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.
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!
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.