CBD raises by 85m during the day - what's going on?
For the last few weeks I've commuted into the city a couple of days a week and I've noticed a strange thing happening when I get home and upload the ride to Strava.
Here's todays ride:
http://app.strava.com/rides/5455573
It would appear that during the day the CBD has risen by 85m! Is my Garmin buggered or is something strange going on?
Phil
Replies to This Discussion
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Permalink Reply by RD6 on -
you might find riding along different sections of the road can actually make the GPS read at different gradients / metres
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Permalink Reply by Tim B on -
Ive driven through the city with my car based Garmin and seen figures ranging from -85m to +500m. I figure the altitude reading isnt always accurate!
Most likely if you are using a barametric assisted GPS such as the Edge500, any change in air pressure during the day will effect the reading, especially if a large front rolls through. My house can move by 10 to 20m over the course of a 2 hour ride. I guess over the period of a work day, there is a fair chance of the air pressure changing and that is what you are seeing. Some days it will be up, some down
If you look at the Adelaide data on the BOM website, you can see the pressure started dropping from 1008 to around 1002 at lunch time until about 5pm when the change started to come through
http://www.bom.gov.au/products/IDS60901/IDS60901.94675.shtml
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Permalink Reply by Michael Fraser on -
Dear Phil
If your Garmin device does not have enough time between starting the unit and starting the ride the elevation may be incorrect. I once did a ride along the beach and was riding at 50m below sea level.
If you continually start your rides at the same location it is recommended that you use elevation set points. I have set elevation points for my home and work. When you start your Garmin this will set the elevation to that value. It ensures the unit starts at the correct altitude. If you have a Garmin 500 use Menu/GPS/Set elevation. For this to work correctly you need to ensure that the Garmin device be given sufficient time and satellites to ensure an accurate triangulation of the elevation.
Michael
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Permalink Reply by Tim B on
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I think the issue is that he logs both legs of the ride (to work then home again) as one 10 hour ride with 2 hours something moving at each end. The air pressure changes while he is at work, thus giving a different reading when he rides home. As suggested aove, you can set various known altitude points for a ride ( home, work, beach, mount lofty etc) but think you need to adjust them manually. I always "zero" mine at home to 101m at the start of a ride, not sure if you can adjust the altitude during a ride on the garmin though?
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Permalink Reply by Phil Crick on -
I'll try setting elevation points and/or recording the commute as two separate rides. Thanks for your responses.
Phil
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Permalink Reply by Gus K on -
Isn't it set by barometric pressure? On Garmin Connect when you've uploaded it you can enable elevation correction on the lower right hand side but that is on the uploaded data. The same uncorrected data would be uploaded to Strava.
I would also set the elevation points of some known locations.
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Permalink Reply by Tim B on
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Barometric pressure is used to measure the change in altitude. In an ideal world if the barometric pressure wasnt effected by changes in weather, then it would remain fairly accurate as you climb and descend. When you "zero" your computer, it is only taking a snapshot of the current pressure reading.
I wonder how accurate the corrected elevation data is in Garmin Connect sometimes, although it is supposed to be fairly good. It often changes a 900m elevation ride to a 1200m elevation gain ride. Looks good in the stats though i guess :-)
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Permalink Reply by Gus K on
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You're right but I think more specifically barometric pressure is used to show the gradient - or as you say the change in altitude.
You'd think with GPS is would measure it by time spent pinging satellites.
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Permalink Reply by Dahondude on -
The Garmin units only use a barometric altimeter. Measuring altitude using GPS on almost all consumer GPS units can be wildly inaccurate. It's important to make sure the holes on the back of the unit are clean and unobstructed and as everyone else has said, changes in air pressure during the day as high or low pressure air moves over your location can lead to altitudes that clearly don't make sense.
On the Garmin Edge you can only enter an Elevation Point (to calibrate the altimeter) for the beginning of your ride, not for other locations that you might stop at during the ride (for example if you stop at Mt Lofty in the middle of the ride you can't then set the altitude without stopping the Garmin and then restarting it).
Elevation correction on the Garmin Connect site is really only necessary for data imported from a device that doesn't use a barometric altimeter. If you want the most accurate elevation data from the Garmin Edge, use an Elevation Point at the start of your ride (you can load up to 10).
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Permalink Reply by Mike Brisco on -
A well known issue, and all to do with geometry. I once spent a cycling holiday, with a companion who knew all about GPSs having studied surveyign at uni. I don't remember much about the country we rode through .. but do remember a lot, about GPSs.
I'll try to explain.
To get height accurate, the GPS needs a a good spread of satellites. Some high in the sky almost overhead. Some low down, on the horizon. Some in front, some behind, some to your left, some to your right.
In the CDB, the buildiings cut out the low satellites . All the GPS can hear, are satellites overhead. And because of that, that is what makes height particuarly inaccurate.
I think Garmin used to quote within 5 m for horizontal distance, but within 25 for vertical, and that's assuming a clear view of all the sky. If you only have clear view of one part of the sky - when the GPS tries to measure altitude, it won't even be able to get it within 25 m. Might be morer like within 100 or 200.
I'll try to explain the geometry - problem is same, if you are navigating a yacht, taking compass bearings off landmarks, and plotting your position on a chart.
Any fule no, - you need three bearings to fix your position. The three lines, define a triangle, you are somewhere within that.
If your land-marks are spread widely over 360 degrees - ie you sight one thing ahead, another behind, a third to your right - the triangle is same length all sides. So you know where you are left to right along the coast. Also how near/far from land you are
If you can only sight things ahead - triangle is same width, but extra long . It is stretched out, in the direction away from land, out to sea. Measure it several times, you get the same answer for how far out to sea you are.
With GPS is exactly the same principle. Except instead of landmarks ahead of you, and you working out distance out to sea. The "landmarks" (satellites) are now overhead, and you work out distance up or down ie altitude.
Anyone wants more detail - you need to buy me a coffee, and bring a pencil and notepad.....
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Permalink Reply by Phil Crick on
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Well that makes sense, I'm surrounded by tall buildings when I come out of work, and it does struggle to initially pick up a signal. It's probably worth setting my workplace as a start point so it knows the elevation.
Phil
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