Altitude on Garmin Edge 305

[redfox]

Just got one of these recently. Anyone know if there is a way to change or calibrate the altimeter, like I can on my other Garmin GPS?

At the start of my ride yesterday it said I was 118 metres below sea level (should have been +35).

It was about right when I got back, so it must have caught up with itself at some point, but I don't want to have to hang around waiting for the altimeter at the start of every ride.

 

[frog]

Page 76 of the manual says it's just a barometric altimeter, nothing about calibration. When I start mine up you can see it search for satalites and the level of accuracy increases with each one obtained. Once you get to the data screen if you have a segment for altitude then you will see that settle down after a few seconds.

 

[domtyler]

Page 76 of the manual says it's just a barometric altimeter, nothing about calibration. When I start mine up you can see it search for satalites and the level of accuracy increases with each one obtained. Once you get to the data screen if you have a segment for altitude then you will see that settle down after a few seconds.

I was under the impression that a barometric altimeter uses air pressure to take its reading rather than using the GPS data.

 

[Membrane]

I was under the impression that a barometric altimeter uses air pressure to take its reading rather than using the GPS data.

It does, but it is inherently unreliable if used on its own due to weather related varying air pressures. The 305 can use barometric or GPS to determine altitude. The barometric sensor is used as a fallback for when GPS data isn't available.

As far as calibration goes, afaik upon initialisation the unit uses barometric data only, then when it finds GPS sattelites it uses that data and it also uses that data to "calibrate" the barometric altometer. This means that during a ride if the GPS signal temporarily disappears the unit can fall back on the barometric sensor which by then will be reasonably accurate.

 

[redfox]

Thanks All!

As far as calibration goes, afaik upon initialisation the unit uses barometric data only, then when it finds GPS sattelites it uses that data and it also uses that data to "calibrate" the barometric altometer. This means that during a ride if the GPS signal temporarily disappears the unit can fall back on the barometric sensor which by then will be reasonably accurate.

Just tested mine with this in mind and it seems to be correct. Altitude slowly rose from from -130 to +35 once the 305 new where it was. Looks like I will just have to switch it on and leave it outside for five minutes before I intend to leave.

 

[bobbyp]

The alternative is just to feel smug about how easy those extra 165m of climbing were.

 

[andrew_s]

If the Edge 305 is anything like the 60Csx, if you know the height of your house, it's quicker to enter a manual calibration that it is to wait for the GPS to autocalibrate (ie average a sufficient number of GPS heights).
So, let it autocalibrate once, note the height, then on future occasions just enter the height it found the first time.

 

[redfox]

The 305 isn't anything like the 60csx, which I also have.

I am well used to setting the altitude on the 60csx and not being able to do so on the 305 seemed dumb, hence the post.

 

[gkerr4]

the main purpose of the barometric part, as far as I know, is for the gradient function. so once known by GPS and self calibrated it can work out the change in altitude fairly well and since it also knows distance travelled it can give you the gradient.

 

[andrew_s]

The 305 isn't anything like the 60csx, which I also have.

I am well used to setting the altitude on the 60csx and not being able to do so on the 305 seemed dumb, hence the post.

Ah, just another example of the uselessness of the Edge series then, along with not being able to tell you where you are in any useful form (like an OS grid ref).

(speaking as someone whose idea of "training" is a longer route to the pub)

 

[redfox]

Ah, just another example of the uselessness of the Edge series then, along with not being able to tell you where you are in any useful form (like an OS grid ref).

(speaking as someone whose idea of "training" is a longer route to the pub)

I thinks that's a bit harsh. As a navigational aid it is pretty rubbish, compared to something like the 60CSx, but that isn't intended to be its appeal. It is a training aid and, in that respect works pretty well.

My concern about getting the altitude correct is because I want to be able to race against myself over known routes. I ride these routes a lot and don't require navigation. However, racing myself (or more probably watching my virtual self disappear over the virtual horizon) may help with motivation a bit.

 

[peterdowning]

According to the wiki at Motionbased, the Garmin combines both barometric readings and GPS data for increased accuracy.

http://wiki.motionbased.com/mb/GPS_Barometric_Altimeter

 

[Gary D]

At a slight tangent to the OP, but I thought I would ask the question.

When I return from a ride and upload from my 305, I use both the Training Centre and the MotionBased sites for comparison.

Does anyone know why I get 2 different readings on total altitude from each site?

The ride I did yesterday was 36 miles and TC said there was 1,217 feet "Total Ascent" and MotionBased said 1,720 feet "Elevation Gain" - quite a difference!

Any ideas? Or am I being thick and comparing 2 sets of data based on different parameters. Which is correct?

I was going to post this on the MotionBased forum but seeing as the topic came up here!

Thanks.
Gary.

 

[redfox]

At a slight tangent to the OP, but I thought I would ask the question.

When I return from a ride and upload from my 305, I use both the Training Centre and the MotionBased sites for comparison.

Does anyone know why I get 2 different readings on total altitude from each site?

The ride I did yesterday was 36 miles and TC said there was 1,217 feet "Total Ascent" and MotionBased said 1,720 feet "Elevation Gain" - quite a difference!

Any ideas? Or am I being thick and comparing 2 sets of data based on different parameters. Which is correct?

I was going to post this on the MotionBased forum but seeing as the topic came up here!

Thanks.
Gary.

FWIW my guess (based on no knowledge of how either piece of software works) would be that motionbased, being a web service that has to support many types of GPS (some with no altimeter) in a uniform way, may look up the elevation of each lat/lon pair in a database as the ride is imported. Whereas Training Centre can safely use the elevation data given by the GPS.

I have found that most route planning software (TrackLogs, Bikely etc) seem to overestimate the amount of climbing involved in a given track, probably because they are all getting their elevation data from the same place. Altimeters also tend to slightly under report the total climbing.

Someone who actually knows whey will be along to contradict me soon.