I know that smartphones gps have problems with their accuracy, especially with altitude. But I heard that the Galaxy Nexus has a barometer that theoretically is used to improve the accuracy of the gps.
Anyone has used it to track any route? It can compare (in accuracy) to a mid-range bicycle gps device?
Barometric pressure is usually accurate to less than 5 meters with even a cheap sensor, provided it is calibrated on the day (A fast changing weather system can trow it out however)
GPS altitude is very inaccurate due to the very nature of GPS. Think about the earth, the GPS satellites in orbit, you and the ground The only GPS satellites you can "see" are those above you. To get accurate altitude, you would need to have a satellite below you - which there are - that you can "see" - which you cannot. Therefore it is easy to get accurate lat/long, and hard (nearly impossible) to get accurate altitude.
The only way to get altitude from GPS (the kind that is useful to a cyclist), is to augment the data from either Barometric source, or location based. Unfortunately the altitude profile data is not accurate enough for most cycling applications.
Although I have not used a Nexus, I rely on a Garmin with barometer when in the mountains - My guess is there is no way a GPS based altitude will be more accurate than barometer.
Barometric altitude is far more accurate than GPS over several hours IF you have it corrected for the air pressure. Air pressure does not vary quickly. So if you calibrate a barometer to a known altitude (using a marker, sign, map, or very good GPS fix) it is much better than GPS for measuring an altitude profile
Accuracy varies between each smartphone and dedicated GPS device. It even depends on how the device is used. For example, many smartphones use cell towers in addition to GPS, which can improve accuracy in a city but have no effect on mountain roads. Furthermore, the type of accuracy you require depends on how you plan to use your data.
With respect to barometers, a GPS with a built-in altimeter will show improvement in altitude accuracy. Whether this is helpful depends on whether elevation is important to you (see: Is it possible to estimate road grade while riding?). A barometer will not improve the accuracy of latitude or longitude readings, and if you don't care about elevation while you're riding, you can fix altitude data after you get home (see: How can I fix bad elevation data in a GPS log?).
In short, a barometer will help with elevation accuracy during a ride but won't affect latitude and longitude. Other factors, such as the quality of the GPS receiver and antenna, are more important for the accuracy of latitude and longitude, and these vary from device to device.
