Mileage Difference

this is why I always have a wheel sensor on my bikes. GPS really isn't for distance accuracy. I use GPS because I like maps (I do care about a reasonably accurate map, though). Distance accuracy is achieved with a wheel sensor (properly calibrated). I also like saving my ride data to look at later, and that's the other place that a GPS comes in handy compared to a regular cyclocomputer.

GPS distance recording is really not that accurate the way we use it for bike stuff. The faster you go, the less accurate it is (the more spread out your recorded points are). The more you turn, the less accurate it is (connect the dots around a curve cuts distance off). The more variation in your devices, the more discrepancy you'll have. Phone manufacturers are absolutely terrible about giving you any kind of specifications for the GPS hardware in them (processor OR antenna). Because that varies from one phone brand and model to another, the differences can be huge from that reason alone.

remanaz said:

Interesting thread. I just ot a small cheap Chinese cyclometer that says it uses 3 satellites to track you. I wonder how this would compare to my phone. I might have to try this.

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Three satellites are the minimum required to location (longitude and latitude) along with elevation. If you don't need elevation, only 2 satellites are required.

Phones actually "can" track several satellites concurrently. Glancing at the GPS Status & Toolbox app I mentioned earlier, I can see my phone currently tracking to 18 satellites (14 with a fix) as I sit inside my home. If I were outside with a better view of the horizon, I'm sure it would track even more. Sounds impressive, but just because the GPS Status & Toolbox app is using all of these fixes doesn't mean other apps, like Trailforks, does. Trailforks may only use the minimum required for tracking position long/lat/elev, which as mentioned, is 3. The less data (fewer satellites) used, the less accurate. The other thing is, a trail map app probably isn't using all of the sat data in absolute real-time. In fact, there's a configurable setting in Trailforks to set satellite refresh, with is a default minimum of one second (it can be set for longer intervals to help slower phones). One second isn't terrible, but that's assuming the phone isn't busy doing something else. Apps like Trailforks have other things to do... displaying a map, logging data points, etc. There's only so much CPU power in a phone. What else is the phone doing? Facebook, Twitter, weather, phone service, messaging, email, etc., etc., all running in the background. Tracking multiple satellites and plotting their data could bog a phone down, so there's some trad-off taking place. All of these variables impact accuracy.

Something like a cyclometer is dedicated to one function, and can likely collect data points more frequently than an app on a phone. All of its focus is on that function. I would guess that it will always be more accurate than a multiple purpose device, like a phone, would be. The same can be said about dedicated GPS devices like those from Garmin.

remanaz said:

This is the satellite info. I am not sure if it means 3 satellites or 3 types of satellite systems.

"the bike computer positioning system is connected to 3 satellite connectivity: GPS, GLONASS, Galileo." This type of stuff is foreign to me.

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Each of those is a constellation of satellites. each constellation has dozens of satellites in it. you can't connect to all of them at once (physically impossible because they're covering the whole globe). it also doesn't tell you how many the receiver can track at once, but it gives you more options to get a quality position fix when conditions are poor.

you have a combination of poor translation in the product description and lack of knowledge of what's actually going on. a potent combination, for sure.

fredcook said:

Three satellites are the minimum required to location (longitude and latitude) along with elevation. If you don't need elevation, only 2 satellites are required.

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a fix on 2 satellites does NOT give you a 2D position. That requires 3 satellites. 4 satellites is required for a 3D position (coords plus elevation).

How Does GPS Work? GPS for Dummies – HikingGuy.com

How GPS Determines Your Position
At a minimum, your GPS receiver needs three satellite signals (aka “fixes”) to determine your position, which is called trilateration.

With one signal, you can be anywhere within the surface of the sphere.

With two signals, you can be anywhere on the blue line where the two spheres intersect.

With three spheres, there are two intersection points. This is called trilateration. One fix will be on the Earth’s surface, the other one will be in space. The GPS chipset will discard the one in space and give back the position of the one on the surface. Garmin also calls this a 2-D position.

Adding a fourth satellite will give you one point of intersection, eliminating the need to discard the point in space. It will also allow the GPS to calculate elevation. This is sometimes referred to as your 3-D position. This fourth signal is also used to sync time. Most GNSS systems strive to have their satellite orbits distributed so that at least four satellites are always visible from any point on the Earth at any time for this reason.

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remanaz said:

You are correct that accurate is not the correct term here. I should have specified larger than a bread box change, lol, as I am not referring to small couple feet variations. Also not mileage. I can upload my data and see the map of my route. If I take a path that slightly different on the way back from the way to it will show it. For example, my driveway vs the side walk it shows this. Again I only got this cheap one because I wanted to see if I even wanted to get a cyclometer. I wasn't going to throw $200+ at something I didn't even know if I wanted.

I am liking being able to track my routes and speed and upload them to strava. I like being able to compare previous rides to my new one and break that down to smaller segments as well. Is that possible with a wired system?

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No. a cheap wired cyclocomputer cannot do these things. The critical thing here is saving data from your ride. What GPS permits is recording where you are and at what time. Speeds and elevations can be calculated from that, and if you have other sensors, you can really start piling on the data. In the heydey of cyclocomputers, the most expensive ones could record, but any data they recorded had no position to associate with. Just time and distance. It gave some rudimentary analysis capabilities if you rode exactly the same route.