Feet Vs Survey Feet

[kechha2060]

I've few questions - Probably somebody can shed some insight
What unit will the field survey data be? Feet or Survey Feet? I work in Texas and coordinates are in millions. The field data in my office is processed in autocad and then converted to microstation for design. AutoCAD's feet is used to process all survey pts while Microstation Survey feet is used for design. The DOT standard is to use Survey feet.

 

[TerryScan]

Survey data in the U.S. is in U.S. Survey Feet.

Plain Autocad is rather ignorant of units. There is no distinction between a standard foot and a U.S. Survey foot. So, when one works with survey data, that unit...in Autocad.. is a U.S. Survey foot.

With Civil3D or Map3D there is a setting which can overlay further "meaning" to the drawing unit (State Plane US Foot, meters etc.) that is only relevant to those programs.

 

[chicopee]

Not true about Autocad being ignorant of units. The default unit of Autocad is inches in decimal form.

 

[TerryScan]

chicopee,

From Autocad Help: "Drawing objects are measured in drawing units. Before you start to draw, you must decide what one drawing unit will represent based on what you plan to draw. Then you create your drawing at actual size with that convention."

Your default may be inches. My default is feet. The "ignorance" I speak of comes to play if you get my drawing in feet and change it to inches (or any other base unit). None of the elements will now measure correctly ((It does not automatically make my 1 foot line into a 12 inch line, it will now measure as a 1 inch line...without one manually scaling the drawing. To Autocad, the unit is just some arbitrary label to the DRAWING UNIT.

This is because Autocad IS ignorant of units.

 

[eea]

I agree with TerryScan's statement, but certainly apply caution if you are converting between the two platforms. I haven't had to maintain a dual CAD platform project since AutoCAD 2000i and Microstation SE. AutoCAD was/is "unitless" but all of the addon software would find me double checking. Even a simple conversion of meters to feet in SPC can produce errors if you don't hold enough significant figures in the conversion. M x 3.2808333333333 ... in my region you'll introduce a 4'+ bust if your calculator uses a truncated 3.2808 conversion factor.

Also check your survey source data, I've had the surveyors acccidentially start a new project in the total station data collector with Feet instead of Survey Feet. We brought the data into the survey point manager assuming Survey Feet and started checking control against independently collected GPS control. What the heck? It became pretty clear, pretty fast what happened but for a few moments it looked like a wasted day of ground work.

 

[Krausen]

"Survey data in the U.S. is in U.S. Survey Feet."

That is not necessarily true. Many states have drafted legislation requiring the International Foot as the standard unit of length measurement for surveying data in the State Plane Coordinate System based on NAD83. The state I live in is one of these. Other states have simply left it open to the surveyor to decide. See attached (source: "Point of Beginning" professional surveyor national publication)

 

[dik]

Terryscan... Autocad is essentially unitless... you draw something one unit and it is drawn as one unit... it's indifferent as to it being one mile or one angstrom... once you have instructed autocad what units to use, it is very sentient... I think that's what chicopee was getting at.

I'm not aware of survey feet or regular feet... can someone explain what the difference is? I used to think that 1"=25.4mm... but, not so 1"=25.400000025908mm makes a big difference when locating a structure on a UTM site.

Dik

 

[IRstuff]

According to NIST:

http://www.bipm.org/en/si/si_brochure/chapter4/conversion_factors#LENGTH

note that the conversion from ft to meters is exact 0.3048 m/ft, so if you have to carry the conversion, this is more precise.

TTFN
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[Krausen]

dik -
The International Foot is defined as 0.3048 m = 1 ft (international) exactly.

The US Survey Foot is defined as 1200/3937 m = 1 ft (US Survey).

The difference is minimal, but can become significant when using the State Plane Coordinate System with your coordinate origin hundreds of miles away.

This was the case on a recent project I worked on, where the contractor used the wrong foot units to set foundation corners. They ended up being about 2 feet off on the Easting and 1 foot off on the Northing. An embarrassing mistake for everyone involved.

 

[dik]

Thanks Krausen... I tried to get additional information on the exact conversion due to a similar problem with UTM coordinates with the building originally being located several feet from where it should. The difference is small and in trying to confirm the 'exact' number I stated, I've found two different values... On one project we had two surface coord systems, a UTM and a below surface coord system and all were different...

Dik

 

[IRstuff]

1200/3937 = 0.3048006096... while the NIST conversion factor is 0.3048006

All sort of close enough for government work.

This is a bit of interesting reading:

http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf

In which, the inch apparently has been willy-nilly manipulated to be exactly 25.4mm. I seem to recall that the inch was 25.4001 or somesuch when I was in school. I'm not old enough to have gone to school before 1959, but apparently, even much, much later, textbooks still had the old conversion factors.

TTFN
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[dik]

Thanks for the information IR... Is this binding International? or just the US and does the authority that 'looks after' the UTM system follow it? (No challenge to your comment, intended.)

Dik

 

[IRstuff]

It's supposedly the law in the US, but then again, so is the metric system... ;-)

TTFN
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