Measuring ascent

[davem]

I have a Suunto T6c. The total climb measured in a number of races differs by a significant amount to that from others I have compared with (as much a s 10%). It uses atmospheric pressure to calculate change in altitude.

 

I have even registered a climb of up to 30m in a spinning class.

 

Do the GPS based units (Garmin) use a different method or is it the same.

 

I have not found a way to calibrate the pressure. I have set the base altitude correctly off Google Earth.

 

My total climb for D2D was 1124m. What did you measure?

 

Maybe I need to have the Suunto serviced.

[NinjaManiak]

Some Garmin use GPS reading for elevation, but the Edge series uses GPS combined with atmospheric pressure.

Ascent is also dependant on the polling/measurement frequency, measurement accuracy, and any smoothing factors applied to the measurements by the software.

 

Example: If you were to ride around and around a flat parking lot and take a measurement every few seconds, you should get no ascent. If you used a more accurate device (say GPS, with base station, altimeter, inclometer, the whole works), and took  a measurement every 2 cm, then you'd start measuring "Ascent" on all the speed-bumps and pavements you ride over.

 

Example2: If you drove from CPT to JHB, and took a measurement at each end, your Ascent would be 1500m. If you took more measurements along the way, it might be a bit more...

Ninja Maniac2009-09-16 02:06:02

[davem]

Thanks Ninja.

 

I measure every 10s which should give a reasonably accurate overall reading as you are unlikely to go up and down (with a smaller net measurement) within 10s.

 

Would be interesting to see the readings from an Edge for D2D.

 

Can an atmospheric altimeter become inaccurate through wear? Or is it one of those devices which either work or doesn't?

[tabasco]

I have a Suunto T6c. The total climb measured in a number of races differs by a significant amount to that from others I have compared with (as much a s 10%). It uses atmospheric pressure to calculate change in altitude.

 

I have even registered a climb of up to 30m in a spinning class.

 

Do the GPS based units (Garmin) use a different method or is it the same.

 

I have not found a way to calibrate the pressure. I have set the base altitude correctly off Google Earth.

 

My total climb for D2D was 1124m. What did you measure?

 

Maybe I need to have the Suunto serviced.

 

Garmin gave me 1024 meters wich is about 10% difference. I am not familiar with the Suunto, but I know with some insruments, the pressure sensor could have dirt in it which can cause problems.

[Daxiet]

Most devices including Garmin Edge only make use of a barometric pressure sensor. Barometric pressure is affected by temp, weather conditions.

 

So in your spinning class if it got warmer in the room, you would register some ascent on most devices. I have compared ascent with a friend who rode the exact same route as me with me, both Edge 705's and we had a descrepency of 60m (total climb ~500m).

 

That is why pilots make contact with the tower to verify the current barometric pressure on the ground, then factor that into the altimeter, otherwise they would smack the ground a lot. There is a special aviation term for this, I just can't recall.

 

For D2D my Edge read 1198m on Garmin Connect site and 1148m in Training Center - Go Garmin, just another bug they do nothing about.

 

Keep in mind the early you started the more climbing you device would falsely read and the longer you stay out there the more climbing you would record - this is because the temp would keep rising and affecting barometric pressure.

 

Daxiet2009-09-16 03:18:16

[Brighter-Lights]

VDO Z3 and Polar 625X eg. gives up to 100m diff for our home altitude with different weather conditions...

Have to know your actual start altitude and set it everytime you go. Even then there can be diffs. - you start in sunny town and weather changes + start raining in the mountain...

 

 

 

 

[Dirtbreath]

I use a t6 with a GPS pod. I find the altitude a lot more consistant when you mount the watch on the bars than on your wrist which heats up more.

 

I mistakenly changed my settings to give baro pressure instead of altitude and it doesnt give you altitude on the download. The GPS pod however still gives the distance. Does anyone know if this is a true distance or just a straight line distance excluding any allowance for ascent/descent?

 

Speak to Steve at Manex Marine if you have any more problems.

[NinjaManiak]

Thanks Ninja.

 

I measure every 10s which should give a reasonably accurate overall reading as you are unlikely to go up and down (with a smaller net measurement) within 10s.

 

Would be interesting to see the readings from an Edge for D2D.

 

Can an atmospheric altimeter become inaccurate through wear? Or is it one of those devices which either work or doesn't?

There are different typres of inaccurate' date=' maybe the whole range will be off (ie. 1000m at Waterfront, and 2000m on Table monutain) which may not effect the ascent reading, but then maybe the scale is effected (ie 0m at waterfront, 5000m on Table monutain), or maybe even jumping around (0m, 166m, 2m, 212m,1m, 234m, 5m, 3423m etc). I'm not sure about this, but I recon that if it ges a reading it should be okay.

 

 

10 seconds should be fine. Just remember that you will start to get "precise error" when your readings are too close together, and in the range of the accuracy of the altimeter. Here is an interesting comparison between Forerunner 305, and Edge 705 for the same cours, showing a bit of the old "Precise Error Overmeasurement phenomenon"*

 

* I haven't written the Wikipedia article yet, so you may not be able to google it just yet.

 

Edge 705

[img']https://cdn.bikehub.co.za/uploads2/20090916_074158_Route_53_30-11-.jpg[/img]

 

Forerunner 305

 

 

The Edge gave 471m or ascent, and the Forerunner 1580m.

However this is with no smoothing. After smoothing is taken into account, the results were Edge 371m, Foreruner 394m.

 

At the end of the day, you need to consider all these factors when quoting the "asccent".

[NinjaManiak]

I use a t6 with a GPS pod. I find the altitude a lot more consistant when you mount the watch on the bars than on your wrist which heats up more.

