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the grade of the climb (“% GRADE”) or how much climbing you’ve done in a workout (“FT GAIN” or “MTR GAIN”).
    Some head units use the Global Positioning System (GPS) for this function, while others use barometric pressure. You may notice when standing at a stoplight that the altitude reading seems to bounce around quitea bit. That’s common with both systems and reflects “handoffs” between newly arriving overhead satellites (GPS) or changes in temperature and atmospheric pressure (barometer). Which is more accurate for determining altitude, GPS or a barometer? There’s a fair amount of disagreement on this matter among experts. For our purposes in riding a bike, it’s not important. What we want is reliability. For example, your bathroom scale is probably not absolutely accurate to the ounce, but (you hope) it is reliable—you are confident that if it shows a change of 1 pound, what changed was you and not the scale. That’s the same sort of confidence we want in your power meter.
    You may even find that when you download the data after a ride, altitude changes such as feet or meters gained and starting and ending altitudes displayed on your head unit don’t agree with what the software says. That’s also common and has to do with the algorithms being used by each to compute altitude. Some software, such as TrainingPeaks and WKO+, correct your elevation profile based upon known coordinates in the U.S. Geological Survey (USGS) database when GPS is used. Of course, the most important piece of altitude data is feet gained rather than actual altitude at any point in the ride. Regardless, I’d recommend relying on the one shown by your software and always using it for workout analysis.
Speed
    I’ve never met a cyclist who isn’t interested in speed, so most riders set up their power meter head units to display speed in “MPH” or “KPH.” In fact, however, there is a good reason to monitor speed besides the simple thrill of seeing how fast you went. In Chapter 5 , I’ll show you how you can use speed in conjunction with power to reliably pace steady-state races such as time trials and triathlons.
Temperature
    Some head units give you the option of displaying temperature (“TEMP”) in a field, while others measure it but don’t display it in real time, instead opting to reveal temperature after the fact in the download to your software. Temperature measurement not only has to do with how warm and comfortable you may be on the ride but also is used to adjust the altimeter on the head unit if it relies on a barometer for altitude display. Older power meters used to be affected by big changes in temperature, but recent self-calibrating models have reduced this inaccuracy.
Other
    There are a multitude of other data fields your head unit may be capable of displaying based on how it’s set up and the power meter you are using. For example, it may provide such options as watts per kilogram of body weight (“W/KG”), the current power zone you are in (“ZONE”), Normalized Power™ (“NORM PWR”), Training Stress Score (“TSS”), and Intensity Factor™ (“IF”). I’ll explain each of these in the following chapters. They are critical bits of data that reflect your performance. You can wait until you know more about them before deciding whether you want to display one or more of them on your head unit.
    There are even more options, again depending on the power system you have. Other data fields could be dedicated to vertical ascent in meters per hour (“VAM”), kiloJoules per hour (“KJ/HR”), and left-right pedal balance (“L-R”). GPS-based head units often provide maps and directions much like those you may use when driving your car.
    With all of these possibilities, setting up your handlebar display can be a daunting task. For now, you may want to keep the settings on the unit’s default, just as it came out of the box. Later on, as you get the hangof what all this means, you can