data points, we do some math to estimate the elevation at that point.)

### Elevation on Ride Recordings

Rides recorded on a device use either barometrically measured elevation (high accuracy, but can be off due to calibration issues) or GPS derived elevation (poor accuracy). Most phones and GPS computers released in the last few years use a barometer for elevation recordings.

In order to calculate gain and loss, we mathematically "smooth" some of this recorded data to minimize error accumulation, and then sum the elevation delta from point to point.

## Compare Rides Between Devices and Software

There are a few factors that contribute to differences in elevation calculations on rides between devices: logging frequency, data collection method, and the software itself.

1. Logging frequency. Most GPS devices allow you to choose how often the device logs a data point. The more frequently GPS data points are collected inherently increases the changes in elevation. Ride with GPS is set to log every 2 seconds by default, and you can manage this frequency in your app settings. Increasing the logging frequency from 2 seconds to 1 second effectively doubles the number of data points recorded.
2. Data collection method. Many GPS units and current smartphones have a barometric pressure sensor. If you are not using a barometric pressure sensor while recording your ride, then you are likely using GPS derived elevation, which is much less accurate. If you have a barometric sensor on your phone, you can toggle it on from the app settings. Things like an incoming storm or entering a building can affect your barometer, but in general this technology provides much greater accuracy than GPS derived elevation. Devices that calculate elevation based on GPS data points leverage triangulation from satellites that are miles away to determine incremental changes in elevation.
3. Software. The data that is collected while recording is first processed on the device, whether that be your Garmin unit or smartphone. GPS is a messy data stream, so the device you are recording with utilizes an algorithm in the software to filter out bad data points. This algorithm process the data and removes outlier points to give you normalized data. Each algorithm has varying levels of thresholds and tolerances for bad data points, and these variations filter out different data points, so the end result is different. The elevation differences between software algorithms should be no more than 10% in either direction.

### Deep Dive on Ride Data

Here's an example to help illustrate why we have to smooth your ride data: imagine you are riding on a mostly flat route, and using a Garmin with a barometric pressure based altimeter. That altimeter has some amount of error in it, so every couple of points it "pings" a value different from the previous point. If we just added up all the elevation deltas from point to point, we'd have a ride that has a claimed gain and loss of thousands of feet even though it's a very flat route.

To avoid this, we smooth out this error using some techniques used in signal processing. It's not perfect, and yields slightly different results depending on how big the error is (GPS vs barometric pressure, different brand sensors, etc) and how frequently the points are logged. Our techniques are consistently within 10% of what a barometric pressure based Garmin unit will state for elevation gain and loss, and recorded rides are consistently within 10% of the elevation our route planner states it will be.

There are deviations from this depending on the quality of the elevation datasets (city vs forest vs flat plains, etc), which are unfortunately unavoidable. We have tried many different methods of calculation over the years, and all that happens is that we make one type of data more "accurate" and another type less. "Accurate" is in quotes, because it's relative to a specific brand of sensor and a specific algorithm used by a specific piece of software. Some people think Garmin is the most accurate source of truth, others Strava, others Ride with GPS, others still Wahoo. The truth is that each system provides an estimate using assumptions, which take into account tradeoffs.

We feel the important thing is to be consistent - if you plan a route with our software, and you see 3,500 feet of elevation gain, when you ride the route you know what to expect. If a different service says it's 4,500 feet, and another says it's 3,000 feet, it doesn't matter, because the important thing is that you can estimate the difficulty of a planned route, and know what it will feel like to ride it.

Using the Replace Elevation feature on a recorded ride can help in some cases, where the ride was recorded with a GPS unit or phone that does not have a high accuracy barometric pressure altimeter. Generally speaking, our elevation datasets are better than GPS derived elevation, and generally are worse than barometric pressure derived elevations. The Replace Elevation feature replaces the rides recorded elevation values with values derived from our elevation datasets.

## GPS Drift

GPS drift can occur when passing over a bridge or through a tunnel. Most people have seen this on occasion - a GPS unit or mobile phone records a line that drifts from the road on a map. Sometimes this drift can look erratic, depending on how poor of a signal the GPS unit received. If this happens while you are riding through the hills where the road traverses a slope, you can see large errors in calculated elevation gain and loss. This is because the GPS point has you off the side of the road by 50 or 100 feet on occasion, which in the case of a road on a scenic hill means the elevation point sampled from our datasets is either down or up the slope!

GPS is highly accurate in the horizontal plane, but very poor in the vertical. This is due to the angle between the line of sight to the various GPS satellites and the ground. Small errors result in big differences in height, but not big differences in location on the earth. As such, a barometric sensor is going to give you the best elevation measurement; using the replace elevation feature follows closely behind the barometer measurements, and GPS derived elevation pulls up the rear. There are some factors that can affect barometric pressure, like an incoming storm or entering a building or riding through an inversion. These conditions don't usually cause large errors from point to point, but usually make the whole ride shift up or down in elevation.

### How do I improve my GPS accuracy?

All GPS receivers (including those on your phone, Garmin, Wahoo, Hammerhead, etc.) will be affected by these building and terrain situations. The most "accurate" conditions are when the device has a clear view of the sky on open terrain.

• Mount your device on the handlebars, and out of bags and pockets (we recommend QuadLock cases and mounts for on-bike phone storage).
• If you must keep your device in a pocket or backpack, stow it as close to the top or surface as possible.
• Stay away from large metal structures like tall buildings, tunnels, and power lines.
• Keep other electronics away from your device. Keeping your smartphone right next to the GPS might cause some drift.
• Try a different case. Some cases with magnetic clasps or metal shielding can interfere.

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