RoboCheme

I do a lot of climbing and the total ascent values that my 705 shows at the end of my ride appear to be pretty accurate based on my knowledge of the actual hill elevations. When I get home and load the ride data into any number of programs (Strava, Garmin Connect, Garmin Training Center), the total ascent numbers are about 10% higher. For example, at the end of a typical ride, the 705 will show 3015 feet (after adjusting for slight changes in the starting point elevation, normally less than 20'), but the programs will show that I did 3405'. Now I know the latter sounds a lot better, but I'd rather stick with the right nos.

I've read explanations on why there are the differences, but my question is why don't at least the Garmin programs use the altimeter based readings (the 3015' no. in the above example)? All of these programs seem to prefer to interpret the GPS routes and calculate their ascent nos. Is the 3015 no. not kept in the TCX file?

Thanks

Tulex

I'm actually having a hard time figuring out what Garmin uses. For example, it appears that the display shows gps numbers for speed even if you have the sensor, yet records the sensor speed. Also, I have let my unit sit for a long period to settle on an elevation. I know I am at 375. It never got there. But, when I hit the start button, the elevation changed, dropping to an even lower number if I recall. But, what it showed in training center was different still.

AMFJ

My 705 will give me different grade percentage for the same hill on the same ride. I did a loop yesterday 2 times as part of a ride. The same hill was reported as 9 and 10% grade. It felt steeper on the second lap, but I'm pretty sure it wasn't. . .

at this point, I take most of the data from the 705 as "generalized" for each ride. It gives me what I want to know, even if it isn't exact.

rankin116

Doesn't the 705 use pressure changes to estimate elevation? Or was that the 305?

RoboCheme

When you push the start button, it looks for a waypoint within 30 feet (I think) and it uses that elevation. For example, I put in my home's location and elevation as a waypoint. When I start a ride from home it always resets to that elevation. I use it to correct the ascent nos. since the elevation reading is a bit different when I get back because the barometic pressure has changed.

RoboCheme

it uses changes in barometric pressure to estimate changes in elevation.

Tulex

I have my home position as a point, but no elevation set for it. It continuously sets to around 240 feet, even though I am at 375, and it used to settle in to around there until I did the 3.1 upgrade. Even though I have gone back to 2.9, it still isn't right.

umd

All the programs use the same data. It's all about how you add them up. The UNIT does some smoothing while it is accumulating. Some programs just add them up raw, and some do their own smoothing. Measuring total elevation gain is a problem like "how long is a coastline" There just isn't a single correct answer, it depends on how precicely you measure it.

chasmm

This is actually getting a bit of discussion over on the Garmin forums. It seems that a recent update may have futzed with the speed calculation algorithm (that's the general consensus). The Edge 500, for example, is supposed to use the the GSC-10 speed/cadence sensor to determine SPEED, based upon the wheel size entered. If you enter "AUTO" as the wheel size, then the unit is supposed to use the GPS to help determine the optimum wheel size based upon the number of wheel revolutions in some distance. If you enter a number manually, then that number times the number of wheel revolutions recorded by the speed/cadence sensor should be your speed. The GPS should only be used (again, with a manually entered wheel size) if the system is not registering the magnet passing by the speed/cadence sensor.

However, since the most recent 2.40 firmware update, there have been several reports of wild speed reading fluctuations when under heavy tree cover which is indicative of the system getting the speed via GPS. Owners of other GPS based Garmin units are reporting the same thing...that it seems their units are using GPS rather than the speed/cadence sensor at the primary means of determining speed.

At this point, we're waiting and hoping for further clarification from Garmin...

Charles

Tulex

icyclist 

Because I've kept data for years on my rides, I can see how consistently my Garmin 500 measures elevation gain, both with the software on my computer, and via the Garmin website. While there is some variance there, it's a consistent variance, so for me, it's just a matter of choosing which measurement to believe, and sticking with that one. Looking back over my ascents with my VDO cyclometer, I see the readings I got then are fairly close to what I'm getting with my Garmin now.

Of course, the connect.garmin.com site measurements include an option for elevation correction, adding another wrinkle; those corrections seem fairly consistent, too, adding about 100 feet for every thousand feet gained.

I just go with the elevation measurements derived from my computer's software.

Tulex

I just did a ride. I went up a hill that has a T that you can turn right and continues up another hill. When I looked at the readout right after I made the turn, I was going up what I know is about a 4%, yet it said -2%. Yet, when I got back and loaded it into TC, it shows a constant climb right after that turn. So, the display simply does not show what the unit is recording for speed and ascent.

RoboCheme

We seem to be getting off track here. My question is still the same - why isn't the ascent no. from the Garmin unit used? It's based on the altimeter and not on some interpretation of the route.

umd

It doesn't store it anywhere

RoboCheme

OK, thanks. That seems rather stupid. Oh well.