We have no idea how big the US coastline really is

CoastlineNOAAWhere would you trace this coastline?

When you’re mapping a coastline, how exactly do you decide what to measure? How many nooks and crannies of shoreline do you trace? What
is a coastline, anyway? 

Coastlines are living, changing entities. The tide goes out, and the length of the shore changes when it comes back in. Hurricanes can reshape the entire makeup of coastal ecosystems. Climate change is drawing the water higher, completely altering what the shoreline might have been 100 years ago. 

This seemingly scientific analysis can get philosophical pretty quickly. While we may trust Google Maps with our lives, maps of course can’t show us a 1:1 representation of our surroundings. They have to be scaled down in order to fit into a space that we can actually use. The ratio of measurement that the National Oceanic Atmospheric Administration uses as its official coastline measurement, for example, is 1:1,200,000

Science TV personality Steve Mould explains the challenge of measuring coastlines in an illuminating YouTube video

First, Mould shows what measuring the coast with one size ruler would look like: 

And then he does it again with a ruler half the size:

But the two rulers give different answers: 

“The smaller I make the ruler, the bigger the answer,” he says in the video. “The question becomes: How small does that ruler need to be?” 

Coast survey 1921NOAASurveyors take coastline measurements of southeast Alaska in 1921.

One little-known secret is that most of our maps today, with all our advanced technology, are based on calculations that were made by measuring the coast on the largest maps available in the mid-1900s. 

The US Census coastline estimates, which measure the general outline, are from surveys done in 1948. And the shoreline estimates, which measure a more detailed path along the shore that includes offshore islands, are from surveys done in 1939-40. 

So how big is the coast?

It depends who you ask. According to the US Census (which compiles geographical data from multiple government agencies), the general coastline of the entire US is 12,383 miles, while the shoreline is 88,633 miles.

But the official shoreline estimate from the National Oceanic and Atmospheric Administration is 95,471 miles. That estimate uses the very same survey, but lands on a larger number by adding in the shoreline from around US territories like Puerto Rico and along all the Great Lakes and surrounding rivers, which was measured in 1970

So even the “official” numbers are different, depending on how exactly the shoreline is defined.

Since we can’t make 1:1 maps, we’ll never really know how long the coastline is. But that doesn’t stop us from making some educated guesses.

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