GPS disagrees with computer and Map My Ride
 

Hi all---a while back I was cruising though the local Wally World and spied a Magellan Triton 400 hand held GPS on clearance. Spontaneous purchase!

Now the irritating part. After a ride the mileage it shows and the mileage on my bike computer, and the mileage that "Map my ride" shows are different. Map my Ride and my bike computer agrees with each other.

As an example I just got in from a Sunday morning cruise and both my bike computer and map my ride showed I went 14 miles. The GPS says 11.5. I know that bike computers and gps's aren’t critical in having fun---but I am a tinkerer by nature and like to play with such things.

Anyway----comments? Similar experiences?

Thanks

Can you dump the raw gps data from your trip into a computer? It's possible your GPS missed some updates or something.

You know, if you have a smart phone, you can use the internal GPS with an application from one of those "my my..." type websites for a mere $20 or so. I use the all sports GPS app with my blackberry, love it. It doesn't do turn by turn stuff, just data grabbing, but it's cool.

For phone type GPS tracking, I use

http://sportypal.com/Home

and best of all its completely free!

As for the GPS being out - I have seen that on my phones GPS where the miles dont match the actual bike computers. When I view the route at the above website, its obvious that the GPS lost signal for a minute or two (heavy woods etc) and then re-synched. The lost mileage is not recorded and lends to an overall lower mileage tally. Have you tested on a route with wide open spaces with no possible GPS blocking issues?

Good things to think about. Can't dump raw data---I have a mac and Magellan's aren't compatible. No smart phone either. Loosing signal could be a possible cause-----will see if I can monitor that better.

My experience with my GPS in my iphone is that it will cut corners. I have mapped runs with mapmyrun.com said it was 4 miles go out run it gps says it was 3.6 miles. go to runkeeper's website look at the data make it follow the roads and come up with 4 miles. so either the roads (I believe they both use google maps) are mapped horribly wrong, or the gps is cutting the corners. like you take a left the last point before the left and the first point after the left are connected cutting out distance. you make a good amount of turns and go through a good amount of curves and it ads up.

I use sportypal on my blackberry, I like it for the most part. I tends to be <.5mls shorter and slower than my bike computer, but I've learned to just ride another half mile before calling it quits...

Its free, has pretty good features on their website as well. A buddy that used mapmyrun.com on his iPhone has ditched it in favor of sportypal.

There are numerous possibilities for error with all of these devices/services:

1) Is your bike computer programmed with the proper wheel size? A size based on the measurement of the tires and wheels that are actually on your bike?

2) Is the Google Map data, which MapMyRide uses, for your area up-to-date and accurate? When I rode down the Pacific Coast, I found that the routes I'd plotted using Google Maps often didn't agree with the maps on my GPS device which often didn't agree with the actual roads

3) How often does your Magellan GPS sample your position? Because runners and hikers move rather slowly, in comparison to cars and bicycles, some hand-held GPS units record location points at infrequent intervals in order to conserve memory and/or power. Depending on how the GPS data is analyzed, this can create error when determining the total distance traveled.

4)Are you losing the GPS signal? As previously mentioned, this can cause loss of data. My Garmin Edge 705 rarely loses signal, even in deep tree cover, and I think most modern GPS units should be pretty similar in performance, but you'll have to keep an eye on your unit to know for sure.

I came across this thread as the result of using Search (imagine that!). My search was motivated by the DRAMATIC differences I'm seeing when plotting out the exact same route on MapMyRide versus RideWithGPS. So...not exactly related to GPS but could explain part of the discrepancy you're seeing.

I just plotted a ride in RideWithGPS that I already had stored in MapMyRide. They both show 50 miles (+/-) but MapMyRide indicates 1394' of climbing, whereas RWGPS shows....3204'! I mean HOLY SMOKES...that's not even close. On another route I plotted, again about 50 miles, MMR has 364', RWGPS has 1323'!

Has anyone else compared the two sites side-by-side? Don't they both use Google Maps?

ridewithgps does a much better job of figuring climbing for a ride. Barring things like bridge crossings over a gorge (where it can add up a LOT of extra vert as it follows the topo map and doesn't figure the bridge), it does a good job and is usually pretty close to what I get with my Garmin 500.

Well, I ride with he triton 500, pretty much the same as the 400 from what I can see. And my distances are spot on. I just went back and checked my log on Vantage point and my logs on map my ride. Maybe your settings are out. They recommed settings for the battery savings mode, I max all the settings and carry more batteries. If you have any questions you can check out the Triton forums: http://tritonforum.com/ Also, maybe you neeed to upgrade you firmwear? You can do that on the forums site too...

Cheers,

Eric...

D strong, measuring climbing can get tricky! Both could be correct. It depends on how small a change in altitude the unit measures. A sensitive unit could measure lots of height on what looks like a flat road.

My GPS and bike computer will often disagree slightly ... but by slightly, I mean by about a mile over the course of a 50 mile ride. The CayEye has the higher mileage of the two. I'm not sure if that's because my tires might be a slightly different size than the ones on the bike when I bought the computer, or any number of other reasons? Occasionally the GPS unit will get int his weird mode where it's computer will think I've gone thousands of miles, but the data in the track log seems accurate.

A GPS chip is generally accurate to about 10 meters, and can get down to 10 feet or less in urban areas with WAAS. But when it comes to altitude, GPS is good to about 150 feet. That's much harder to calculate from some overhead satellites. If you're under a forest canopy or have tall buildings or bridges around, altitude will suffer most, because you need more satellites to figure it out. A good GPS unit will give you a choice between using elevation from satellites or from a barometer. But, don't discount your lat/lon readings because the ele is off.

My bicycle computer and my GPS have never matched. I have had the same GPS for years, but I am on the second set off bicycle computers (road bike and MT bike). With the first computer, the GPS went further than the bike computer. With the second computer the GPS lists less distance. (Yes, I checked the wheels were the proper size in the computer.)

For me, I decided that I like the GPS data and map capability (dump to computer), so I decided that is the number I will keep in my logs. I use the bicycle computer for cadence and MPH.

My Garmin Edge 705 did this once on a charity ride. For some reason, it glitched and added a single point that was thousands of miles away from my actual location. Amusingly, this caused my average speed to jump from 15mph to 650+mph. Sadly, my riding companions that day were pretty slow and brought the average down to around 590mph by the time we finished the ride :D After updating the software on the GPS device to something a bit more recent, I haven't had the problem again.