us survey foot

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us survey foot

Postby steve » Thu Nov 09, 2006 10:47 am

does anyone have info on us survey foot vs. international foot. the reason i ask, we just had our data collector repaired (recon, tds survey pro, i know i should get a field genuis but it was a packaged deal). it was set back to us in the "us survey foot" setting . apparently, unknown to us, all of our imported data was adjusted on import. we did not find out the difference until we noticed different coordinates in the raw data files vs. our computer files.

what foot does mscadd use? this probally doesn't matter within the cadd files; but when sending to a data collector it could make a difference.

any thoughts???

thanks
steve
 
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SURVEY FOOT

Postby John Gallagher » Thu Nov 09, 2006 11:46 am

Hi,
A quick Google search brought up this site http://www.dot.co.pima.az.us/gis/data/about/units.htm.
From this site the following :

ESRI's ArcInfo documentation provides this additional explanation:

In 1959, the directors of the National Bureau of Standards and the United States Coast and Geodetic Survey agreed on a redefinition of the inch-centimeter relationship. This redefinition defined 1 inch as equal to 2.54 centimeters, exactly, or 1 foot as equal to 0.3048 meters, exactly. However, their agreement stipulated that the older value for 1 meter equaling 39.37 inches,. exactly, be retained for identifying the ‘U.S. survey foot’. One of the reasons for this retention was that the State Plane Coordinate Systems, which are derived from the national geodetic control network, are based on the relationship of 1 meter equaling 39.37 inches, exactly. The difference between these two values for the foot is very small, two parts per million, which is hardly measurable but not trivial when computational consistency is desired. Fundamental survey units, such as rods, chains, statute miles, acres, sections, and townships, all depend on the relationship of 1 meter equaling 39.37 inches, exactly.

Hope this is of some help.
John Gallagher
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Postby ianw2 » Thu Nov 09, 2006 2:15 pm

MSCAD (or any other CAD program) does not use a foot, US or International, or a meter, per se. The software does not care what dimension you want the drawing to be in; it simply works with numbers.

The difference between the dimensions appears only when measuring an object or laying out an object in the real world. Then, you better make sure you know which one your measuring instruments are set for and what dimension should be used for the task.

The difference between an International Foot and a US Foot (also called a US Survey Foot) is the conversion factor from meters to feet. The US Foot is defined as 39.37 inches equals 1 meter. The International Foot defines 1 inch as being 2.54 centimeters.

The difference is that the US Foot is 0.30480061… meters while the International Foot is 0.3048.

There’s not much effect until you’re using big numbers such as State Plane Coordinates. 100,000 meters is 30,480.00 Int’l Ft but 30,408.06 US Ft.
Ian Wilson
Ian Wilson Land Surveying, Inc.
Temecula CA 92591
(951) 684-1044
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Postby Glen Cameron » Fri Nov 10, 2006 1:06 pm

As has been mentioned already, a foot is a foot is a unit of measure. It really does not matter what it is until you have to convert it to something else or combine different units of measure into the same job.

Essentially you can call our units Imperial Feet or meters. We do not do any conversions of data unless told to do so by the user or the collector file (in the rare occasion it has an option to do this).

Cheers,
Glen W. Cameron, C.E.T.
City of North Bay, Ontario
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