If you have never had the misfortune to deal with the difference between US and International feet, consider yourself lucky. When the issue presents itself, it can make things difficult to understand. No worries, the curtain will now be pulled back and the mystery solved.
First a bit of history, this will be abbreviated to keep you awake. The United States joined the Treaty of the Meter in 1875. It took the group 5 years to redefine the metric system, which made the length of the yard measurement used in the US at that time different from that used as the standard dictated in the Treaty of the Meter.
In 1960, the US changed the yard to .9144 meters exactly. This shortened the length of the new US Yard by 2 part per million. We will go over what this means in a moment, for now here is the conversion factors. US to International feet, multiply US feet by 1.000002000004000008000016000032 (approximately) to get International feet. Yes, that’s five zeros followed by a 2 and a lot more zeros. In other words not a lot. To go from International feet to US feet, multiply International feet by .999998 (exactly).
The take home message above is two parts per million. In other words, if you were working on a 2 million foot long road, (378.79 miles) your error from end to end would be 2 feet. My point is that using either system will not affect your job. If you are using International feet and the job was surveyed in US feet, you will be OK. With exceptions, read on.
I will use a simple but powerful example to explain the issue. Our imaginary job has a corner point of N 2,000,000 and E 2,000,000. The surveyor localized in International and I build the job in US feet. The corner point I mentioned is the same coordinate value for both of us. It does not matter what coordinate system I use to localize the site with my rover. I am going to occupy the points staked by the surveyor. This includes our subject 2 million by 2 million coordinate. The job will fit and perform fine. I refer you back to the fact the difference in these two systems is 2 parts per million.
With the above information known to you, how can someone ever have a problem with the different systems? In a word; conversion. Let’s use the coordinates above. The plan notes read the job was built in International feet, we decide to make it US feet. Applying the conversion we get the following;
2,000,000 x .999998 = 1,999,996.00 In this example we can easily see the 2 parts per million. Our job is now 4 feet off to the north as well as the east. All you need to do is move the job and verify the rotation of the job from our 2 million, 2 million point. We always check at least 3 points. Here is the hard part to understand. You effectively shrunk the job, why won’t this affect the actual size of the site? I refer back to the 2 parts per million factor.
Let’s say you are on a big site, a big truck merchandise transfer facility off an Interstate highway. The job is a mile across. How will the job size be affected by the conversion? Here is the math.
1 mile, (5,280 feet) x .999998 = 5,279.98944 That translates to a hundredth of a foot over a mile. This difference is impossible to see on paper, or in the field.
What do you need to do? Here are the guidelines for success in international feet to us survey feet conversion;
- Try to use the native units the job was designed in.
- Never convert units, just know what you have and what coordinates the site was localized in.
- Make a note of at least 3 points that you can find on the CAD file and the site. Refer to these and verify they are in line with each other.
- When in doubt, ask somebody to verify. A question now is cheaper than a screwed up job.