Garmin vs.BT route tracker

The corners and turns can get off some with the Garmin, it takes sample points, if there is a turn in between the sample points it gets cut off a little...the more turns the more the Garmin comes up short. On a really straight course its very close.

On the BT tracker I don't know why its different unless BT tracker is putting the Garmin route on the roads in the map making it slightly longer than the Garmin.

Edited by rockrunner 2009-08-09 6:43 PM

ahhh, that explains things for me too

I did a track workout and thought I'd be cool/lazy with my 305 and set the auto-lap for every 1/4 mile, it seemed to lap short by about 20 yards every true lap

Corners that makes sense..and on BT routes it looks like all the corners are right.

My Garmin sometimes does odd things I can't quite figure out when the pace gets slow all the sudden I'm not running 2 m/m slower.

Any one else have some input?

a GPS is not 100% accurate, so yes, it can be off.  Also, you won't run the exact line around a marked course, so your measured distance will be off from the 'known' distance. 

On the route length:
I don't use the BT tracker, I use sport tracks and have noticed that at times they disagree about the total distance. Sport tracks just downloads the GPS points taken by the garmin and the time of each one and then uses it's own algorithm to determine what distance that was. The distance difference is usually the part of the algorithm that deals with "lost points" or "corners" or "turning" etc. I basically always look at the sporttracks value as opposed to garmin for consistency, especially whe comparing paces. In a perfect world you get 100% consistent readings for the whole run and you have 100% accuracy but that's just not true in real life.

On live pace:
I find my garmin can lag just a slight bit when looking at "live" pace when running especially if it fails to get a good data read for a split second, it'll appear to bounce. It happens on the bike at times too but moreso on the run for me. I don't use live pace too much.

Altitude
The forerunner 305 uses GPS based altitude instead of a barometric altimeter. GPS altitude has it's own flaws but it supposed to be correct 95% of the time. I find it's good at course profile and on a relative basis but on an absolute level it can be a bit wrong. If you use sporttracker there is a plugin that will correct the course altitude based on published data.

Also wanted to add, there is a very well established and LONG formula to calculate the distance between two gps points but that gives you the straight line distance between the two points. Again, ideally you have tons of gps data points so that the distance between each one is so small that you can a nice curve out of all the points but if you miss some points along the way, you can see how you would need an algorithm to deal with that issue.

Last thing is, don't forget that courses are supposed to be measured using a specific set of rules but if you zig zag or ran wide at all the corners, you can have a different distance compared to a measured course. I did a 7 mile road race yesterday garmin was spot on at 7.07 miles. I believe the governing bodies (USA track & field has a paper that discussed how they measure a course).

[ok geek off]


Edited by merlin2375 2009-08-10 1:11 PM

Elevation

On the elevation, it is because the Garmin has lots of accumulated error.  It is taking data, say, once per second.  Every time it takes a data point, if that point is considered to be higher than the previous point, the Garmin adds the difference to 'total ascent'.  Since it is measuring total ascent, there are never any subtractions from this number.  So, every time it makes a tiny mistake, that mistake gets added to total ascent, and after a couple of hours, a lot of erroneous additions are made.

Example:

  Actual elevations at points 1,2,3, and 4 are:  100', 101', 102', 103'.  (I.e., you are climbing a hill!)  Total ascent (measured at this resolution) = 3'

  Suppose that the Garmin measures these elevations as:  100', 99', 102', 104'  It will report total ascent as 5'

You might suppose that since the errors are 'random', everything would cancel out, but it doesn't.  First, the errors aren't totally random -- they have a lot to do with stuff like how far off course the GPS satellites happen to be at the moment.  (Attempts are made to correct for this, but they are imperfect, and if the Garmin can see only 3 or 4 satellites at a given time, then they are even less useful.)  Second, because we are talking about total ascent, the errors tend to accumulate rather than cancel out.  For example, even if the Garmin were, for a few seconds, systematically 'missing out' on your climb, by, say, measuring the first three elevations as 100', 100', 99'.  Still, eventually it would get the elevation right, or close enough, and then suddenly it would add a number too large.  For example, if it got the fourth one right, it would suddenly add 4' instead of the correct 3' to your total elevation.  Basically, if the Garmin ever underestimates your elevation and then some time later gets it right, it will add too much.  (That's not 100% correct, but close enough for government work.)

Distance

I don't know the answer to your question about distance, but back when I had a (functioning) Garmin, I noticed the same thing.  I don't know how the Garmin calculates distance.  What I can tell you is that any GPS reciever calculates a 3-dimensional location for you (relative to some 'fixed point', such as the center of the Earth), and then turns this into a geographic location.  It is possible (but I just don't know) that the Garmin is using the 'raw 3D location' data to determine your total distance covered, in which case there is room for a difference between what it calculates, and what would be calculated (for example on Google maps or whatever) from the geographic location data.

This might have been fixed over the years, but when I was looking at making a purchase, I had some friends with a Garmin. We live in a heavily wooded area and so the gps would lose connection through some of the thick stuff. When we would get to the other side, the gps would straight line from the last point it recieved. Sometimes this would cut a quite out a bit of distance. This was the main reason I went with the Polar footpod. Had I lived in a less forested area, I would have gone with Garmin.