The standard unit of
length in the national spatial reference system, or NSRS,
is the meter. But many surveying and mapping applications in the United States
still use feet, notably when working with state plane coordinates.
It sounds simple enough, but due to a variety of historical reasons,
the foot as used in surveying and mapping, has
two different definitions and conversions. So which is the right foot?
These units are close in value but not identical.
This difference is so small that our eyes can't distinguish it. Even over more
than a mile the difference would be an insignificant 100th of a foot. But if you
are using a coordinate system with
very large values, the different definitions will change your position significantly.
This is a common issue when using state plane coordinates.
For example, here are the decimal degree coordinates for a point near the border
of Kentucky and Virginia.
Let's transform this location from latitude and longitude into state plane
coordinate zone Virginia south.
State plane coordinates are commonly used map projections used by U.S.
surveyors and can be computed in meters,
U.S. survey feet or international feet. Next let's convert for meters as
provided by NGS
to U.S. survey feet, which is the standard in the states of Kentucky and Virginia.
Finally let's see what would happen if we converted our coordinates to
international feet by mistake.
In this example, the differences between U.S. survey feet and international feet
would result in two entirely different positions — disagreeing by seven feet
in the north-south axis and twenty feet in the east-west axis...
and placing the two positions 22 feet apart.
This is significant enough to notice. And in this example could put you in a
different state!
So before making your conversion from meters to feet,
check what conversion, if any, was legislated by the state you are working in.
Many states have adopted legislation defining what type of foot is to be used
in that state.
And well over half have chosen the U.S. survey foot. Also take care when using
today's survey grade GPS receivers.
They may seamlessly provide output in feet, but you need to understand
if they are U.S. survey feet or international feet.
If you always do, you'll be sure you are getting started off on the right foot!