Complex Numbers

Getting the Drift

Today's article, from Nautilus, is a few years old, but it's not like plate tectonics has stopped since then.

    What Happens to Google Maps When Tectonic Plates Move?  
Earth’s tremors can tweak your GPS coordinates.

It's not just Google Maps, of course; this applies to any GPS receiver.

This is a thing I've often idly wondered about, but never enough to go and seek out the answer.

As a writer on physics, I’m always seeking new metaphors for understanding Einstein’s general theory of relativity, and while working on my last book, Spooky Action at a Distance, I thought I’d compare the warping of space and time to the motion of Earth’s tectonic plates.

Oh look, a book plug. What a surprise.

Regardless, this is the kind of thing I mean when I say there's no such thing as useless knowledge: you can more easily find similarities between disparate things. The irony (or whatever) here is that GPS wouldn't work worth a damn without relativistic corrections for the satellites' speed and distance.

I discovered a sizable infrastructure of geographers, geologists, and geodesists dedicated to ensuring that maps are accurate. But they are always a step behind the restless landscape. Geologic activity can create significant errors in the maps on your screens.

As if paper maps are more accurate.

Several factors produce these errors. Consumer GPS units have a position uncertainty of several meters or more (represented by a circle in Google Maps). Less well known is that maps and satellite images are typically misaligned by a comparable amount.

All things considered, an uncertainty of a few meters, compared to the size of the planet, may as well be sorcery. I'm pretty sure military hardware has less inherent uncertainty, but it would also be affected by tectonic shifts.

For the most part, misalignments don’t represent real geologic changes, but occur because it’s tricky to plop an aerial or orbital image onto the latitude and longitude grid.

And this makes sense when you think about it. Not only do you have the Earth's curvature (it is, in fact, roughly spherical) to consider, but also elevation—another parameter that can change, albeit usually slowly.

NGS and other agencies recheck survey marks only very infrequently, so what a stroke of luck that a community of hobbyists—geocachers—does so for fun.

I had no idea geocaching was going on. I was a guest at a large gathering of theirs many years ago, and they seemed... obsessed. But I suppose we all have our obsessions.

Confusingly, the U.S. uses two separate datums. Most maps are based on NAD 83, developed by NGS. Google Maps and GPS rely instead on WGS 84, maintained by a parallel military agency, which has a considerably larger budget.

It should be kept in mind that GPS, like the internet itself, started out as a military project, and only later became accessible to civilians.

When NGS introduced NAD 83, replacing an older datum that dated to 1927, it was the geographic version of the shift from the Julian to the Gregorian calendar. If you’d been paying attention, you would have woken up on Dec. 6, 1988, to find that your house wasn’t at the same latitude and longitude anymore. The shift, as large as 100 meters, reflected a more accurate model of Earth’s shape. Vestiges of the old datum linger. You still see maps based on NAD 27.

This change was relevant to my work, as well. As I recall, the datum (the zero reference) for the two systems was different by nearly a foot; I wasn't quite as concerned with the horizontal references, because the parcels we worked with were relatively small, a couple hundred acres at most, and the important thing was relative position, not absolute. Yes, we used inches and acres.

“Most surveyors and mapmakers would be happy to live in a world where the plates don’t move,” Smith explained.

Considering that plate tectonics is probably linked to the processes that generate Earth's magnetic field, which helps protect us from effects such as solar radiation and the atmosphere floating away into space, no, no, they wouldn't be happy to live in that world, not for very long.

Anyway, the article does get a bit technical, but I find the discussion of how this works entirely fascinating... if irrelevant to my quest to find the nearest brewpub from wherever I happen to be at the moment.