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From Fellrnr.com, Running tips
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The 235 is not a good choice for ultrarunners, as its battery life is too short. You can charge the 235 while you're running, but I'd recommend getting a watch with a longer battery life so you don't have to charge it so often. See [[Best Running Watch#Watches for Ultrarunning| Watches for Ultrarunning]] for more details.
=Garmin 230/235/630=
Garmin produces three similar watches; the base 230, the 235 that adds optical heart rate monitoring (OHRM) to the base 230, and the 630 that adds extra functionality to the base 230. I don't see that the additional price of the 630 is worthwhile, so if you don't want the OHRM then go for the 230.
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* The Garmin Connect smartphone app and website are both excellent, and Garmin is continuing to improve them.
=Garmin 235 Cons=
* The [[GPS Accuracy]] is terrible. I'll add more ; see below for details in the near future.
* The [[Optical Heart Rate Monitoring]] is also terrible, but it's broadly similar to other systems. See below for details.
* I love the idea of having continuous hear rate monitoring, especially to get things like [[Resting Heart Rate]]. However, the poor accuracy limits the value, and the continuous monitoring seems to add problems where the 235 gets "stuck" at a particular value. I'd expected the heart rate data to be used for calorie calculations, but it doesn't seem to have any effect.
* There's no support for displaying your current [[Pace From A Footpod]] while getting all other data from GPS. For situations where pacing is critical, such as running a marathon, this is a must have feature. If you need to know how fast your running, look elsewhere.
* The 235 includes an internal accelerometer to give you an idea of pace and distance while running on a treadmill without a footpod, but I found the accuracy was rather poor. I've yet to come across a watch that can reasonably estimate your pace from the movement of your wrist, nor does it seem likely that this functionality is practical.
=GPS Accuracy =
The Garmin 235 did a rather worse than even I expected, and I had remarkably low expectations. The images below back up the more rigorous statistical analysis to your find on my [[GPS Accuracy]] page. Given that the [[Garmin 225]] did worse than the [[Garmin 620]], and the 225 is essentially a 620 with Optical Heart Rate Monitoring, it's possible that the 230 or 630 might do a little better than the 235. However, even if those invariants are proportionally better, it's still a really grim level of inaccuracy.
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|[[File:BridgeGarmin 235.jpg|none|thumb|x300px| You can see the 235 does terribly in the middle twisty section, which is a difficult section for most GPS watches. However, they rarely do quite this badly. The 235 also does quite poorly under the bridge, which counterintuitively is a section that most watches are able to handle without issues. You can see a few tracks where the 235 seems to get significantly lost, and the spread of the blue lap markers is further indications of some fundamental issues. If you compare this image with the equivalent on my [[Garmin 225]] review, you'll see how much worse it does than even that rather mediocre watch. To show how the tracks should look, check out my review of the [[Polar V800]] or [[Suunto Ambit3]]. (This diagram has tracks color coded with green indicating good accuracy through to red indicating poor accuracy, and the lap markers as blue dots.)]]
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|[[File:AccuracyGarmin 235.jpg|none|thumb|x300px| This close up section of a zigzag shows the 225 failing to track the shape of the trail, with far too many red colored lines, and the blue lap markers widely distributed. (This diagram has tracks color coded with green indicating good accuracy through to red indicating poor accuracy, and the lap markers as blue dots.)]]
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|[[File:ZigZagGarmin 235.jpg|none|thumb|x300px| Most GPS watches tend to show the tracks that have the green lines shifted slightly down and to the left, blue up and to the right. The 235 does not show this typical pattern, though it may be that the green lines are a little closer to the actual trail than the blue lines. It's not unusual for a GPS watch to be more accurate in one direction than the other. You can see the lap markers shifted based on direction which is also typical. (This image has the tracks color-coded for direction, with green coming from the right, blue from the left.)]]
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=Optical Heart Rate Monitoring=
To analyze the accuracy of the 235's optical heart rate monitor I gathered a little over 20,000 readings and analyzed them in comparison with an ECG-based chest strap heart rate monitor. I created to cross to visualize this comparison. The first graph shows the heart rate measured by the ECG HRM on the horizontal against the Garmin 235 on the vertical. If the two systems match exactly then the point will be on the green line of equality. You can see that many of the points cluster around this green line, but are distributed more widely than I think is acceptable. In addition you can see a cloud of points well away from the green line that represent times when is the 235 was reading a wildly high heart rate. There is some discussion within the running community that these high readings might be the optical system becoming confused by the impact of foot strikes as they tend to be vaguely in the vicinity of [[Cadence]]. This graph uses transparent points to give a better impression of the density of data, with black areas having at least 10 data points lining up. There is a blue regression line, which will be aligned with the green line if the system is accurate, but as you can see it is distinctly misaligned. I've also included two red lines that are 25 bpm away from the true value. The second histogram shows the distribution of errors, and includes the erroneous cloud of points is a slight bump to the right of the main spike.
