# Total ascend calculation of Garmin



## eiri (Oct 11, 2009)

When I calculate the total ascend of a trip it is a lot different from the display on my garmin. 

My assumption is that the total of ascend is the sum of all altimeters during my trip. My calculation takes the difference in height of two trackpoints and adds the ascend if there is a positive height. This results in a optimistic total of ascend difference with my Garmin device of about 25%.

Is there something I should consider in my calculation, eg. only calculate after a specific distance or time?

Thanks.


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## Harold (Dec 23, 2003)

eiri said:


> When I calculate the total ascend of a trip it is a lot different from the display on my garmin.
> 
> My assumption is that the total of ascend is the sum of all altimeters during my trip. My calculation takes the difference in height of two trackpoints and adds the ascend if there is a positive height. This results in a optimistic total of ascend difference with my Garmin device of about 25%.
> 
> ...


You need to factor in the error of the altimeter, and it's quite possible that the GPS does similar. As in, if there's a positive difference between points, that difference must be greater than the potential error for the measurement (a threshold) before it's considered "ascent".

One way to determine this would be to do a statistical analysis for each pair of points to find out if the difference in elevation is statistically different from no change. It would be easier to do this sort of analysis if you had the GPS recording a point at a regular distance of something like 100ft or so. If your GPS is set to 1sec or auto recording, it will occasionally be putting points VERY close together. Even if you are on a very real hill, if the points are too close together, the small difference in elevation may not wind up being significantly different from no change between adjacent points.

Another thing you could do would be to plot your points on an elevation profile. You will see some noise in the data. Some programs will allow you to do some smoothing of that data in an attempt to process out that noise (measurement errors) especially if you can export that data into a spreadsheet program. You could also do a rudimentary manual smoothing of that data before doing a climbing analysis.


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## slocaus (Jul 21, 2005)

eiri said:


> When I calculate the total ascend of a trip it is a lot different from the display on my garmin.
> 
> My assumption is that the total of ascend is the sum of all altimeters during my trip. My calculation takes the difference in height of two trackpoints and adds the ascend if there is a positive height. This results in a optimistic total of ascend difference with my Garmin device of about 25%.
> 
> ...


Basically you are doing a base (point A) to top (point B) elevation difference. In reality, there are ups and downs along the way, so every bit of climbing is included in the Garmin recording, whereas you ignore it all as if it was a straight line slope base to top.


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## eiri (Oct 11, 2009)

NateHawk said:


> You need to factor in the error of the altimeter, and it's quite possible that the GPS does similar. As in, if there's a positive difference between points, that difference must be greater than the potential error for the measurement (a threshold) before it's considered "ascent".
> 
> One way to determine this would be to do a statistical analysis for each pair of points to find out if the difference in elevation is statistically different from no change. It would be easier to do this sort of analysis if you had the GPS recording a point at a regular distance of something like 100ft or so. If your GPS is set to 1sec or auto recording, it will occasionally be putting points VERY close together. Even if you are on a very real hill, if the points are too close together, the small difference in elevation may not wind up being significantly different from no change between adjacent points.
> 
> Another thing you could do would be to plot your points on an elevation profile. You will see some noise in the data. Some programs will allow you to do some smoothing of that data in an attempt to process out that noise (measurement errors) especially if you can export that data into a spreadsheet program. You could also do a rudimentary manual smoothing of that data before doing a climbing analysis.


Thanks for this answer. I know the height measurement is sometimes a bit "confused" and might have some incorrect values. Best seen is when you are at a one position, and see the height is still different between two measured points. I will look for some smoothing algorithm that might bring me closer to the Garmin results (because I want to declare that measurement to be "true" to have consistent results when I report my total of ascend afterwards).



slocaus said:


> Basically you are doing a base (point A) to top (point B) elevation difference. In reality, there are ups and downs along the way, so every bit of climbing is included in the Garmin recording, whereas you ignore it all as if it was a straight line slope base to top.


No, that was not what I did. I summed all up's of all trackpoints and that gave me more ascend then the value of my device.


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## Harold (Dec 23, 2003)

eiri said:


> Thanks for this answer. I know the height measurement is sometimes a bit "confused" and might have some incorrect values. Best seen is when you are at a one position, and see the height is still different between two measured points. I will look for some smoothing algorithm that might bring me closer to the Garmin results (because I want to declare that measurement to be "true" to have consistent results when I report my total of ascend afterwards).
> 
> No, that was not what I did. I summed all up's of all trackpoints and that gave me more ascend then the value of my device.


You misunderstand. The altitude measurement isn't "confused" at all. There is simply error inherent in the measurement of altitude.

I really don't understand your reason for trying any of this. Trying to tease out some "super accurate" numbers from the altimeter on your GPS is pointless. It is likely that the GPS is already making some kind of correction assuming the raw numbers have inherent error.


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## eiri (Oct 11, 2009)

NateHawk said:


> You misunderstand. The altitude measurement isn't "confused" at all. There is simply error inherent in the measurement of altitude.
> 
> I really don't understand your reason for trying any of this. Trying to tease out some "super accurate" numbers from the altimeter on your GPS is pointless. It is likely that the GPS is already making some kind of correction assuming the raw numbers have inherent error.


My reason is simple. When I do calculate and total of ascend, I want to have the same result as Garmin does. If I get back of a trip, i read from my device that i have climbed 1600 hm (5249 ft). I store my gpx files and display them on some personal webpage (height profile, summaries, etc.). There I calculate the GPX file runtime but don't want to display 2000 hm (6561ft) from my calculation. The value of Garmin calculate is used at many places and seem to be accepted as correct. I want to normalize to that same calculation method.


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## Harold (Dec 23, 2003)

eiri said:


> My reason is simple. When I do calculate and total of ascend, I want to have the same result as Garmin does. If I get back of a trip, i read from my device that i have climbed 1600 hm (5249 ft). I store my gpx files and display them on some personal webpage (height profile, summaries, etc.). There I calculate the GPX file runtime but don't want to display 2000 hm (6561ft) from my calculation. The value of Garmin calculate is used at many places and seem to be accepted as correct. I want to normalize to that same calculation method.


Can you post a link to your website so I can see an example of what you're trying to do?

I post my stuff on my own website, also. I usually do not use the GPS file upload sites. I use GMap4 to display the map, and when I want to post an elevation profile, I generate one with GPS Visualizer. If I'm only interested in the numbers, I use the numbers generated by Topofusion from my GPS track. After I finish my ride, I don't really care what the GPS says. I only use the GPS readout when I'm on a ride and particularly interested in my climb data.

Garmin's method is only "accepted" as correct because it's easier to just report those numbers rather than try to calculate them on your own. It is widely reported that not every GPS receiver reports the same values here even for the same ride, so it is widely accepted that these numbers should not be taken for more than what they are - estimates.

I say save your energy and don't worry about it.


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## eiri (Oct 11, 2009)

The website is on my own private intranet, so I cannot give you a link (yet). I have send you a private message with a link to an (unfinished) tool that I wrote a while ago, with the same issue. In the zip are some examples.

Same issue is in this program: http://www.2steger.de/gac/ (with exact the same values as my calculation if I recall correctly).

I still would like to solve this. Sites like GPSies.com, GPS-tour.info and Garmin Connect are much closer to the results of the device.


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