Does US Survey Foot have inches?

Question

From time beyond my remembrance, there are 12 inches in 1 foot. Now later in life, I get introduced to the US Survey Foot and Mile. I see all these calculations for converting between Meters and these other units. And once in a while, I'll see somewhere, someone has taken the foot and divided it by 12 to get inches. My head goes, wait, is that a survey foot or an international foot.

So the question to any surveyor out there is. If the unit is in survey foot or mile, should any of the other conversions(inch, acre, yard) be allowed or should you convert to a known standard first?

For example, I know that an acre is 66 chains by 1 chain or 43560 square feet. But just because I have 43560 square survey feet doesn't mean I have an acre. Yes, I realize the difference is so small. But one thing that happens a lot with software is conversions back and forth, I would call it roundtripping. Errors have a way of creeping in.

Answer 1

For reference, the difference between the two kinds of feet is 0.6μm, which is smaller than most bacterium.

In measuring a distance of around 3 miles, the difference between using the two kinds of units is about 0.01 foot (in surveying terms, one "hundredth"). Of course, error tolerance is always dependent on the application, but you could say as a rule of thumb that this is when the difference becomes significant. You probably would not measure a distance of more than 3 miles in inches.

The main reason to differentiate between U.S. survey feet and international feet is in coordinate systems such as NAD83, where coordinates are measured from an origin point which may be many hundreds of thousands of feet away. In this case the error from using the wrong unit could be multiple feet. However, these coordinate systems typically use feet - certainly not inches or yards.

In short, the U.S. survey foot and U.S. survey mile are only used for very specific applications in surveying and engineering for consistency with historical data. Inches are not used in surveying. So there are (I suspect) no instances of anyone using "U.S. survey inches" for a serious purpose.

U.S. survey acres and U.S. survey yards are indeed units that are used, although much more rarely than U.S. survey feet. Even for the purposes of surveying, the list of cases where the difference matters is very short.

By the way, using one kind of foot over another has nothing to do with conversion error or anything of that sort.