Sea level on my garmin

D

Deleted member 23692

Guest
Sea level in the UK is taken at Newlyn in Cornwall (and is a mean value) and depending where you are it'll vary. Added to that GPSs aren't that good at altitude
 

ColinJ

Puzzle game procrastinator!
My GPS showed minus a few metres at Knott End on one of my forum rides when a bunch of us went down to the sea to have our photos taken. Judging by how far up some nearby concrete posts the barnacles (?) went, we actually were about 3 or 4 m below the high tide level.

slow-train-2-jpg.8281.jpg


(I really want to ride that route again! I look forward to being well enough at some point in the future.)
 

Shut Up Legs

Down Under Member
I've had my Garmin Edge 705 show -200m at 'sea level', and as mentioned, it's due to extremes of atmospheric pressure. When starting a trip with the Garmin (I've done this on the 705 and the 800), you can partly work around this by creating a waypoint at your start location, and setting the waypoint's altitude to what you know it should be for that location (there are web sites that can give you this information, by the way). Extremes of pressure may still cause the altimeter to give inaccurate readings, but starting the ride by setting a waypoint as I described makes the altitude readings more accurate over the trip.
 
D

Deleted member 23692

Guest
It's not extremes: any change in air pressure will affect the reading on an barometric altimeter. Assuming you're not moving, a I mbar change in pressure will show as an approx 8m change in altitude.

If maintaining accuracy is important to you, do as victor suggests at the start of the ride and then check (and adjust) regularly using the spot heights on OS maps. TBH I can't see the need whist cycling, although it's a different matter when you're stuck in a whiteout up a mountain. If your GPS uses the satellites to work out altitude then forget and level of accuracy :smile:
 

ColinJ

Puzzle game procrastinator!
If maintaining accuracy is important to you, do as victor suggests at the start of the ride and then check (and adjust) regularly using the spot heights on OS maps. TBH I can't see the need whist cycling, although it's a different matter when you're stuck in a whiteout up a mountain. If your GPS uses the satellites to work out altitude then forget and level of accuracy :smile:
My old Etrex only uses the satellites to calculate altitude and is usually pretty accurate as long as it has a good lock on 4 or more of them. I have checked against an OS map and the reading is usually within about 1%.
 
D

Deleted member 23692

Guest
You must have a magic one then :smile: I've used eTrex for a good few years for capturing spacial data to load into GIS at work for legal orders, so I may be looking at a higher degree of accuracy than most. Whilst the X & Y axis data has become more accurate with each version, I still don't trust the Z axis data. I use a Suunto barometric alti for that every time.
 

ColinJ

Puzzle game procrastinator!
You must have a magic one then :smile: I've used eTrex for a good few years for capturing spacial data to load into GIS at work for legal orders, so I may be looking at a higher degree of accuracy than most. Whilst the X & Y axis data has become more accurate with each version, I still don't trust the Z axis data. I use a Suunto barometric alti for that every time.
Having read a lot of comments about how bad the altitude function was supposed to be, I was pleasantly surprised!

The highest point on Cock Hill between Hebden Bridge and Oxenhope is at 432 m and my GPS has indicated 430-434 m nearly every time that I have checked, and that must 20+ times.

The times when the reading was way out were when I switched the GPS on and set off before a proper satellite lock was achieved. My house is at about 105 m but I have the GPS indicate up to 160 m if I rush it!
 

Shut Up Legs

Down Under Member
I've had very few altitude accuracy problems with the Garmin Edge 705 and 800 units I have. I very rarely have to set waypoints as mentioned above, and that -200m at sea level only occurred once, and was the most extreme inaccuracy I've seen in these units :smile:.
 
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