Hello, welcome to the Weekly Mailbag. I'm Ray, from DCRainmaker. com. If you've ever searched for reviews on sports technology - you've probably come across my site. I write about my triathlon/running training in general, along with sports technology and whatever else seems interesting to me. I started the Weekly Mailbag series earlier this spring as an opportunity to share some of the answers of the many e-mailed questions I get each week.

If you enjoy what you find here, then feel free to click the links to find even more in depth information on each topic. And if you have any questions, you can always e-mail me directly. Thanks for reading!

1) To use or not to use elevation correction
2) Heart rate measurements while underwater combined with distance
3) How to setup run/walk intervals

Question #1: To use or not to use elevation correction
From Rick-

"I've had my Edge 705 for about 2 years, and am very happy with it. However, I have a question. Do you believe the elevation gain measurements are accurate? There is a way to "enable" the elevation feature, I believe, from the internet. Is this more accurate than the Edge 705? I'm questioning it because a century I rode listed the elevation gained as substantially less than my 705. Maybe they used MapMyRide?"

The Edge 705, like all Garmin Edge units, uses a barometric altimeter, which means it's dependent on having an initial elevation be correct, and also dependent on correctly reading the barometric pressure. This is different from a GPS based altimeter, such as what's most common on the Garmin Forerunner line, which uses GPS location data combined with reference data about the earth's curvature to determine your elevation. The problem with GPS based elevation data is it can be less than accurate, at least in non-aviation and non-military grade equipment. By the same token, the Edge's barometric altimeter can also be temperamental. For example, if you have something otherwise stuck in the four little holes on the back, like water, it won't measure correctly.

That said, usually it does measure correctly.

The 'elevation correction' feature that you see on Garmin Connect and other sites takes NASA elevation data that's mapped to the earth using lasers, and adjusts against that. It's not quite 100% perfect, but it's usually very accurate - often far more accurate that the Edge 705's unit. And about as accurate as you're going to get without scientific data out on the road itself measuring it (or known elevation points).

As for the elevation data listed, most sites - like MapMyRide - use elevation or topographic data from a variety of sources. So it depends heavily on if those sources are accurate, as well as if the course creator accurately listed/drew the route.

The nice part is you can turn on/off the elevation correction feature. I often look at it both ways and decide which one I think looks more accurate. For example, on an out and back course the two elevation halves should look like a mirror image. But in general, with the Edge units, I'll stick with the barometric data that the unit recorded.

Question #2: Heart rate measurements while underwater combined with distance
From Sebastien-

"I've bookmarked your reviews of the Swimsense and the Pool-Mate Pro, but unless I'm mistaken, they do not record heart rate. Would you by any chance know a device which does record heart rate while swimming, but without the strap? I've heard about monitors taking your pulse at the wrist, but never found any information on them."

As noted, neither the Swimsense nor Pool-Mate Pro can monitor or record heart rate data, which is a shame since they both track distance, stroke and speed data. On the flip side, well established solutions like the Polar heart rate monitors that do read HR data underwater still require the strap, yet lack any swimming related data.

In short, there's no device today that I'm aware of capable of doing both. There are however a number of devices that can take your HR using your wrist, or your ear - but I've yet to try them.

Nonetheless, the MIO Drive watch allows you to read your HR data - and is fully swimming friendly (waterproofed), but it's unclear if it can both read and swim at the same time.

The FINIS Aqua Pulse works in much the same way as many of the different underwater MP3 players do - and can measure pulse and even audibly tell you about it. But, despite a fair bit of searching, the device doesn't appear to be made anymore, nor listed on the FINIS website.

**Update 6/6/11: I just received a note and call from the head of FINIS to let me know that indeed Aqua Pulse is not dead yet...but in fact, not actually out yet. They'll be launching the Aqua Pulse in the next few weeks. They'll also be updating their website shortly to include the product. In addition, they'll be sending me over a unit to try out.**

And there are other wrist measurement ones as well from a variety of different watch companies, but the no longer available product trail left behind in the wake of these looks much like the carnage of paper cups after an aide station. The general challenge with all of these devices though is the lack of recording options, none offer any downloading to computer, or any way to review the details of the data. That said, I'm optimistic that we'll see some of the major companies in this space (Swimsense/Pool-Mate Pro) innovate a bit more here. Further, I think we'll see some outside solutions integrate with existing watches - in much the same way that the Lapview folks are bringing swim distance data to existing ANT+ watches.

Question #3: How to setup run/walk intervals

"My mother in law is starting a run/walk strategy for the Chicago marathon and has a 305. I can't decide the best way for her to set up a run/walk (4:1 minute ratio). I would like there to be a warm up, so many of these 4:1 repeats (depending on the day and how far she wants to go), then a cool down. This can be setup on the watch if you know exactly how many repeats you want to do. Therefore she would need many workouts setup specific to how far she wants to go. But what if it is hot, or she decides mid run to double her mileage??? Is there a way to set it up so that you can just press lap whenever you want to begin the cool down? When I press lap within the workout mode it just skips to the next run or walk in sequence. I think you said the new FR610 can do this better, but i am hoping you have a better idea to get us going."

Unfortunately, there's not a lot of good options for the FR305 here that do 100% exactly what you want. As you noted, in the FR610 they added the walk/run feature, which would allow you to do exactly what you want.

But in the FR305 we have to improvise a bit. I think you're on the right track with respect to creating a run/walk workout with different targets for each one - even if there is no target at all, as it's merely a reminder to run/walk.

What I'd do is create a workout in Garmin Training Center that simply repeats forever. You can do up to 99 iterations per set, though I think 75 sets of 5-minute each (4m run, 1m walk) should probably cover her. The only problem here is that it'll still be in run/walk mode for the final cool-down. But from her standpoint she could just ignore that piece and simply start her cool down on a given set…or whenever she wants. That would get her 99% of what she wants (run/walk reminders) with only slightly sacrificing the clear delineation of data for the cool down.

Now, one sorta geeky last hail Mary option to officially trigger the cool down to start is that you could use a spike in heart rate. It's definitely not ideal given the whole point is to cool down, but technically it would work. See, Garmin Training Center allows you to trigger the end of a repeating interval if your heart rate goes too high. So, one could theoretically do a 20-second hard sprint to trigger the cool-down piece. Just simply set a high enough HR that she wouldn't hit it in normal marathon long runs and you're set. Out of the box thinking, and probably not ideal…but still technically an option.

Recent Mailbags:

Weekly Mailbag - May 30th, 2011
- Is Edge 500 reliable with PowerTap?
- Garmin FR310XT and where to wear it during swim
- Training with Heart Rate Resources

Weekly Mailbag - May 22nd, 2011:
- Comparing Garmin GTU-10 vs Spot Tracker
- Creating CompuTrainer courses from GPS files
- Heart Rate Strap Combo Platters

Weekly Mailbag - May 8th, 2011:
- Connecting multiple ANT+ units to a single HR strap
- Screen protectors for Forerunner devices
- HED3 wheel speed sensor magnet options?

You can find all past Slowtwitch Mailbags here, and all prior ones here.

[Editor's note: our capable editor-at-large for electronics Ray Maker is the publisher of the online sports tech blog DC Rainmaker, one of the top-ranked sites by Google for extremely in-depth reviews of advanced GPS and Heart Rate Monitors for triathlon, cycling, and running.]