The Foot Controversy and the
State Plane Coordinate System
Many of our readers provided more specific
feedback regarding our March tip on the difference between the
International Foot and the Survey Foot. Click here
to see the original article. Below are two letters to the editor
that provide some additional insight on this issue.
Dear Editor,
While in one respect you are correct, surveyors using electronic
distance measuring devices can not measure a distance to a precision
of 2ppm (parts per million, or 0.002 feet in 1000 feet), that
is not the issue. The real issue is when this 2ppm difference
is applied to State Plane coordinates in the N2,000,000 and E6,000,000
range! This 2ppm literally moves a State Plane coordinate position
4 feet by 12 feet, using the coordinate values listed. [Clarification:
The coordinate of N2,000,000, E6,000,000 is arbitrary, for the
purposes of simple math. It is a location near Half Moon Bay,
California.]
With more and more projects being based on State Plane coordinates,
this issue is going to be more and more prevalent. Working on
projects in many States, CH2M Hill is well aware of the problems
presented when users mix Survey Foot and International Foot units.
In nearly all cases it is simply lack of user awareness or a project
that has not been set up correctly.
Imagine you complete a design in rural California using State
Plane coordinates based on International Foot units. This information
is then passed on to a surveyor for staking.
Surveyors are aware that California uses Survey Foot units. Many
survey software packages convert the foot units on the fly, but
because of this discrepancy, you will then find out that the design
being constructed will be 4 feet by 12 feet out of position. You
would quickly come to the realization that 2ppm matters!
Joe Feyder,
CH2M HILL
Sacramento, California
Dear Editor,
United States Geodetic Survey coordinates given in meters need
to be converted to the "old survey foot" with the proper
conversion factor (12"/39.37") on a computer calculator
with as many decimal places as possible. This increases the accuracy
of the measurement. Also, take great care in mixing the MicroStation
conversion (by referencing by coincident world, a metric drawing,
for example) because it uses the international factor. The only
reliable way to reference a metric drawing is to make sure it
also lines-ups to a similar point or alignment converted and displayed
(in both the foot drawing and the metric drawing) by InRoads,
because InRoads geometry transformation (feet-meters or meters-feet)
employs the "old survey foot" factor.
David Mild,
STV Group
Douglassville, Pennsylvania
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