Day Hiker wrote:How many different numbers have we seen for Mount Whitney? 14494, 14495, 14497, and "over 14500." WTF? And that peak I am sure has had more precise efforts on it than most others.

I believe you said surveyed within 5' of precision, not 5' of accuracy. If you went back and repeated the optical surveys from the same reference points, with the same methodology, the offset from those reference points would be precise within much less than 5', not sqrt(5^2 + 5^2).

The correction floats all boats; it's a little more complicated than just adding a number, but for present purposes lets say that's what they did. When you add the same exactly known value to elevation for both base station and summit, you do not affect the accuracy of the difference between the two. The correction comes from an algorithm that is repeatable and always gives the same answer for the same inputs.

If you read the PID file for Whitney, you'll likely find that they are just changing the mode of calculation, and applying the new geoid to the optical survey data. I haven't read the Whitney PID for a while, but I know that the NV peaks that "grew" did so without any further measurements; they just took the old data and applied a new baseline. There was no precision involved.

Ironically, your GPS likely has a newer geoid programmed into it, than the geoid used for calculation of that number on the Whitney summit.

So we probably agree on this point: obsession with the elevation number printed on a map, published in the 1970s or 1980s, is rather pointless.

Day Hiker wrote:

sixfingers wrote:is there a GPS that calculates global warming induced sea level rise as a zero point in measuring summit elevation?

Ha ha! Has it even risen a foot? Only the Everglades would notice that.

Elevations are based on a mathematical model of Earth and not on the actual day-to-day or year-to-year level of the ocean.

(Yes, I know you were joking.)

Might be worth throwing in that the sea level varies by up to something like 200 m from place to place due to things like local variations in gravity in turn due to geological variations, irrespective of what the waves / tides / thermal expansion effects are doing etc.

MoapaPk wrote:

Day Hiker wrote:How many different numbers have we seen for Mount Whitney? 14494, 14495, 14497, and "over 14500." WTF? And that peak I am sure has had more precise efforts on it than most others.

I believe you said surveyed within 5' of precision, not 5' of accuracy. If you went back and repeated the optical surveys from the same reference points, with the same methodology, the offset from those reference points would be precise within much less than 5', not sqrt(5^2 + 5^2).

By "precision," I meant that of the human race's measurements of a given peak. One year, it's 14497; another year, it's 14495, and so on. It doesn't matter to me that it's because they changed methods. They could change methods again next year and get yet another number, even though this year's number is supposed to be taken seriously to the exact foot.

I do understand that with any given methodology, they might repeatedly get the same number to within a foot. But they got many different elevations over the years because they changed something in the methodology each time. Well, why should I believe the latest measurement is correct when all the previous ones were wrong, and I was supposed to believe those? And they'll probably change methods again next time and get yet another number that supersedes this one.

And if those different values are the result of a geoid that's changing each time, it's still the same end result in terms of the peak's ever-changing official elevation.

So maybe I believe the numbers to, at best, the nearest 10 feet, because the official measurements don't usually seem to swing more than that.

Here is another issue, and it is regarding accuracy. Anyone with a brain, two working legs, and a barometric altimeter can calibrate to 14500 on Whitney then run over to Keeler Needle in 30 minutes and check the elevation there before any real atmospheric changes take place. You will get a reading of 14320 to 14330 (or 170 to 180 below whatever you were calibrated to on Whitney's summit). But the stupid official number for Keeler is 14260 feet. How totally lame.

So the accuracy of Keeler Needle's official elevation is off by about 60 feet. Or Whitney is off by 60 feet. Or they're off by +30 and -30 or +20 and -40 or something to that effect. In any case, they both can't be right, and at least one is WAY off.

So, back to the topic of centimeters of precision, how is that possible if one can't even see the actual summit high point from the surveyor's viewpoint? Looking in an upward angle from below, it is not possible to see the actual benchmark or actual 1cm hunk of granite that is the true summit of a peak.

It would seem a survey of the actual high point is possible only if a device on a tripod of some sort is placed on the summit, in some exact relation to the true high point, and that device is sighted from the reference point or points. And then they would have to subtract the difference in height between the device and the true high point. But this doesn't seem as though it would be feasible on a lot of peaks, particularly ones you can't stand on.

In some places, there is snow or a glacier on the top of the mountain, like on the highest Swedish mountain Kenekaise.

The height was once 2117 m, and is now 2104 m. The exact height of course varies with the season and the year.

Day Hiker wrote:It would seem a survey of the actual high point is possible only if a device on a tripod of some sort is placed on the summit, in some exact relation to the true high point, and that device is sighted from the reference point or points. And then they would have to subtract the difference in height between the device and the true high point. But this doesn't seem as though it would be feasible on a lot of peaks, particularly ones you can't stand on.

They use a chain of viewing and often do have an object of known height to sight, and the height of the object is accounted. That is, they did; there aren't too many optical surveys these days.

To add to the confusion, the shape of the earth actually has changed in the last 100 years, as the more mobile continents have moved over the asthenosphere.
EDIT: Oops -- wrong reference. Anyway, the datum we now use is earth-centric, and assumes that the NA continent does not move relative to the earth's interior. Maps often used a surface-defined datum. The actual heights of the mountains shouldn't change that much, but their position on the earth can change -- so the DEM which has z(x,y) must take this change into account.

I have used my USA based Garmin eTrex Legend and GPS60C in Australia without any problems. The accuracy was what I consider normal for these units, somewhere between 10 and 30 feet.

The one minor annoyance is that both GPSr units took approximately 10 minutes to acquire their new location (stationary with good sky view) but I guess that after travelling 9000+ miles I can overlook this.

Ciao.