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CAD drawing accuracy

Anonymous

CAD drawing accuracy

Anonymous
No aplicable

When comparing measurements on a CAD drawing to the actual measurements of what is being drawn, how accurate should a CAD drawing be?  Is there a standard for the level of accuracy?

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ed57gmc
Mentor
Mentor

There's a lot of variables in that question. What industry are  you talking about? Is it for construction or as-built?

Ed


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JDMather
Consultant
Consultant

@Anonymous wrote:

When comparing measurements on a CAD drawing 


Dimensions are the controlling information.


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dgorsman
Consultant
Consultant

Accuracy on a drawing should reflect what is being designed, how it's being manufactured, and what it's being used for.  You wouldn't dimension a ditch to the nearest micrometer, nor would you dimension a support bracket for an optical scientific instrument to the nearest centimeter.

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Anonymous
No aplicable

I apologize for not providing sufficient detail to explain my question.

This would be for an as-built plot plan of a facility. The facility is about 15 acres and has several buildings and structures.

When I take a measurement on the CAD drawing and compare it to the actual physical measurement, I find differences of a few percent to as much as 50%.  The title block indicates that the drawing is to scale.  I've  sent the drawing back to the draftsman for corrections. 

I realize that a 50% variation is unacceptable, but how accurate should we expect measurements on a drawing to be?

 

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Anonymous
No aplicable

There are no dimensions on the drawing, only a scale.

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ed57gmc
Mentor
Mentor
Solución aceptada

If  you're the customer, "you get what you pay for". You have the right to determine the accuracy, but be prepared to pay for it.

Ed


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leeminardi
Mentor
Mentor

CAD drawings are not always constructed to scale and when created with great precision the value is the nominal size and does not reflect the allowed tolerance.  There are rules for tolerancing in drawings of manufactures parts.  The tolerance may be specified explicitly in a dimension line or implicitly via the precision of the dimension line value.  

 

Buildings are a different story.  I would not expect to ever measure distances on a drawing and use them to build something with any precision.  

 

You might find this discussion interesting: http://archinect.com/forum/thread/86328/drawing-precision

 

lee.minardi
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Kent1Cooper
Consultant
Consultant

@Anonymous wrote:

.... for an as-built plot plan of a facility. .... how accurate should we expect measurements on a drawing to be?


I have no simple answer, but some thoughts....

 

This may not be exactly what you're asking, but in my experience with such things, if I lay out a property boundary in AutoCAD using metes and bounds numbers along the edges in a drawing like that or from a deed description, an "accurate" survey will not come out at the end of going around exactly at the starting point unless it's something simple like a rectangle, but it is typically within about 1/16" [in US Imperial units -- that would be about 1.6mm] of the starting point, maybe twice that much in something as big as 15 acres.  If the drawing comes from modern surveying techniques, you should expect that kind of accuracy, at least in property boundaries, if not maybe quite that precisely in existing features locations.

 

I agree with others that it depends on what it's for.  If you need to work in parking spaces and a lane to get to them, and there's a regulation about the size of the spaces and the width of the lane, then it really matters how much space you have to work in, possibly to within as little as an inch or so if it's a close fit.  Or if you're trying to fit another building in, it matters from the point of view of Building and Zoning Code requirements, when there are setbacks and fire-separation distances to account for, and I would want things to be similarly drawn within inches of truly precise.

 

There is a certain degree of "reasonableness" to the issue.  Even if a drawing of existing conditions has the corners of buildings [what a Surveyor would typically locate] very accurately positioned, it may have the walls between those corners assumed to be, and drawn as, straight lines, but they may not actually be, so the distance between walls away from the corners may not be accurately reflected everywhere.  That's not likely to be very far off, though -- they're not likely to assume a straight wall unless it's pretty close to that.  Or, for certain features, you can't expect too much precision, for example, where do you draw a tree with a sloping trunk in a plan?  At the ground plane?  Four or five feet up from the ground, where they typically measure the trunk diameter?  If the ground rolls gradually from a more level area into a bank, where is the "bottom" of the bank?  Etc., etc.

Kent Cooper, AIA
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gotphish001
Advisor
Advisor

We don't do the survey and civil drawings as we send them out to consultants. That doesn't happen until later so I need to have something to show some kind of site plan for schematic drawings. Most of the time it's very pre-lim so I just will trace over some google map views to show roads and parking lots and buildings. I've noticed that the accuracy of my site plan mock up depends greatly on the angle the satellite is taking the picture from. Certainly it's not off by 50% but I've seen it be off by 10%-20% in one direction because of the angle the picture was taken.  I'm just doing them to show clients in planning stages so if it's off 5-10 feet on a 300 foot property line it doesn't matter much. The drawings are just to show clients the existing buildings and roads are here and the new building will be here. The civil guys will make an accurate site plan to build off of. If something is really close to something else I might go out and actually measure to make sure it fits, but usually things are spread out enough. 

 

I'd ask where they are getting the dimensions from because they seem off to you.  On a remodel I've had a original site plan from like 1966 that I scanned and scaled to be the right size.I traced over that for my pre-lim plan. After the real civil plan came back it was no where even close to the old plan I had nor the new plan I made from that. It was like a guy in 1966 was just standing in the field drawing it by looking around. haha



Nick DiPietro
Cad Manager/Monkey

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BeKirra
Advisor
Advisor

@Anonymous wrote:

There are no dimensions on the drawing, only a scale.


I noticed that you have marked your thread as solved.

However, I add my comment here hoping it may also help others with similar situation.

 

There are two kinds of drawing accuracy.

1) Accuracy when comparing with the dimension and the measurement on the geometry on drawing.

    1st of all, this is depended on the precision set on DWG file.

    We all expect drafter (architect/engineer/surveyor, etc.) draws everything accurately. But this is not happen all time in real life.

    For example, the length of a line or the dimension on this line is 10.0555 as designed. When the precision is set to 0.0, the measurement returns 10.1.

    We also say this is a reading error.

    On the other hand, If the length of a line is designed as 10 and "actually" it is drawn as 10.0555. (obviously the precision is set to 0.0000)

    (BTW I have seen this a lot in my work place. And I am wondering why the drafters drawn something with decimal points when it supposed to be an integral length.)

    The designer will now have to determine if the drawing is acceptable or not - it depends on the tolerance is set.

2) Accuracy when comparing with the dimension on drawing and the measurement on site.

    This seems straightforward. Once the tolerance is set, the designer should make his judgement easily.

    The scale on the drawing and your scale ruler would be the only reference when there are no dimensions shown.

 

HTH

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