Just Purchased a Garmin Rino 120

[mcontreras]

We just purchase a Garmin Rino 120 and I was wondering how these things measure distance. Like if I just walked over two mountains, does it measure how far I walk or does it measure as if I flew over the mountains?

[joebartels]

It measures where you walk.

[chumley]

GPS devices are capable of determining elevation as well as latitude/longitude, so it will measure your travels in three dimensions. But you should know that it is much more accurate on the lat/long than on elevation, especially in lower reception areas. If you've got only 3 satellites tuned in, your spot on the map might be pretty close, but your elevation could be off by 1000 feet or more. But if you are receiving signals from 6+ satellites, your elevation is probably within 50 feet or so. (At least with my little eTrex). So, while it will measure your route over mountains and through valleys, I've found that the actual distance can be off depending on the reception while you are hiking.

Similarly, I've found the distance to be spotty when hiking tight switchbacks with a good canopy of trees. Because the each switchback is virtually "on-top of the other" if I don't have great reception, sometimes the track skips part of the ascent and connects points of the trail with a straight line which bypasses switchbacks. This really only happens when the switchbacks are very close together.

[joebartels]

I think it's two satellites for lat/lon and three to triangulate elevation. Anything over three isn't receiving extra signals as they take the top three strong signals.

This was my understanding from long ago. Perhaps I was totally misinformed or the newer units have actually been improved.

[chumley]

If you have an hour of time and a degree in astrophysics, read the wikipedia article on gps. Since the Earth is not flat, it actually takes THREE satellites just to determine lat/lon (trilateration?) Typically, four satellites are required for an accurate reading in three dimensions. Fewer than three satellites can specify a location if additional information is added. As an example, a ship may determine its location with only two satellites because it is known that it is at elevation 0 ft.

Apparently, determining location is all about receiving the time and location of each broadcasting satellite and your GPS unit processing the error correction of both sets of data it receives. Wiki says that using more than 4 satellites will never result in less accurate results than using the best 4, and will usually result in more accurate data.

My GPS has a "signal strength" screen which shows up to 12 satellites, their location in the sky, and the clarity of signal being received from each of them. When I'm receiving 6+, I'm usually accurate to 8 or 9 feet. If it's only 3 or 4 satellites, my accuracy may be 15, 20 or even 50+ feet, so I'm fairly certain that it uses as many satellites as it can.

And we all get to use this technology because the Soviets shot down Korean Air Flight 007 in 1983. Who knew :

[joebartels]

Yep, straight from Garmin

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude).

Couldn't find anything on the max number of signals simultaneously processed. I just recall the sales pitch back when 8 & 10 satellite receivers where being bombarded with 12 satellite receivers. They would say that 12 is a gimmick as only 10 satellites are in view at any one time. Then they'd go on mentioning a 12 satellite unit couldn't process more data than 10 stating only the highest strength signals were used. Not sure what prompted me to rely on a salesman in the first place

[mcontreras]

We returned the Garmin Rino 120 and purchase the Garmin GPSMAP 60 instead. This one did come with a CD and I can create waypoints and routes on my computer then send it to the GPS. I downloaded the Route for Grand Falls for this weekend. Taking the kids there to see and hang out for a bit. Thanks everyone.

[big_load]

Measured distances when hiking are often quite a bit low (up to about 10%) since the unit samples at a finite rate and cuts corners between samples. It's time-consuming, but you can get a better estimate by editing the track afterward to get all the wiggles back in.