Dear Sir or Madam,
I have been experimenting with changes in a CAD file when the coordinates/datums/projections are changed to best understand Civil 3D behavior.
I noticed that if I do not set a coordinate, the lat-long also changes from what they were with the SPC set in the file. If I send the surveyor an export of the points with only N_E values and no coordinate set in the cad file, how does the surveyor know there is an issue with the point and its lat-long or northing-easting location, especially if the N-E values did not update, in the Civil 3d software?
You are asking your questions in a rather academic-technical manner and I'm not really an academic-technical guy. So I'm not sure that I fully understand your question..... but .... it is really helpful if there are hard check points included in the data set, ideally control monuments, that the surveyor can check into to confirm that they are on the proper coordinate system relative to the works to be staked out. And if, for example, some new curb that joins up to some pre-existing curb is to be staked, the surveyor should be checking into that old curb before proceeding. And so on.
The ways of the blunder are many, and varied. Success has a thousand fathers, failure is a bastard. There are too many ways to bollux things up to leave these matters unconfirmed.
Going back to the 1960's at least the BIG RULE "don't change coordinate systems in the middle of a job". That's a no-no!!!!
Why would you do such a thing?
MightyMoe,
Why would you do such a thing? There are some designers and engineers in my department who do not set coordinates or set the incorrect coordinates in the file, and I wonder, as an academic exercise for my understanding, if this lack of concern for coordinates has had any impact since the software does not seem to update it anyway. As engineers and designers, we do not report or show lat-long values, only northing-easting values. So if the northing-easting is not updating, there is no way to know if they are incorrect or not, and they will remain as imported. Unfortunately, as non-survey practitioners, we do not subscribe to the Trimble Business Center.
Thank you for your time. If there is anything more to add to this subject, I am all ears.
The first task for any project is setting up the coordinate system. If that means a crew takes a total station to the field, sets on a point of some sort and calls that point 10,000N 10,000E then that's the coordinate system. I haven't done something like that for a very long time, but it's an acceptable way to do tiny surveys such as lots or blocks. Autocad handles that type of survey easily, but it won't have a geographic connection.
If you wish to be on geographic coordinate systems, then you need either GPS or a set of existing control points that are connected.
Once you do pick what type of projection; then it shouldn't change without an extraordinary reason. The choice for a coordinate system may come from the client, or you might be free to decide yourself. Either way before crews leave the office that should be worked out. Both horizontally and vertically.
Other than that, I can't help you much, I've seen massive lawsuits because of changing coordinate systems in the middle of projects.
Autocad is not the vehicle to administrate coordinate systems, it's a user of them.
You seem way over your head with this, I would step back and do lots of research.
As Harrison Ford said; "It ain't that kind of movie".
Autocad ain't that kind of program.
Then I would pick a projection and stick to it through thick or thin. Since you don't have any programs that administrate projection acquisition, I don't know how you're going to do that. If you can't locate geographic coordinates on the ground the idea of projections, changing them, the XY coordinate number, all becomes irrelevant.
Setting the datum and projection correctly in C3d allows the user to bring in some awesome tools. The problem is, you are captive to the least educated user on the team. When the team is a few hundred or thousand people things get dicey. It isn't laziness, it's fear. That fear is usually well founded...
Changing datums, projections, or units may or may not impact your project. There may be no visual clue of mismatched information until trades or surveyors begin layout using the Cadd file. At that point large amounts of cash are required to make it right.
We set our drawings at the beginning and publish complete metadata for use down the line. If the drawing comes from us with no datum or projection set, there is a reason.