Ming the Merciless
There is no mercy
- Location
- Inside my skull
All within the margins of gps error
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All within the margins of gps error
Ajax Bay
Guru
- Location
- East Devon
Or as Mr Garmin says:
"GPS heights . . . main source of error has to do with [where the satellites are and their elevation angle] . . the accuracy permitted by geometry considerations remains less [by orders of magnitude!] than that of horizontal positions. It is not uncommon for satellite [GPS derived] heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating."
"GPS heights . . . main source of error has to do with [where the satellites are and their elevation angle] . . the accuracy permitted by geometry considerations remains less [by orders of magnitude!] than that of horizontal positions. It is not uncommon for satellite [GPS derived] heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating."
Ajax Bay
Guru
- Location
- East Devon
Modern GPS units have a calibration arrangement where the display on switch on uses barometer data to deduce altitude (height above datum). So really good idea to force calibrate (as previously discussed) by having a waypoint with associated altitude (per map) close to front door. After a while (?20 minutes) the altitude is auto-calibrated from GPS signal (if that's been set in settings), if it's been consistently good enough, I read.
People might "expect that it works immediately after having turned on the device but this is not true. Due to the problems of GPS-based elevation measurement, the device must wait a long time and must do excessive averaging and filtering before even thinking about calibrating the barometer. Furthermore, the device can only do that when reception conditions are good."
ColinJ
Puzzle game procrastinator!
- Location
- Todmorden - Yorks/Lancs border
When I have switched my Edge 500 on away from my front door (or my sister's house in Devon - the 2 locations that I have set as known elevation points) I have noticed that the elevation displayed can be way out. If I hang around for a while it eventually gets much closer to the true figure. If I set off before then the GPS never seems to get anywhere near the true elevations during the ride.
I have also noticed that if I switch off at a cafe stop, when I switch it back on later the device returns a completely different elevation value. I think that is really sloppy programming - how exactly was the elevation supposed to have changed while the device was stationary?!
I have also noticed that if I switch off at a cafe stop, when I switch it back on later the device returns a completely different elevation value. I think that is really sloppy programming - how exactly was the elevation supposed to have changed while the device was stationary?!
Sea of vapours
Guru
- Location
- Clapham, Yorkshire Dales
Whils true for any instantaneous elevation value, that is somewhat harsh as an unqualified statement.
As you've described in a later post, many gps receivers will deduce a static elevaton value over a time period by, broadly speaking, averaging gps elevation values during that time period, including (presumably) weighting the readings based on any variations in gps signal quality over the sample period. This will give a single digit (metres) accuracy over a few tens of minutes in most circumstances, in much the same way as an accurate location could be determined, even using civilian gps receivers, when SA (selective availability) was active, up until the early 2,000s. If devices with a barometric altimieter hand over elevation values to the barometer once the device is moving, the results are good.
For example, I've never calibrated my gps receiver to a known elevation, but it reliably puts my house within a couple of metres, vertically, of the correct elevation if it's been on for a short time. Similarly, when stopping on a ride at a known (to me) altitude, I've often seen movement to within a few metres of that altitude over a short time in cases where overall barometric pressure changes have occurred during the ride such that the elevation is 'off' when arriving at the known point (meaing I know it, not that the device does).
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HLaB
Marie Attoinette Fan
When I have switched my Edge 500 on away from my front door (or my sister's house in Devon - the 2 locations that I have set as known elevation points) I have noticed that the elevation displayed can be way out. If I hang around for a while it eventually gets much closer to the true figure. If I set off before then the GPS never seems to get anywhere near the true elevations during the ride.
I have also noticed that if I switch off at a cafe stop, when I switch it back on later the device returns a completely different elevation value. I think that is really sloppy programming - how exactly was the elevation supposed to have changed while the device was stationary?!
I think with my old Geko when you booted it up, it would ask you to set the elevation.
Barometric altimeters can't cope with cafe stops
They seems to keep the elevation the same in newer units though.
OP
OP
Deleted member 121159
Guest
Blimey, I've opened a can of worms, haven't I?! I went from confused to clear to somewhat confused, then to very confused.
I suspect that's an altimeter reading.
The profile of the Bridge is a beautiful curve (hump at the beginning is the descent to the south side track). But the Elemnt does use GPS for zero point calibration, so that combines with weather drift over the day.
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Dogtrousers
Kilometre nibbler
That curve is lovely, and very like the one I got for the Humber Bridge.
I think we should pause for a moment and just marvel at the pocket sized
What software/service are you using in that snapshot? It's very nice.
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That's through Veloviewer
Alex321
Veteran
- Location
- South Wales
That is a very good point.
I have been working in IT for over 40 years now, and when I started, a room sized computer would not have had nearly the processing power required to do that.
Now a 3" x 2" device is doing all those calculations, taking input from multiple sensors to add to the data, and keeping a record of it all to upload when finished.
biggs682
Touch it up and ride it
- Location
- Northamptonshire
Cause it goes up and down the gradients
Dogtrousers
Kilometre nibbler
I don't like Mondays either.
Dogtrousers
Kilometre nibbler
I can remember riding my bike with an old Ingersoll wristwatch (that my dad gave me as he didn't use it any more) strapped to the handlebars keeping half an eye on my clicky clicky mileometer and working out my speed in my head. If I'd calculated the total length of my route beforehand using one of those little wheels that you roll along a map then a bit more mental arithmetic gave me my ETA, which I might communicate to my mum the next time I saw a red phone box, provided I had an adequate supply of 2p coins.
The only measurement of amount of elevation was how much my legs hurt. Plotting gradient profiles was something you might have to do for Geography homework.
I also remember the first time I saw a GPS that gave instantaneous readout of gradient. None of my GPSs did that and I immediately turned bright green with gadget envy. (And soon rectified it with a bit of gadget-related retail therapy).
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