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Coastal Measurement

October 13 2009 | 1 comment
Categories: Map Data

When the length of a coastline is stated by cartographers or geographers, what is the interval of measurement or the precision used? (It seems the coast will be longer and longer the smaller the interval used to measure)

Is there a national or international standard for this?

Thank you.

Mapping Center Answer:

Well, in over 15 years of being a geographer and cartographer, I've never been asked to express a coastline in terms of length.  I'm therefore fearfully full of curiosity wondering why that would be relevant?

To get to the question though, units has little to do with it, though I think I understand what you're really asking.  If the coastline were measured in feet, would it be longer than if in miles.  Only if you were relegated to using integer values, and if you rounded the miles down. 

The real issue is what is the minimum mapping unit, i.e., the length of the shortest line segment one would digitize or extract from an elevation dataset to represent a portion of a coastline with.  The smaller the minimum mapping unit, the more detail can be captured, and therefore the tendency is to have a longer line.  That said, the more detail one adds, the more features like small coastal islands, rocks, and such creep into the work, along with whether the tide is in or out, and soon you find out why mapping coastlines is a really tough job.  Either way, I would not use line length as a determinant of quality, in any unit. 

I don't know of any standard, but NOAA would be the U.S. Agency to check that out with.


Survey posted by Chris Mathers on Oct 14 2009 10:29AM
The way coastline is measured is with a mean high tide survey. the units will be in what ever projection they survey in. Also, this changes. Depending on how you are presenting this would depend on how you measure it. If it is large scale, I would use aerials and digitize a line for any area that is unsurveyed. If it is small scale, I would still use the areials and just be a lot more general about it. ESRI also has some polys of county and state shapes that you could use if you need a "low resolution" answer. So should your DEP equivilant agency. The reason it shows up for some people and not others is where you work. Here at my county we GIS folks do a lot of work for the public works dept. so measuring things becomes important.

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