 

I mistakenly changed my settings to give baro pressure instead of altitude and it doesnt give you altitude on the download. The GPS pod however still gives the distance. Does anyone know if this is a true distance or just a straight line distance excluding any allowance for ascent/descent?

 

Speak to Steve at Manex Marine if you have any more problems.

Straight line

 

The difference should be minimal, over 1km:

 

At 1% gradient = 5cm

At 5% gradient = 1m

At 10% gradient = 5m

At 20% gradient = 20m

[NinjaManiak]

For D2D my Edge read 1198m on Garmin Connect site and 1148m in Training Center - Go Garmin' date=' just another bug they do nothing about.

 

[/quote']

 

 

This is due to the smoothing factor. You'd think they'd be consistent accross their own products, but then again...

Because Garmin have not published their formula, or calculation methods, third party software like SportTracks will also have differences, not only in altitude, but distance and even time.

 

Again it's in the interpretation.

[Bos]

As long as we know no one make is correct theres no standard, haha- just take it with a pinch of salt

[DirtyFrank]

My CS 600 Claims ascent was 1050m. It always seems to under read by approx 10% when compared to the GPS based units. Anyone have similar experience with these units and know how to calibrate these units or is this simply a matter of the barometric units being less accurate?

[davem]

This is fascinating. Thanks for the replies.

 

At the Soweto race, I compared against two chaps who finished at the same time, so many variables were eliminated. They were both using Garmins mounted on the bike, not sure which model.

 

I don't recall the numbers exactly but they both had almost identical readings which were 10% greater than the Suunto wrist mount.

 

Compared to D2D, my Suunto read more than most did.

 

The comment on the temperature effect of a wrist computer versus a bike mount is interesting. A bike mount would be more effected by the wind chill factor on a fast downhill.

 

Perhaps the moral is that this is just a guide.

[Minion]

A bike mount would be more effected by the wind chill factor on a fast downhill.

It won't be affected by wind chill.

Provided it's not in direct sunlight it should read the same air temperature whether you're moving at 1km/h or 100km/h.

[NinjaManiak]

At the end of the day, units with altimeters (polar, suunto, etc) would generally produce better results than units using GPS only (Forerunner, nuvi etc.), and GPS units which also include an altimeter (Edge, Colorado, etc) will produce the best results of all.

Units combining both GPS and barometric altimeters are able to provide the most accurate altitude readings, using absolute location provided by the satellite to help auto-calibrate the barometric altimeter, then the barometric altimeter is used to provide a more stable elevation change. The barometric altimeter also allows elevation readings even when GPS signal is not available."

 

Lastly, as much as "ascent" can depend on many variables, so too is the "altitude". Height can be defined in two ways. The ellipsoid height [h], which is the height above the reference ellipsoid that approximates the earth's surface), and the orthometric height [H], which is the height above the geoid, which is an imaginary surface determined by the earth's gravity and approximated by mean sea level (MSL). 

 

GPS receivers measure ellipsoid height, and then use an approximation of the geoid height to estimate the orthometric height from the geoid height. Depending on what approximation you use (WGS-84 is normally the standard approximation used) you will get different heights.

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Ninja Maniac2009-09-16 12:51:21

[ecocar]

<FONT face="Arial, Helvetica, sans-serif" size=3>At the end of the day, units with altimeters (polar, suunto, etc) would generally produce better results than units using GPS only (Forerunner, nuvi etc.), and GPS units which also include an altimeter (Edge, Colorado, etc) will produce the best results of all. </FONT>

 

<DIV><BR><FONT size=3><FONT face="Arial, Helvetica, sans-serif">Units combining both GPS and barometric altimeters are able to provide the most accurate altitude readings, using absolute location provided by the satellite to help auto-calibrate the barometric altimeter, then the barometric altimeter is used to provide a more stable elevation change. The barometric altimeter also allows elevation readings even when GPS signal is not available."<BR><BR>Lastly, as much as "ascent" can depend on many variables, so too is the "altitude". </FONT><FONT face="Arial, Helvetica, sans-serif">Height can be defined in two ways. The ellipsoid height [h], which is the height above the reference ellipsoid that approximates the earth's surface), and the orthometric height [H], which is the height above the geoid, which is an imaginary surface determined by the earth's gravity and approximated by mean sea level (MSL).</FONT></FONT><SPAN style="mso-spacerun: yes"><FONT face="Arial, Helvetica, sans-serif" size=3> <BR><BR></FONT></SPAN><SPAN style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt"><FONT face="Arial, Helvetica, sans-serif">GPS receivers measure ellipsoid height, and then use an approximation of the geoid height to estimate the orthometric height from the geoid height.<SPAN style="mso-spacerun: yes"> </SPAN>Depending on what approximation you use (WGS-84 is normally the standard approximation used) you will get different heights.<BR><img src="https://cdn.bikehub.co.za/uploads2/20090916_125113_1.JPG" height="279" width="298" border="0" /><BR></FONT></SPAN>

 

<P ="Msonormal"><SPAN style="FONT-SIZE: 12pt; mso-bidi-font-size: 10.0pt"><FONT face="Arial, Helvetica, sans-serif"> <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" /><O:P></O:P></FONT></SPAN></P></DIV><edited><editID>Ninja Maniac</editID><editDate>2009-09-16 12:51:21</editDate></edited>

 

Hi to everyone,

I found this discussion very useful, even if I have a question:

 

How I have to set my suunto watch? Considering the GPS altitude that in my location is of 80 meters + 40 meters(geoid height) or the altitude given by internet for my lat\lon that it is of 80 meters ??

 

And which reference use my watch altimeter, the elipsoid height or orthometric height ?

 

thank you very much!

For more details, read here: http://www.esri.com/news/arcuser/0703/geoid1of3.html