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Then How Squished IS Squished?



In a previous blog entry, I had described geographic datums; how they are a "squish factor" (flattening ratio) applied to a sphere to represent an approximation of the shape of the earth (because it bulges at the equator) for mapping purposes.

A colleague of mine here at IDV, John Dougherty (boss, actually), asked me how much of a squish factor datums actually apply.  Good question.  My answer was, "it depends on the datum" which we both knew was a desperate stall.

I sent an email to someone who would know: Dr. David Patton at Central Michigan University.  This is what he said…

"The flattening ratio used for the reference ellipsoid associated with WGS 1984 is 1/298.257223563.
It is almost identical to GRS 1980, but quite different than the Clarke 1866 (1/294.9786982) that is used for NAD 1927 US topo sheets."



If the general diameter of the earth is around 7,927 miles, that means the datum WGS 84 approximates the earth’s shape as being about 26.6 miles wider than it is tall, while NAD 27 comes in at 26.9 miles wider than tall -a bulge difference of just under a half mile.

To put that into perspective (or out of perspective, really), a vice would have to squeeze a 12-inch globe about one millimeter.  That is the thickness of about two stacked credit cards or a thinly sliced piece of deli meat.  If a globe were to actually be the shape of a datum ellipsoid, the difference between WGS 84 and NAD 27 would be indistinguishable to the eye.  For that matter, the difference between a globe that referenced a datum and one that did not would be too small to notice as well.  However, the world is actually a big place and when it comes time to pick a datum, the difference is significant.


John Nelson / IDV Solutions / john.nelson@idvsolutions.com



5 responses

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    October 23, 2008 at 8:47 am

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