Total Ascent and Descent No Longer Match

This is a topic that has often been debated. Their always seems to be decrepancies with total ascent and descent not agreeing.

One of the oddest things I cannot work out is, I have the very start point at my front door marked as a location with its elevation, 68' and that is what usually shows when I turn the 530 on, but as soon as I press "Start", it flashes up calibrating elevation, but despite it showing what should be the elevation, it can be about 40' out. It does not make sense.

Usually at the end of the ride and I finish at the same spot, the start and finish elevation are different, probably about 30' and sometimes quiet a bit out. During my ride, places may show as only 6', but the next day it can be 100+.

Things that I know affect it is changes in air pressure whilst riding, so if it falls or rises, you will have a variation. Also I have found, riding into a strong gusting wind can cause variations as I think the "ripples" of air will be detected as changes in air pressure and will thus get recorded as feet acscended etc

One way I used to test mine was put it in a clear plastic bag with it showing datafields and elevation profile etc and try and fill the bag with air, not compressed, and seal the bag with a knot and make it air tight with the Edge facing upwards. Take a note of the elevation, then very lightly press on the bag and watch the elevation rise, then slowly take the pressure off and watch the elevation datafield fall. If it is working correctly, you should have the same elevation that the Edge had before you put pressure on the bag. Doing that over a course of time may give different results as it would factor in the air pressure itself altering the pressure on the bag.

Thanks for that. Agreed, an age old discussion, but the potential resolutions I read out there didn't work for me.

I appreciate that elevation is calculated using, in part, air pressure, but where it has been reasonably consistent over many years, and comparable to other people's results using different hardware, of late, it has been terrible.

I have a similar issue to your start point, where it is a known location with known elevation, but then it think it would better to change it by 50 metres. 

I will try the airbag trick and see if at least the instruments on the 530 are reasonably consistent. Perhaps this is how it goes in its old age.

Do you know if elevation accuracy has improved?

I would have thought GPS information for location and elevation would be far more accurate.

To be honest, over the years I have not took much notice of elevation when riding, never really had a need to know it and besides, when I have uploaded my ride, it all seems to correct itself probably to the mapping and elevation corrections.

A few years ago I started to make a note of total amount of a ascending on my events from the organizers notes so I would have an idea of how much climbing there would be left, but gave up when I could find that I had exceeded that amount and there could still be a long way to go.

Today I turned my 530 on, it showed my elevation was just 1' of what it should be at my location and after 3/4 of an hour it had drifted another 3' off. But when I pressed "start", it recalibrated and added another 40' despite the elevation data stored in the 530 for my location. 

But today, I only had a 17' error between ascended and descended which is acceptable.

I did some thinking past two days and wondered if the elevation getting recalibrated at the start was getting corrupted by riding a course and using the start elevation of the course which is also the same position recorded as a location instead of the location that you actually are stood on when you press "Start"

So after my ride yesterday, I did some tests and found that if I did not have a course loaded ready to ride, the 530 would recalucualte the elevation to within 3' of the elevation stored for the location, but, if I tried it with a course loaded, it was over 30' out.

So I looked at the start elevation of the course and found it was 30' above what it should be, and as I am always saving activities as courses, it must happen that the elevation is out when I do it.

This morning when I got up, just before my ride, it tested out perfectly again, only a 4' error.

But 3/4's of an hour later, the 530 had been left turned on, when I set off for my ride, the elevation was showing 115' instead of 66' and when I pressed "start" with my course loaded, it jumped to 150'. I tried it a couple of times more and once with no course loaded and got the same results. So what had changed, it does not make sense.

Basically, I give up, the 530 has a mind of its own regarding elevation. Perhaps, Garmin would have done better if you could set the elevation manually like an aircrafts altimeter with just a simple display and a "+" and a "-" option to increase or decrease it.

"lightly press on the bag and watch the elevation rise" - incresaing the pressure in the bag will lower the elevation reading.

While performing this experiment, you'd have to ensure the temperature inside the bag remains constant: rise in temperature -> higher pressure -> lower elevation reading.

Your only giving it a quick test to see if the elevation falls when pressure is applied and rises when pressure is reduced to check the 530 is working, any change in temperature would only be minimal and not noticed.

5ºF change -> ~1% pressure change -> ~80ft elevation change

When you do, calculate the percent differences.

Just went for a 60km loop. The 530 said 500 up and 357 down.

Fenix said 466m up and 470 down.

4 metres seems a reasonable error. 140 metres is rather incorrect.

4/466=~ 1% error.

140/500=22% error!