Do you know what the difference is between the “U.S. Survey Foot” and “International Foot”? Don’t learn the hard way. This setting matters in Civil 3D. Especially for projects up around State Plane coordinate values (most projects).
After the jump see how this setting, if not attended to, can cause a 5-foot shift in your project…
If you don’t know what the difference is and you work with the Civil 3D Survey tab you need to know! There are a couple critical settings you need to be aware of, set, and watch out for with each new project. Especially when first creating and populating the Survey database and inserting those points into the survey drawing.
Long story short, the difference is:
304,800 meters = 999,998.000 “U.S. Survey Feet”
304,800 meters = 1,000,000.000 “International Feet”
AKA: a 2 foot difference in length when you are looking at millions of feet.
Why is this important? Well, with Civil 3D you can set the Survey point coordinate system independently from the drawing coordinate system.
In doing this, Civil 3D will re-project the points – on the fly - into a drawing that has a specified coordinate system that is different than the Survey database.
Example: if the field survey is based on State Plane NAD 83 (North American Datum 1983) but you want to do some Floodplain analysis on State Plane NAD 27 you can simply set your drawing to NAD 27 and the points will re-project AUTOMATICALLY to NAD 27 when re-inserted into that drawing.
Nice option eh…? As shown above this can be particularly useful for projects that are surveyed on the State Plane coordinate base NAD 83 but do have a F.E.M.A. (Federal Emergency Management Administration) Floodplain Analysis component that is required to be on NAD 27.
This is all fun and good BUT for any given job it is CRITICAL, and I mean CRITICAL, that you set the coordinate system in the Survey database to what it was surveyed on. At a minimum you need to set the “Distance” values (shown above) to the same method of determination as your company template.
Why? Well, if they are NOT set to the same distance method (US Foot OR International Foot) you will see the 2-foot – or more – re-projection difference come up between the points in the drawing verses the points in the survey database. The shift in distance for State Plane values in Pennsylvania is ~5 feet. So, if you don’t watch your Ps & Qs you could inadvertently have the project shift off of State Plane when inserting the points for the first time and not even know it. If this goes un-checked and you are at the end of the project (months of design complete on the wrong coordinate base) and send everything out electronically you can see how this could cause a MAJOR issue for your company.
Below are some more details on the differences between the “US Survey Foot” and “International Foot” if you are interested in the history:
“The U.S. Metric Law of 1866 gave the relationship one meter = 39.37 inches. From 1893 until 1959, the yard was defined as being exactly equal to 3600/3937 meters, and thus the foot was defined as being exactly equal to 1200/3937 meters. This definition of a ‘yard’ and a ‘foot’ was set down in the 1893 document entitled ‘Fundamental Standards of Length and Mass’. On 25 June 1959, the definition of the yard was changed to bring the U.S. yard and the yard used in other countries into agreement (National Bureau of Standards made it official in a document called ‘Refinement of Values for the Yard and the Pound’). Since then, the yard has been defined as exactly equal to 0.9144 meters and thus the foot has been defined as exactly equal to 0.3048 meters. At the same time it was decided that any data expressed in feet derived from geodetic surveys within the United States would continue to bear the relationship as defined in 1893, namely, one foot = 1200/ 3937 meters. The name of this foot is ‘U.S. Survey Foot,’ while the name of the new foot defined in 1959 is ‘International Foot.’ One International Foot [equals] 0.999998 U.S. Survey Feet exactly.”
Per the U.S. National Oceanic and Atmospheric Administration’s National Geodetic Survey (defines and manages the U.S. national coordinate system), the following are the accepted calculations for each of the feet: The U.S. Survey Foot is defined as 0.30480060960125017024227597156924… meters. This calculation comes from the definition of a “foot” as established in 1893 (1200/3937 meters).
The International Foot is defined as exactly 0.3048 meters. This calculation is verified by the following definitions: one inch = 2.54 centimeters and 12 inches = one foot. Therefore one foot is made up of 30.48 centimeters. Divide that by 100 and you get 0.3048 meters.
National Geodetic Survey also states, “these two conversion factors produce results that differ by 2 parts per million; hence for most practical work it does not make any difference to the average surveyor which one is used since [surveyors rarely] encounter distances [large enough for this to be a factor]. Converting a distance of 304,800 meters to feet using the two conversion factors, these are the results: 304,800 meters = 999,998 U.S. Survey Feet and 304,800 meters = 1,000,000 International Feet. A difference of 2 feet in one million feet.” [Editor's Note: A million feet is approximately 189 miles.]
Another reference:The U.S. Survey Foot is defined as: 1 meter = 39.37 inches. If you divide 39.37 by 12 (12 inches per foot), you get the conversion factor: 1 meter = 3.280833333… U.S. Survey Feet.
The International Foot is defined as: 1 inch = 2.54 centimeters. If you convert this to meters and feet, you get the conversion factor: 1 International Foot = 0.3048 meters.
The two conversion factors produce results that only differ by 2 parts per million; hence for most practical work it doesn’t make a difference which one is used since we usually don’t encounter distances this large. For example, converting a distance of 304,800 meters to feet using the two conversion factors, these are the results:
304,800 meters = 999,998.000 U.S. Survey Feet
304,800 meters = 1,000,000.000 International Feet