Cecil’s column «How is the city elevation shown on road signs and maps determined? (03-May-1996)», which says
reminded me of my visit to Denver, where I saw the mile-high marker at the Capitol Building.
Colorado State Capitol Virtual Tour - Mile High Marker
How are altitudes determined that exactly? At the Colorado site you see that some students determined that the original mile marker was three steps too low.
GPS allows very fine resolution now, especially since selective availability has been turned off. But older surveying techniques were capable of high precision–they just weren’t as “easy” as GPS.
A related question is, what is the Denver capitol 5280 feet above? That would be the North American Datum–a best-fit ellipsoid that matches time-averaged sea-level and is extended mathematically to the interior of the continent. The datums are revised occasionally, and the current revision is moving to a world-wide reference datum.
I’m from a land-locked state …
How much does “sea level” fluctuate from high tide to low tide? It seems we might be quibbling over 20 feet …
Four feet or so–but sea-level is “time-averaged” to compensate for the variation.
Difference between high and low tide varies greatly with region, as well as with the lunar cycle. The Bay of Fundy, for instance has 50 foot tides:
http://www.scottwalking.com/quickfacts.html
San Francisco Bay’s largest difference between high and low tide is about 7 feet, IIRC. Significant since much of the bay is very shallow.
Four feet is probably a typical range for a long, straight coastline.
I am reminded of my first trip out west. A passenger in the car looked at Bronco’s stadium and remarked, well it’s awfully tall, but no way that’s a mile high!
Ah, NJ public schools…
Jack
I guess that was my question. What are these older surveying techniques? I imagine that it’s more involved than walking around with a barometer.
As a Denverite, I would recommend that you use the more recent measurement, the purple row of seats at Coors Field, for a standard. They sure seem to be higher than the steps, but it’s all relative.
Well, you can determine the relative height of any two points within line of sight with a theadolite, a plumb bob, and the horizontal distance between the two points. Perhaps you just iterate this procedure from the location of your standard datum?
Do you want theoretical, or practical? The construction of the roman viaducts depended upon being able to traverse long distances and know the exact rise and fall of the terrain. George Washington dragged a surveyor’s chain when he was younger, and Henry David Thoreau was compelled to include his survey observations of Walden Pond in his famous book. The world has been crisscrossed (or triangulated) many times over. Most expeditions are surveys, not just folks tramping through the mud. They established benchmarks of concrete and iron, and generations later they’re still in use.
Mistakes get made of course. And some observations turn out to be paradoxical–which sometimes lead to new theories of the earth. When the mass of mountains turned out to not deflect surveyors’ instruments as much as they would have thought, the theory of isostasy and the study of the earth’s interior was born.
Maybe it is just incredible that people are capable of making exacting measurements over long distances? It is. My favorite example of this is the precession of Mercury. But that’s astronomy.