I've been trying to wrap my head around how C3D approaches this apparent issue with the difference between International Feet and US Survey Feet and the use of state plane coordinate systems.
From what I've read it appears that if you are going to use a state plane coordinate system you should set your INSUNITS to US Survey Feet, but why? This is where the wheels in my head fall off.
I know it has something to do with how Autocad scales drawings when they are being inserted into another drawing. I just don’t understand why you need to set your INSUNITS to US Survey Feet when you are using a state plane coordinate system, because you’re still drawing everything in International Feet. At least I do when I'm working on a site plan. US Survey Feet is not an available DWGUNIT for me. Does it have something to do with the point files that your survey people give you?
I think the source of my confusion is based on what I understand (or think I understand) about how INSUNITS is used.
For starters, I understand that we have DWGUNITS and INSUNITS and you can actually set them to be different. I don’t understand why you would ever want them to be different, but ok. This is another thing I can’t wrap my head around along with US Survey Feet being an option for INSUNITS, but not for DWGUNITS.
From my understanding the DWGUNITS needs to be set to a unit of measure you desire, right? If you set your DWGUNITS to Feet and draw a line at a unit of "1" then you are drawing a line 1 foot long, and if you set your DWGUNITS to Inches and you draw a line at a unit of "1" then you are drawing a line 1 inch long.
And, from what I can tell the INSUNITS need to be set appropriately if you want “automatic scaling” to occur when you are inserting (xattach) one drawing into another when the drawings are using different units. Like feet to inches and inches to feet or mm to inches, ect. So what's happening with US Survey Feet? Is there a scenerio where you need to go Feet to US Survey Feet or US Survey Feet to Feet when inserting drawings into other drawings?
So as an example if I have a drawing where the DWGUNITS is set to Inches and I'm drawing everything in inches and I have a different drawing that is set to Feet and I'm drawing everything in Feet, and I want to insert the Feet drawing into the Inches drawing and I do that, what do you think will happen in terms of scaling? Before I understood how to leverage the INSUNITS setting there would be no automatic scaling and the following would occur.
- In the case of the Feet drawing being inserted into the Inches drawing 1 foot in the Feet drawing would end up being equal to 1 inch in the Inches drawing essentially inserting the Feet drawing 12 times smaller than what you desire and you would have to manually scale it up 12 times.
- In the case of the Inches drawing being inserted into the Feet drawing 1 inch in the Inches drawing would end up being equal to 1 foot in the Feet drawing essentially inserting the Inches drawing 12 times larger than what you desire and you would have to manually scale it down 12 times.
But if I set the INSUNITS for the Feet drawing to Feet and the INSUNITS for the Inches drawing to Inches then Autocad sees these settings in each drawing and calculates a scale factor for how the inserted drawing will be scaled. Awesome! I get it. Thanks Autocad. That's is an awesome feature.
Here is an example of a drawing with INSUNITS set to Feet being inserted into a drawing with the INSUNITS set to INCHES. As you can see Autocad will apply a scale factor of 12 and scale it up 12 times.
And, here’s an inverse example. Drawing with INSUNITS set to Inches being inserted into a drawing with INSUNITS set to Feet. As you can see Autocad will apply a scale factor of .08333333 (1/12) and scale it down 12 times.
So, what’s happening when you have a drawing where you’re using a state planes coordinate system? It’s not like you can set your DWGUNITS to US Survey Feet or draw in US Survey Feet.
Is it because thesurvey people are giving you stuff in US Survey Feet because when they are out in the field recording points they are doing it over a state plane coordinate system and the state plane coordinate system is in US Survey Feet? Am I on the right track?
I’m obviously missing a key point here. Why is US Survey Feet an option for INSUNITS, but not for DWGUNITS. It must have something do to with using state plane coordinate systems.
Please help me to break away from this paradigm thought pattern I am expressing.
I appreciate it - Mike
