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The standard unit of

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length in the national spatial reference
system, or NSRS,

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is the meter. But many surveying and
mapping applications in the United States

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still use feet, notably when working with
state plane coordinates.

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It sounds simple enough, but due to a
variety of historical reasons,

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the foot as used in surveying and
mapping, has

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two different definitions and conversions.
So which is the right foot?

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These units are close in value but not
identical.

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This difference is so small that our
eyes can't distinguish it. Even over more

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than a mile the difference would be an
insignificant 100th of a foot. But if you

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are using a coordinate system with

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very large values, the different definitions
will change your position significantly.

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This is a common issue when using state
plane coordinates.

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For example, here are the decimal degree coordinates
for a point near the border

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of Kentucky and Virginia.

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Let's transform this location from
latitude and longitude into state plane

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coordinate zone Virginia south.

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State plane coordinates are commonly
used map projections used by U.S.

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surveyors and can be computed in meters,

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U.S. survey feet or international feet.
Next let's convert for meters as

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provided by NGS

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to U.S. survey feet, which is the standard
in the states of Kentucky and Virginia.

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Finally let's see what would happen if
we converted our coordinates to

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international feet by mistake.

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In this example, the differences between
U.S. survey feet and international feet

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would result in two entirely different
positions — disagreeing by seven feet

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in the north-south axis and twenty feet
in the east-west axis...

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and placing the two positions 22 feet
apart.

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This is significant enough to notice.
And in this example could put you in a

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different state!

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So before making your conversion from
meters to feet,

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check what conversion, if any, was
legislated by the state you are working in.

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Many states have adopted legislation
defining what type of foot is to be used

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in that state.

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And well over half have chosen the U.S.
survey foot.  Also take care when using

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today's survey grade GPS receivers.

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They may seamlessly provide output in feet,
but you need to understand

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if they are U.S. survey feet or international feet.

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If you always do, you'll be sure you are
getting started off on the right foot!