I occasionally use a Garmin wristwatch type of GPS while biking or hiking. Today I finally put it on my sea kayak; I wanted to measure the distance between launch point and dam. Not wanting to study the online user manual, I left the display as usual: Time of day, miles traveled, avg speed, and altitude gain.
Weeeellllllll…didja know that a reservoir somehow has 220 ft elevation gain in 10 miles round trip? Hmmm! Funny, I thought flatwater was, well, FLAT.
Could those 220 ft result from a few dinky boat waves and tiny wind waves?
Or is it barometric pressure change? The weather was fair, though there is a slight chance of showers overnight into Sunday.
Would the “altitude gain” indicate dropping barometric pressure?
You can have outlier data points that are due to errors acquiring your location. This can be due to less number of satellites visible or other factors you can also see this kind of error with max speed. If you have a track recorded and sit in one place it will sort of draw a squiggly line around the spot. Back when GPS was first allowed for civilian use the military only allowed a degraded single with enough accuracy to prevent others from using it for pinpoint targeting… if you had a graphic of your altitude over time the outliers would be obvious.
GPS does well for horizontal, but relatively poorly for vertical. Our lake is at ~2100’, but recently my GPS reported 1946’. I don’t go that deep on my rolls.
Years ago paddling 50 miles of the Snake River in the Yukon one day we were tild we had gone up a hundred feet
The gradient on the Snake is about 25 feet per mile the other way.
Wrist band mounted GPS watch…… which went UP and down as you paddled. I see that all the time with the wife’s watch. I used to wear one of those step counters. Got a lot of steps counted while mowing grass, on a riding mower. Could get more step counts if I swung my arm some too. Smart watch but not too smart. .
Another interesting thing is to be sitting still and it shows horizontal motion and distance traveled.
Used the Strava App on my phone during a 4-5 miles walk in Orlando. Very flat Orlando. Table top flat Orlando. After I was done, the app showed 1000 feet of climbing. I must have stepped up over a few more curbs than I thought.
@Overstreet said:
Wrist band mounted GPS watch…… which went UP and down as you paddled. I see that all the time with the wife’s watch. I used to wear one of those step counters. Got a lot of steps counted while mowing grass, on a riding mower. Could get more step counts if I swung my arm some too. Smart watch but not too smart. .
Another interesting thing is to be sitting still and it shows horizontal motion and distance traveled.
as mentioned above altitude measurements on non military GPS are notoriously inacurate. This is partially designed into the devices so they are not accurate enough to aim weapons and partially due to the software that converts the semispherical surface of the earth into two dimensions for distance measuring,
I’ve heard that topo maps are used to determine elevation gain and loss. Did you lose elevation going towards the dam, and gain on the way back? I paddle on a reservoir and my Garmin Edge 500 seems to work that way.
I’ve always exported my GPS data to my computer, then used Google Earth to plot it against the terrain elevation. This yields the most accurate results I’ve been able to get so far. Most of the time the elevation only varies by a metre or two (3-6 ft) on flat water. Usually on the low end.
Not only does this give you up-to-date elevation data, it also gives you visual confirmation of your track’s accuracy compared to landmarks you take note of along the way. This is because it’s overlaid on whatever map or satellite imagery Google has decreed to be most recent and/or accurate.
@SeaDart said:
as mentioned above altitude measurements on non military GPS are notoriously inacurate. This is partially designed into the devices so they are not accurate enough to aim weapons and partially due to the software that converts the semispherical surface of the earth into two dimensions for distance measuring,
Yeah, last night I watched as the GPS gave several altitudes for the exact same spot—the floor of a room off the garage. There is also a graphic display that gives what is apparently a dynamic estimate of accuracy. And the accuracy was low. But still, the range of altitudes does match my eyeballing of a topo map. Building is a bit lower than the highest point (6400’) on the property. The GPS gave numbers from 6345 to 6396, IIRC.
I’ve noticed that most people’s “rating of accuracy” for altitude is less forgiving than the same regarding map location. People generally seem willing to accept, for example, a 100-foot leeway in map location without batting an eye (after all, the real location is “right over there” and there’s no problem finding that spot), but the same 100-foot margin of error in altitude will be thought of as a lot, at least in cases where altitude is known with fairly good precision, such as on the surface of a lake.
@SeaDart said:
as mentioned above altitude measurements on non military GPS are notoriously inacurate. This is partially designed into the devices so they are not accurate enough to aim weapons and partially due to the software that converts the semispherical surface of the earth into two dimensions for distance measuring,
Yeah, last night I watched as the GPS gave several altitudes for the exact same spot—the floor of a room off the garage. There is also a graphic display that gives what is apparently a dynamic estimate of accuracy. And the accuracy was low. But still, the range of altitudes does match my eyeballing of a topo map. Building is a bit lower than the highest point (6400’) on the property. The GPS gave numbers from 6345 to 6396, IIRC.
My wrist unit made by garmin is accurate for distance assuming you give it time to calibrate before starting the trek. Average speed appears to be accurate but my maximum speed is always off. Elapsed time also seems about right. Elevation is accurate as well. Meaning you can compare what it says to a known mapped elevation and it will be accurate. When paddling with the unit I strap it to my pfd. What I don’t trust it to do is determine elevation changes. While an average gradient between starting and ending points is probably okay, just like with max speed, there are just too many little “jumps” (signal loss or variations) to be accurate.
As far as temperatures, air or water- it’s now drysuit season in wv.
Most of the newer watches have a built-in barometric altimeter. Those are very good for detecting small elevation changes with accuracy. They of course suck for absolute elevation unless you calibrate them frequently, since the barometric pressure will be different every day.
But when wearing such a watch during a roll, you will get quite some altitude meters. Under water, the pressure will increase by 1 millibar for each centimeter of water depth. Above water, the pressure will increase by 1 millibar for each 8-9 meter of water depth. So during a roll, where my watch is a few cm below water, I often see a sudden altitude change of more than 200 meter.