Afternoon,
I am not sure if this is the right forum for this discussion but I am needing more clarity on the importance and effects of survey data and setting your coordinate system in Civil 3D. I have been an engineer for awhile now and have worked with 3 different companies. The first 2 i worked at were very BIG on always setting your coordinate systems in your drawing. Especially when working with Survey data. I am now at a company that just leaves everything at "No datum, No projection" from what i have read and researched setting a coordinate system in Civil 3D is a crucial step, but the information is almost to wordy and seems to be a huge technical rabbit hole to go down and i am kind of needing a more simplistic explanation.
So, as an example lets say the surveyor goes out and gets the data in a coordinate system like NAD 83 Iowa state plane South US Foot. I then go into the survey database on Civil 3D and instead of setting my database to the coordinate system it was shot in leave it at "No Datum, No Projection" during import.
So what type if any affects does this have on the accuracy and location of the data during design and construction?
Should engineers always be setting the coordinate systems? I am thinking in terms of sharing with surveyors for construction layouts and GIS mapping. To me this was a big YES, and the research keeps saying yes, but i have not really been able to produce any misalignments or differences when using the correct coordinate system NAD 83 Iowa state plane South US Foot vs a 'No datum, no projection" .
So what type if any affects does this have on the accuracy and location of the data during design and construction?
None. No effect on the accuracy of the coordinates. But there are other good reasons. There are a number of geospatial tools and resources available when you have an appropriate coordinate system set. My favorite is background aerial photography.
How about the problem of the surveyor collecting the design topo on state plane. Then the engineer designing his project on state plane. Then when it is time to be constructed you have 0.07' of error in every thousand feet when you put these points on the ground. Granted in some places the difference between state plane and ground is negligible. But when your project is a big Google data center comprised of 12 buildings over 4000' from one end to the other it is relevant. Design topos should be collected on a coordinate system that reflects the ground. Engineer's designs should be on a coordinate system that reflects the ground.
How about the problem of the surveyor collecting the design topo on state plane. Then the engineer designing his project on state plane.
I really like the idea of using an LDP, but it's the rare case indeed where even that seven hundredths in a thousand is meaningful.
.07' in 1000' is 70ppm. I dream of that kind of accuracy. 800ppm Is more likely here than 70ppm within a state coordinate system. Normally it floats between 200-350ppm for most projects. Still .35' in 1000' can easily be achieved chaining.
There's not really a simple answer to your question other than, it depends. If you're in a large firm, reach out to the individual ultimately in charge of setting up CAD templates and pick their brain. By design, there are multiple paths within C3D to achieve the same thing, so it might be that both, having and not having a projection are correct within the context of how other background settings are managed.
I have multiple C3D templates with some projected to State Plane with a scale factor of 1 USft, others No Projection No Datum, and one that's setup to show unscaled GNSS coordinates along with an inverse combined factor for ground coordinates. I then have all of these in International Feet as well.
Projection or no projection can be important, but probably not as important as having unambiguous metadata associated with all survey data used in your projects. It's a great way of grading surveyors too. If they don't know what metadata is or why it's important, or seem put-off by the request, find another surveyor. Below is an example of the metadata you should see in a report of survey or noted on a plat or any time you receive new survey data (excuse the copy/paste formatting).
H/V Positional accuracy 95% CL: 0.05’ / 0.07'
Type of field procedure: GNSS Network-RTK
Dates of fieldwork: XX/XX/XXXX
Horizontal Datum / Epoch: NAD83(2011) / 2010.00
Vertical Datum: NAVD88
Published/Fixed-control use: State-VRN
Geoid model: Geoid18
Combined grid factor(s): 0.9999XXX
Units: US Survey Feet
If you use no projection, no datum, then available canned photography and some other easily connected data sources usable for AutoCAD can't be accessed, because the program doesn't know where the coordinates are geographically.
You're stating in the set-up.
If you want the program to know where the coordinates are located; then define them in the program. I can see a shop that doesn't care, if that isn't a concern then no projection, no datum works fine. You're simply working in cartesian coordinates which can have an origin point anywhere with any coordinate pair. AutoCAD handles that very well.
I look at XYZ as a function of LLH. We will always have a cartesian coordinate system that's connected to geographic coordinates. It makes it so much easier.
As far as state coordinates, they can be scaled, many don't like that, but AutoCAD does it easily. LDP's work, but can be difficult to connect them using AutoCAD, there are few available in the pull down menus.
The bigger question would be why not set the coordinate system? The advantages are clear. When we receive files that we know are on a specific system but the cad files are in the no projection system I know those producing the drawings or managing their systems have no clue what they are doing. There is no reason to not set the coordinate system if you're on one.
When I was a survey manager the call for help I most often got was from CADD people working on an engineering project who imported our survey data and it fell in the Atlantic Ocean on their photo backdrop. 99% of the time it was because they had not chosen a correct datum in the CADD file. If GIS or backdrop or any use needs a geographic location rather than arbitrary it needs a correct datum setting.