Elevation change of Logan Pass in Glacier National Park 1977-2018

November 9, 2018 10:38 AM

My parents visited Glacier National Park when they were dating in 1977, and my dad took a picture of the Continental Divide sign in Logan Pass. This past summer, they went back, and he took another picture of the same sign. In the intervening 41 years, the elevation listed on the sign decreased by 8 meters. Is there an explanation for this change?
posted by Plutor to Science & Nature (10 answers total)

My thought would be measurement accuracy. There are 3 different plaques on the steps of the Colorado State Capitol building in Denver showing where the 1 mile above sea level mark is, based on increasingly accurate measurements over the years.
posted by tiamat at 10:44 AM on November 9, 2018 [1 favorite]

Well, a cursory google doesn't turn up much, but it hadn't changed yet as of 1985 (PDF). Fourth column, third paragraph.
posted by sagc at 10:47 AM on November 9, 2018

A lot of measurements for things changed with the advent of accurate GPS.
posted by bondcliff at 10:51 AM on November 9, 2018 [2 favorites]

Agreeing that what almost certainly happened is that technology improved, new measurements were taken, and the official elevation was revised to reflect the new, more accurate measurements. It happens more than you'd think, and as bondcliff says it happened a lot after GPS hit the scene.
posted by Anticipation Of A New Lover's Arrival, The at 10:59 AM on November 9, 2018 [1 favorite]

GPS is part of it, but also increasingly accurate digital elevation models, based on the shuttle radar topography mission.
posted by rockindata at 11:15 AM on November 9, 2018

Just to elaborate on my brief answer, this same thing happened in the White Mountains in New Hampshire after they shut off selective availability of GPS and the park service and other agencies were able to re-measure peaks and trails. There is a list of > 4000 foot mountains in New England and after they re-measured a couple mountains that were previously thought to be under 4000ft (or otherwise not qualifying due to insufficient prominence) were added to the list and a couple others were removed.

As others have said, there were other contributing factors but they're all due to technological improvements in measuring.
posted by bondcliff at 11:18 AM on November 9, 2018

Response by poster: This certainly sounds like a plausible explanation. Was this a coordinated effort? Did the NPS or USGS or someone else spend a bunch of time and money re-signing all of their parks and landmarks? Is this is a project that happened at a certain time everywhere and has information I can read?

Or did someone local just decide "hey that sign is getting old, let's replace it, oh and by the way let's see if the elevation has changed"?
posted by Plutor at 11:37 AM on November 9, 2018

Related: I'm amazed at the precision of surveying:

Surveyor Radhanath Sikdar measured Mount Everest in 1852, with a height of 29,002 feet. Modern measurements indicate the height is 29,037 feet.

especially considering the array of measurements made
posted by tinker at 12:11 PM on November 9, 2018 [1 favorite]

Also possible that the change was related to the switch from the North American Datum of 1927 to NAD1983--the timing is right, but I don't know if that's the explanation in your case or just a coincidence.
posted by agentofselection at 6:59 PM on November 9, 2018

Might be a change in the spot they are measuring. Is it at foot of the the sign, which is a little uphill from the road, or is it at the highest point driving on the road or is it the lowest spot on physical saddle of the pass forming the continental divide which is a little below the road.
posted by JackFlash at 7:22 PM on November 9, 2018