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Author: Daniel Armstrong AutoCAD 2017, AutoCAD Map 3D, AutoCAD Civil 3D, AutoCAD

As a Civil Engineer and Infrastructure Application Specialist at Ideate, I'm celebrating along with Surveyors, Mappers, and Civil Engineers in nine states… AutoCAD 2017 based products now support US Survey Feet!

It's been something we've wanted for years because many of us assign our drawings a State Plane Coordinate System (SPCS) using AutoCAD Map 3D or AutoCAD Civil 3D. US Survey Feet is preferred by California, Texas, Colorado, Connecticut, Maryland, Indiana, North Carolina, Nebraska, and Wyoming. The issue up until now, was that although Map 3D and Civil 3D functionality understood coordinate systems, and the difference between a US Survey Foot and an International Foot, underlying AutoCAD did not. AutoCAD tools to attach External References, insert blocks, and ReCap point clouds, auto-scale based on International Feet. The difference between a US Survey Foot (~0.3048006 meters) and an International Foot (0.3048 meters exactly) is quite small. Yet it makes a significant difference when you are working in large state plane coordinate values for mapping and civil engineering projects.

Now the sobering news… because 'US Survey Feet' is a new unit to AutoCAD (INSUNITS=21), potentially undesired automatic scaling of Xrefs will occur if we are not careful. Why do I say that?

Externally referencing a ‘legacy’ engineering drawing (pre 2017), set to Intl Feet, into a 2017 'US Survey Feet' units drawing will now be automatically scaled, regardless of whether the application performing the reference is AutoCAD 2017, Map 3D 2017 or Civil 3D 2017. This will also occur even if the referenced drawing is assigned a coordinate system based on US Survey Feet. Remember that attaching an Xref is an AutoCAD based tool. The auto-scaling will not be obvious in the Attach External Reference dialog because the default Drawing Units length precision in most drawings is set to four decimal places.

 

 

Now, if you set your Drawing Units length precision to six decimal places, you'll see a scaling factor of 0.999998.

 

 

This is the ratio of Intl Feet to US Survey Feet, which comes to about 2 feet for every 1 million feet. It's negligible for low coordinate value drawings, but not for our state plane coordinate drawings.

In the below screen shot, I've externally referenced an Intl Foot drawing into a new 'US Survey Feet' drawing. Note that the coordinates of the same point differ by -12.51, -4.07.

 

 

Also note that 'US Survey Feet' is not understood by AutoCAD versions prior to 2017. You'll get the below warning upon changing the units. An AutoCAD 2017 created drawing can still be opened by AutoCAD 2013-2016.

 

 

Lastly, some of you may be aware of an old undocumented AEC command called -DWGUNITS. It's occasionally used to address block insertion scaling of 12x or 1/12 even when INSUNITS is correct in both drawings. Because it's old and undocumented, it does not include US Survey Feet.

Although it's great news that Autodesk finally added US Survey Feet to AutoCAD's supported units, we may not fully realize the benefit for a few more releases. Until then, I recommend setting the Drawing Units length precision to six decimal places for all drawings assigned US Survey Feet. Doing so will give you the visual prompt of the auto-scaling AutoCAD is about to perform.

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    Daniel Armstrong
Daniel Armstrong
ENI Application Specialist

Daniel is an Application Specialist for Infrastructure Solutions at the Ideate Sacramento office. He is a California licensed Civil Engineer and LSIT with over 9 years land development design experience and 6 years Civil–GIS software consulting. Additionally, he provided 6 years of engineering support at a municipal public works department. At Ideate, Daniel assists Civil Engineering and Surveying firms in maximizing their utilization of AutoCAD Civil 3D through consulting, training, mentoring, and technical support. He conducts standard classroom and custom training on AutoCAD, Map 3D, and Civil 3D.