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Civil3D & Us Survey Feet vs Feet

ryancj31
(@ryancj31)
10+ posts Member

At the company I am with currently we have been using the "Feet" units in our Civil3D drawings despite our data collectors using "US Survey Feet".

C3D has made available the use of "US Survey Feet" as a unit since 2017 but we just never bothered to look into changing our drawing use that unit.

Despite overlooking this issue, I consider myself to be pretty savvy with both C3D & GIS Data (and maybe slightly less so with coordinate systems and projections). Whenever discovered I will always chase down discrepancies of hundredths. Yet I have never noticed any problems with our current configuration (C3D = "Feet", Data Collector = "US Survey Feet").

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My current dilemma is to decide if I should change the units in all future C3D drawings to "US Survey Feet" knowing that it may (or may not?) have some repercussions.

A. Does it pay to change all new projects to "US Survey Feet" if the new coordinate systems released in 2022 (maybe delayed a year or two now?) will not be using "US Survey Feet". My thinking here is that we have not used "US Survey Feet" in our drawings for 4 years now with no know problems so why change now only to change back to "Feet" again.

B. What issues may I encounter with copy and paste from old drawing with units set to "Feet"? I often use copy and Paste to Original Coordinates to move objects from one drawing to another.

C. Why have I not noticed any issues? Seems like there should have been issues encountered somewhere unless my particular workflow has miraculously steered clear of them. Particularly with some of my county wide drawings where I have coordinates for all of our shots on section corners.

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I know there is a INSUNITS system variable in C3D that may eliminate all of my concerns - or not??? I am currently trying to look into this system variable but just need to figure out a good test scenario.

 

Thanks in advance. Just thought I would throw this question out there before I started scratching my head too hard.

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Topic starter Posted : January 22, 2021 2:03 pm
Tyler Brown
(@tyler-brown)
10+ posts Member

Check out this link: https://www.caddmicrosystems.com/blog/ussurveyfeet-vs-feet-autocad/ It may help you understand the issues encountered from this.

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Posted : January 22, 2021 3:11 pm
Norm Miller
(@norm)
500+ posts Member

In CADD a unit of measure is a unit of measure. 1:1

If the input and background files are all the same unit it makes no difference if its foot, meter or miles. The problems start when the input files are not the same unit. The more we georeference from various sources the more important it becomes to set the CADD file correctly and know the input file metedata upon import. 

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Posted : January 22, 2021 3:29 pm

John Putnam
(@john-putnam)
1,000+ posts Member

Unless you are assigning coordinate systems in C3D and bringing in data from other sources then it really does not matter much.  You are talking 0.02 feet in a mile.  Not much if you are doing a simple lot topo but about 14 feet in state plane coordinates around Portland.

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Posted : January 22, 2021 4:49 pm
Bill93
(@bill93)
5,000+ posts Member

It's 2 parts per million different (0.011 ft per mile).  If you don't use a thermometer and barometer on each setup, then you already have much more error than that in the EDM.

When you go to state plane coordinates then it is important to use the right foot because of dealing with millions.

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Posted : January 22, 2021 5:04 pm
leegreen
(@leegreen)
1,000+ posts Member

This is much more serous than depicted above.

Your coordinates can change by several feet going from Usft to International feet.

I see this error often when receiving CAD files from other surveyors working on State Plane Coordinates. 

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Posted : January 22, 2021 8:13 pm

leegreen
(@leegreen)
1,000+ posts Member

With coordinates in the millions, here is an example of the difference between USft and IntFt.

image

For this reason, I always have a text label of the control point coordinates in the CAD file for a quick check.

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Posted : January 23, 2021 5:52 am
dms330 liked
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