Garmin elevation readings

pgmcpq

I am seeing very strange results for elevation reading from my 305. This has been going on for a while but I had not noticed. From my training log :

I decided to incorporate a few hills repeats this morning having done no explicit hill work for while. Looking at the results on Garmin connect I was surprised at how little elevation was shown. Yet when I look at Sunday's recovery run around the playing fields of the local park I see an elevation gain and loss of 240 feet . This cannot be correct ! Played around with switching off elevation correction but the results still seem out of whack for a six mile flat run.

When I switch on elevation correction (which if I understand correctly takes elevation from somewhere other than the Garmin?) the results are at least consistent ( ie runs I expect to have more elevation gain do so) but the results seem very exaggerated - I have not turned in SJ overnight.

All updates have been applied - is there a setting I am missing, is it a known issue, or is my park sliding off the earth (I can see the scifi movie now). It now leaves me doubting my earlier race profiles. Any ideas ?

gerard65

The Forerunners are not great for measuring elevation. I can't remember the exact reason but it has something to do with needing mercury in them for reading elevations.

pgmcpq

gerard65 wrote: »

The Forerunners are not great for measuring elevation. I can't remember the exact reason but it has something to do with needing mercury in them for reading elevations.

Ahhh - I wondered how it was done. Not good news but good to know - thanks.

tunney

Commercial GPS units are many more times inaccurate in the Y-plane than they are X and Z.

Condo131

I sometimes use an eTrex Vista, using atmospheric pressure setting, and find it much better for elevation. However it's not 'runner friendly' at all - you carry it in your hand - and arm swing gives an error.

I haven't used it for a while but, because it isn't relying on satellites for elevation, find it significantly more accurate, than my 305. Another downside is that it is a pain to download the profile, convert to an image, crop, adjust and then probably paste it into something else.

Compare these profiles for the same race (note the first 0.5 mile is different in each (we moved the start!))
Old Eagle 5 course
Current Eagle 5 course

I prefer the eTrex profile, if only because it results in a better vertical range on the graph.

The big bonus with the eTrex Vista is that it eliminates all the hassle of erroneous 'hills' from trees, buildings, etc.

robinph

The Forerunners only get elevation data via the satelites and due to the way that thr triangulation works, some crazy maths needed to figure it out, so it's a lot less accurate in the vertical than in horizontal positioning.

The bigger mapping/ walking gps units, like the etrex/ gpsmap/ oregon/ colorado, use barometric readings to figure out the elevation although that is kind of synced with the readings it gets from the satellites as well because setting it to a known elevation point each time you turn it on would be a major pain.

I think the Edge cycle units have barometric elevation as well, but not sure.

Slogger Jogger

I tend to use Sportstracks in conjunction with my 305. I recently downloaded an elevation correction thingy for Sportstracks which I apply to my data and the results then tend to make sense each time. Not sure why garmin can't have a more correction elevation gizmo for their own hardware.

xebec

Noticed a significant difference between elevations for different Garmins at the Irish Runner race at the weekend:
405: 161ft
305: 189ft

These are just two that I found searching Garmin Connect now. Maybe the 305 isn't as good for measuring elevation as the 405?

Krusty_Clown

Slogger Jogger wrote: »

I tend to use Sportstracks in conjunction with my 305. I recently downloaded an elevation correction thingy for Sportstracks which I apply to my data and the results then tend to make sense each time. Not sure why garmin can't have a more correction elevation gizmo for their own hardware.

They do now, in Garmin Connect. They don't for the watches themselves, as the software needs to consult a very large database of survey data to get the corrected elevation information. It's still at its best, an approximation.

Krusty_Clown

xebec wrote: »

Noticed a significant difference between elevations for different Garmins at the Irish Runner race at the weekend:
405: 161ft
305: 189ft

These are just two that I found searching Garmin Connect now. Maybe the 305 isn't as good for measuring elevation as the 405?

I'd just put it down to different methods for calculating elevation across the various devices. don;t think they've ever been consistent. Just compare runs from some of the lads doing the IMRA races and you'll see huge discrepancies. We should all go back to candles and pieces of string, instead of these rubbish Garmins. It's opposite day today, isn't it?

pgmcpq

Thanks for the feed back on this.

Curious and curiouser. So that's what elevation correction is then. Otherwise it is really triangulating off the satellite ?

When I enable elevation correction the results are at least consistent - i.e. a run where I know there were hills does indeed show up as having hills but the figures seem very large to me. I run often do 6 miles around a set of playing fields - each loop is about .0.8 of a mile. If my math is correct this implies a 40 foot in dips and rises between one end of loop and the other. I am going to see if I can verify this -

Well - I'll be looking more sceptically at those GPS profiles in the ART Events forum from here on!

robinph

The elevation correction data is just replacing the altitude data that the gps device collected with the data that the map the trace is overlayed on top of has for elevation. For the likes of SportsTrack that is radar topography collected from the shuttle, and that is much less accurate once your outside of the US because NASA haven't bothered releasing the high res data for the rest of the world. Cannot remember the numbers but it's something along the lines of only one elevation point recorded for each ten meter square area of ground, but for the US there is an elevation point recorded for each three meters. *

* Made up numbers.

Krusty_Clown

pgmcpq wrote: »

Curious and curiouser. So that's what elevation correction is then. Otherwise it is really triangulating off the satellite ?

Technically correct, but I believe you need to be receiving information from at least a fourth satellite before your GPS receiver can calculate any form of elevation information (I guess this is still called triangulation though, just using more vertices? Polygon triangulation?).

Here's a pretty clear explanation of how it works:

extorian wrote:

GPS systems (of the type in your phone anyway) use a roughly spherical model of the Earth to calculate your position on the ground. This works very well for things that tend to be on the ground (such as people). Altitude, however, is much more difficult to calculate, as there is no "surface" with which you can assume the GPS is located.

Simplifying things somewhat, to calculate elevation, the GPS unit measures the distance from the centre of the Earth, and then applies the radius of the surface model (the sphere I mentioned in the previous paragraph), to give you the height-above-centre-of-the-Earth value at that position. The height of the ground, based on this distance from the centre of the Earth at your current location, is called the "mathematical elevation".

The mathematical elevation is around sea level in some locations, but in others it is way off. In fact, even in the places this does happen to be at sea level, this is purely coincidental.

Now... there are some tables of values that can be applied to the above calculation, to get a better idea of ground level. These tables of correctional values have been made by satellites, surveyors, and geologists. Some are more accurate than others.

You also have to take into account the density of the Earth's crust in your location, as this affects gravity, and in turn pulls the ground and sea more or less in different locations. This both affects sea level and ground level.

So, because the elevation calculated by your GPS is based on the mathematical elevation (possibly with a correction applied from a table), and combine this with the fact that the distance from the GPS satellites is around 12,500 miles (20,000 KM) and you're expecting them to figure out distances from them accurate to a few metres, the GPS uses an "error bias" towards the direction of the Earth's centre for safety reasons. This is similar to the reasoning behind the speedometer of your car having an error bias towards higher speeds (i.e. it'll say you're going faster than you really are) - with altitude you don't want planes or missiles hitting the ground unexpectedly, so the GPS tells them they're lower than they really are. This is why you see negative altitudes more often than altitudes that are too high.

As a rule of thumb, if you look at your accuracy value of your GPS at a given time, the elevation will have an accuracy error of around three times that figure.