Altitude Questions

corvus13

Right then, if I set my home altitude at say, 75m, will my garmin 500 reset its altitude to that preset irrespective of barometric pressure and satellite info when I return home? If not, why not? If it does then surely populating all available presets would be a no brainer if you want to know what your realistic altitude gains are?
Q2, When you press "Altitude Correction" on say, Strava etc, does this mean that say, Strava etc, checks the barometric pressure for that location, for that time and correct what your particular, Garmin thought
it was reading?
I'm only asking because I missed my ride today and am feeling cranky. No ride tomorrow. Ish.

Grill

Not certain about the first question, but for the second one Strava doesn't take barometric pressure into account. Keep in mind that the barometric pressure sensor isn't the most reliable.

corvus13

Grill, then what would involve this 'correction'? This is one of many things I'm unsure off. Amongst many others I might add.

Grill

They use elevation databases.
https://strava.zendesk.com/entries/20965883-Elevation-for-Your-Activity

corvus13

I'm being cheeky here by not looking at your link but asking, why doesn't Strava look at that info first off, without our request?

corvus13

Wow, I may be gone some time.......

Grill

Because elevation databases aren't the most accurate either. Essentially barometric altimeters are the most accurate as long as the sensor stays clear and elevation points are set (so a yes to your question one).

corvus13

Grill, stay with me, So .... So.... Why doesn't Garmin 500 and Strava look at the same Elevation Data? And where does Barometric pressure involve itself? If it comes from Garmin 500 but is over ruled by Stravas own elevation....am on tawny port!

Grill

Dude, just visit the link I posted.

How Altitude data is collected:
Barometric altimeters determine altitude by measuring atmospheric pressure. Measurements can be affected by changes in the weather or the sensor holes getting blocked by water or other debris. Device manufacturers may provide basic calibration procedures. For example, Garmin devices with barometric altimeters allow a certain number of manual elevation points to be set; starting an activity near an elevation point causes the device to use the known elevation as the starting point. Consult the manual for your device to learn about calibration procedures.

GPS altitude measurements are derived from GPS signals and can have relatively high vertical error. They also depend on how many satellites are being used and where the satellites are located in the sky.

We generally consider the data from barometric altimeters to be of higher quality than the data derived from GPS signals and prefer that when processing activities. For devices without barometric altimeters, we consult elevation databases to determine the elevation at each point in the activity. The resolution of these databases can vary based on location. For example, the USGS National Elevation Dataset (NED), which we consult for activities located in the US, generally has a 10 meter resolution though some small areas have a 3 meter resolution. For activities located outside of the US, we consult the ASTER and SRTM databases which have a resolution of 30 meters and 90 meters respectively.

We discard outliers and smooth the elevation data when calculating gain to reduce noise. The amount of smoothing depends on the source of the data - data from barometric altimeters is smoothed less while data looked up from elevation databases is smoothed more. When computing gain, we use a threshold to determine whether or not to include a change in elevation; in this way, small variations in elevation change are ignored while larger changes are included.

corvus13

So Garmin 500 ........guesses altitude by Barriometic pressure? and/or mapping info, then sends to Strava which then applies these infos on to their
own data elevation/Barriometic structures? Getting late now. Ont morra.

bompington

You don't guess altitude from pressure - it's a very accurate way of measuring it: my phone (Note 2) will tell you the difference if you hold it in your hand and move it up and down by a metre, or even less.
The inaccuracy comes from the fact that atmospheric pressure goes up and down with the weather - the difference between typical high and low pressure areas is worth a few hundred metres, so an altimeter using it needs to be reset frequently at a known altitude.

DHA987S

Garmins use GPS data to determine initial position and also altitude, I am not sure how regularly in a ride they will perform a "check" on altitude - it might never do it once it has initialised. It will then use the barometer to record changes in altitude. Setting the home altitude will just give it a known reference point to base values off of.

The sensor, as mentioned above, is not the most accurate.

Now for slightly boring bit - air pressure can vary massively from day to day and from place to place. 10 miles down the road the air pressure may be vastly different from where you started, this could fool the sensor into thinking climbing or descending has occurred. There would hopefully be some cross reference against GPS data at some point (perhaps at home when the data is entered into Strava or similar). In summer and winter extremes of pressure (low and high) is likely to make the sensor less accurate as well as I guess it has an optimum operating pressure window. The barometric sensor is more accurate than GPS alone, but far from perfect. But good enough for cycling I would say.

marcusjb

The need to calibrate is really important on longer days in particular, where obviously the weather conditions (and air pressure) can change substantially.

It is quite easy if you are riding in somewhere mountainous as there will be a sign at each summit. It isn't so easy in the uk where we don't have that many summits to celebrate! Have to hope that your ride visits the seaside a few times.

If leaving from home, I know the altitude, so can calibrate it as I leave (it can be +- 60-70 metres out).

It is accurate enough for willy waving purposes.

I think the most accurate method without masses of expensive equipment, is contour counting on OS maps - but that is extremely time consuming.

rolf_f

Strava elevation data is fantasy. If two people ride the same route, they can end up with significantly different elevation totals (I've seen close to 20% discrepancies!) even if they use the same type of unit. What Strava should do is use digital elevation mapping to provide the basis for the calculation (thus guaranteeing a reasonable match) and apply the barometric data to that for the fine detail.

Ohterwise, it would be better to just use the digital elevation mapping. They are only innaccurate in as much as you tend to crop the peaks and troughs a little bit - and as time goes by that impact will lessen as the mapping resolution increases. Furthermore, you know that the digital elevation map can only underestimate your effort - so again, it is consistent. For commuting I use a GPS without a barometer. The recorded elevation of each commute never varies by more than a few feet in a thousand or so.

Ultimately, feet and metres are arbitrary measurements invented by man. It doesn't matter what the number is as long as it enables a consistent measurement of how high you have climbed. In this respect, the digital elevation maps are superior to barometric data.

marcusjb wrote:

It is quite easy if you are riding in somewhere mountainous as there will be a sign at each summit. It isn't so easy in the uk where we don't have that many summits to celebrate! Have to hope that your ride visits the seaside a few times.

Of course, land moves up and down a bit so really you need to hope your ride visits the seaside at Newlyn in Cornwall a few times which is where sea level is measured from.

Mikey23

Newlyn you say? I never knew that!

My 500 seems to under record elevation compared with others on the same route using the same device by between 10% and 20%. I'm never quite sure why and not overly fussed except that it makes me look like a climbing lightweight on the leaderboards

I suppose it's good enough for cycling but not for landing a jumbo jet

florerider

I use both Mapmyride and Ride with GPS. The elevations for the hilly rides are pretty similar, but on flatter rides they can vary by a factor of 2.

I think it comes about from the contour mapping - if the base map uses a contour interval every 50m say, then then the ups and downs over 50m are calculated, but if the interval is over 200m, then there is enough distance for some ups and downs to be missed entirely from the calculation. With a large elevation difference this does not matter, but on an undulating road it can make a difference.

Funny thing is, whilst Map my ride may underestimate, it is probably gives a better indication of the relative climbing effort.

On GPS devices it might depend on the pressure sampling interval.

monkimark

Rolf F wrote:

Of course, land moves up and down a bit so really you need to hope your ride visits the seaside at Newlyn in Cornwall a few times which is where sea level is measured from.

And you'll need to pack enough fig rolls to allow you to stay there for a while and calculate the mean level.