Elevation Maps

[Kyle]

Hey- I don't know anything about elevation maps, but are they all based on a constant sea level that was established a long time ago? So do we have to redo or relabel all those maps when the sea levels change?

[Mike]

One would think so. But since sea level (or Mean Sea Level) varies by place and by sea, and since we haven't had any globally accurate way to measure sea levels in a way that could be consistent from place to place until the last few decades, I don't know how precise most maps are in the first place, and the sea levels have probably risen less than 4 inches since the space age and information age gave us the capacity to get correct numbers. But yeah, eventually, I suppose. (Anything accurate in the above statements came from Wikipedia).

Although, as a mental exercise, I wonder how far sea levels would have to rise for most local elevation maps to no longer be useful for most practical purposes. I assume they're mostly used for relative differences in elevation, and those won't change with sea levels.

Short version: I have no idea.

[Kyle]

I think they're used very precisely in survey maps and for purposes like that. I suppose if you're laying a pipeline (heh, heh) you need it to be accurate-- but they probably go out and get their own elevations prior to construction.

[Mike]

Right. But my point is that if I'm laying pipe all the way across Nebraska (you're right, that works well), does it matter so much that the elevations are off by 2.7 inches if they're ALL off by 2.7 inches? It's the relative elevations that matter more than an absolute measure above sea level.

Although I imagine it's probably moot as I would guess that if it's important, they're using up-to-date computer generated maps based on the latest satellite data... or whatever it is those fancy engineering types use.

[Elle]

Well, you need some kind of vertical datum as reference point, and that could be a mean sea level (which is like Mike says, I think) or it could be some sort of geodetic datum. And these can be converted into one another in local areas. I know there is a North American standard reference point but not sure how global elevations are determined for comparison purposes. Kyle I am sure you know more about GPS than I do and there's a standard on which it's coordinates will be based that E.g. the military uses, but that can be "off" if some other standard is being used.

My dad knows all about this kind of thing I guess. For purposes of measurement the earth can be conceptualized as a geoid with its surface in equilibrium and there are different modes of doing that. I bet Peter has a detailed answer to this one. Autocorrect has been an active hindrance in the writing of this post.

[Irish Fan]

If you're laying pipe all across Nebraska, I hope you are doing it safely! Although, if you are off by 2.7 inches, I suppose you have nothing to worry about. Wait ... are we still talking about elevation maps??

[Elle]

It is true that rural red state Nebraska stopped the darned Keystone XL pipeline. And birthed me into the world, so...

[Kyle]

[Elle]

Seriously. It can be difficult when the 'size matters' cultural message is all about wanting things bigger, but in reality that's not always comfortable or desirable for the job at hand.

[Tahlvin]

[Phoebe]

The old Keystone pipeline has leaked a big ol' leak all over someone's land. This is why they don't want it near the water, people. This is why landowners shouldn't be forced to bend over for a foreign private company. What the brick do sovereignty and property rights even MEAN when you own land and have to let something like this be put on it against your will? Also: file under Hypocrisy. Unless you're against this, and btw also against the Kelo v. New London travesty, please do not bother calling yourself a traditional conservative or libertarian. Just shut up and say "I'm a grifter who wants to take whatever I can grab and let others plunder as well."

[FlameBlade]

Now I know why that 2.7 inches is pretty frigging important.