Most simple, cheapest GPS receiver recommendation pls

[Bob Gross]

Nathan, if you hike to the very top of the Copper Creek Trail, you are at Granite Pass. On the east side of the trail there is a metal benchmark. Try using that to calibrate.

Due to plate tectonics and uplift and erosion, elevations like that are not perfectly constant. I believe there is a web site that lists the benchmarks, the last date of survey, and what the corrected elevation was at that time.

On second thought, hiking up and down the Copper Creek Trail is way too much work.

I've studied lots of 3D topo maps, and I haven't found any that are particularly accurate for elevation.

–B.G.–

[Nathan Hays]

Yup, there are benchmarks all along the trail. Been over Granite Pass 5 times now. Last time we followed the string of lakes NE of Goat Pass and Glacier Lakes. No evidence anyone had ever been there. Left it that way.

[R K]

I've studied lots of 3D topo maps, and I haven't found any that are particularly accurate for elevation.

Bob, perhaps the differences you've found could be explained by the differences between the geoid and the ellipsoid? Just a thought.

[Jerry Adams]

Uplift is a fraction of an inch a year

If it was 1 inch, in 100 years it would rise 8 feet – in our lifetimes this is not significant

[Bob Gross]

"Bob, perhaps the differences you've found could be explained by the differences between the geoid and the ellipsoid? Just a thought."

Yes, the different models each yield different elevation numbers for peaks and other well-surveyed points. Part of the problem is that when your GPS receiver shows you some piece of map, you don't really know which model it came from. Or, if you access some model's maps on the net, you don't really know there, either.

Back in the old days, consumer GPS receivers first showed up in the marine market (yachts, fishing boats, etc.). There, they weren't too worried about elevation since they figured that the ocean was at sea level, and fresh water lakes and rivers didn't matter. So, the early NMEA protocols didn't really address high elevation details so well. Now, we pay the price with elevation confusion.

Tectonic uplift is significant in many areas. I think it is very pronounced in the Himalayas. The summit elevation on Mount Everest keeps getting higher, so you better get over there and climb it before it gets even worse. Some of the other mountain ranges are actually sinking due to their own weight.

–B.G.–

[Paul Hatfield]

> The basic position fixing operation is cut and dried, but lots of the small details are not.

The normal consumer GPS receivers with the integrated displays are doing some sort of averaging when they display GPS coordinates. It's probably an exponential moving average, but as you say, the manufacturers tend not to disclose what they do, which is unfortunate.

BlueTooth GPS receivers (example: GlobalSat BT-359) tend not to do such averaging, and will give you the raw coordinate data, non-averaged.

I don't believe that consumer GPS receivers have improved much in the last 5 years, but I haven't been watching the field closely.

[Bob Gross]

"The normal consumer GPS receivers with the integrated displays are doing some sort of averaging when they display GPS coordinates. It's probably an exponential moving average, but as you say, the manufacturers tend not to disclose what they do, which is unfortunate."

Kalman filtering gets used a lot in the normal front end to mid-parts of the receiver. By the time the position data is streaming along, some manufacturers seem to have different ideas about the best way to do averaging.

A good experiment is to put a GPS receiver in your car, and drive down a flat highway at a fixed speed with the cruise control set on. Let's say it is 50 mph. Then watch the speed displayed on the receiver. It will likely be floating around from 49.5 to about 50.5. I don't think that is a sign of an imperfect cruise control. I think it is something about the sampling rate and round-off error mixed with a display rate issue. Constant data display updating is not a top priority.

Military GPS receivers, like in a jet flying at Mach 2, use a much faster sampling rate, so they get different kinds of errors. They also use a lot more power.

–B.G.–

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