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In Superman Returns, Lois Lane hurriedly scribbles the geographic coordinates of the yacht she is held captive aboard and faxes it to the Daily Planet.  "40N 73W Help us!!"  Richard White flies to the rescue in his pontoon plane, and quickly.  But Richard got really lucky, because coordinates with this level of precision would cover over three and a half thousand square miles of choppy North Atlantic -an area twice the size of Rhode Island.

Lets Talk Specifics
If you’re talking decimal degrees (rather than degrees minutes seconds), every decimal place worth of precision zooms in ten-fold.  Those decimal places get pretty specific pretty quickly.
It can be tempting to work with latitude longitude numbers that have ten decimal places of precision.  But just elongating the decimal tail does not a precise geographic application make.
Most of the big public map providers display their coordinates to five decimal places.  That’s a precision of about 3.5 feet.  Nice.

How Precise Can I Get?
From the perspective of online Composite Applications, it is possible to continue to zoom-in indefinitely. That means I could start with a world map, zoom in on a troubled factory, identify the offending piece of installed hardware, and focus in on the malfunctioning part schematic (then conceivably launch a maintenance history, manufacturer contact, and warrantee information).  Precision like that would require some pretty detailed map tiles (check out the cool example from the Virtual Earth MapCruncher team) or vectors with lots of decimal places.

Fun With Decimal Degrees
At the equator, one degree of latitude (the horizontal lines that tell you how far north or south you are) is about 68.708 miles.  With every additional decimal place in the coordinate, that refines the precision x 10.  Here is a breakdown for your reference

1 degree = 68.7 miles
0.1 degree = 6.87 miles
0.01 degree = 0.687 miles
0.001 degree = 359 feet
0.0001 degree = 35.9 feet
0.00001 degree = 3.6 feet
0.000001 degree = 4 inches
0.0000001 degree = 0.4 inches
0.00000001 degree = 1 millimeter
0.000000001 degree = 0.1 millimeter
0.0000000001 degree = 0.01 millimeter

Precision vs. Accuracy

Just because something is precise, does not make it accurate.  And visa versa.  Accuracy is the degree to which something matches an actual, true, value.  Precision is the degree to which something can be repeated or reproduced.  In the world of geographic coordinates, great care should be taken to not interpret precision as accuracy, but strive for a level of precision that is commensurate with a coordinate’s known accuracy.  For more reading on this, check out Wikipedia’s great entry:



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


2 responses

  1. Morten

    Who said Lois lane wasn\’t at this exact location?
    If I was panicking and I was at that exact location, I wouldn\’t bother with adding all the decimals 🙂

    February 26, 2008 at 7:18 pm

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