Ditto on the Corpcon! That's the only way I transform points!! Export all the point from UTM, convert with Corpscon and Import to a new State Plane drawing. It helps if you have description keys an point groups to get the points back to their layers.
Can I trust Map 3D to allow me to accurately line up the 2 coordinate systems, without having a few common points on both coordinate systems to confirm?
The basic problem is that most people don't understnad what coodinate conversions inolve. In situations like this, you can't assume ANYTHING.
The only way to be SURE you know things correlate is to have some field observations made, and have some common control points in each system. Otherwise, you're shooting in the dark.
Neither C3D nor MAP3D actually has provisions to help with this, so you're pretty much on your own... All I can say is hire a good Survyor who understands this stuff... Otherwise, you may be liable for the consequences. If you're lucky, it will be in the .1' or .2' range, and no-one will really care. At the worst, you can be liable for ????? I hope you have E&O insurance.
Thanks for all the responses. I just wanted to give an update on what we ended up doing.
We checked to see if our client was aware that the ALTA Survey and the topo they were providing to us are on 2 different coordinate systems. For all we knew they may have already done something about converting the 2 drawings to the same coordinate system and inadvertantly provided the wrong files. After being informed about the 2 different coordinate systems our client has instructed the engineering/surveying firm that prepared the ALTA Survey drawing to convert it to UTM Zone 12 coordinate system, the same as the topography they have. (...hopefully they won't just use Map 3D)
I feel all warm and fuzzy inside now!
I may be wrong but I believe converting to a UTM system could result in a larger Grid to Ground scale factor vs. a state plane system as the UTM zones cover larger areas. I would investigate the difference before committing to the UTM system.
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I agree with Neil. You need to see what affect that will have on the Survey. I've seen problems following strict ALTA guidelines and producing maps in true State Plane. If you're in an area with a large conversion factor all the Boundary Line distances can change. Angles can change too. Converting to UTM may make these differences even larger. This sets off alarm bells with lawyers who want to roll over the previous description and often do anyway despite revised Survey information.
We usually try to work in a ground system based on a project. Right now I'm lucky to be working for a County where Ground and Grid have very little variance over a few miles.
I'll let you know what happens.
We are also an engineering/surveying firm but we didn't provide the survey for the ALTA or the survey for the topography. We are the company that will be preparing a site grading plan and need to use both and need them both on the same coordinate system.
When we first got the data for the ALTA and the topo, and saw they are on the 2 different coordinate systems I did some checking and saw the ability of Map 3D to convert from one coordinate system to another, which is maybe good for planning purposes and some other uses but on further investigation would be risky at best to use for construction documents without field verfication, as it has been pointed out in this thread and also discussing with our survey department.
I know which engineering/surveying company prepared the ALTA but I'm not sure who provided the survey for the topography. Since the client is asking the compay that did the ALTA survey to convert to the other coordinate system, as a professional courtesy we can give them a heads up on the possible issues (which they are probably aware of) but it's up to that company to do what is the right thing to do for the client.
I'm not entirely sure what can happen when you try to transform an ALTA to another coordinate system. I think it varies, and there are no hard-and-fast rules... In theory, you should be able to do that, and still maintain the required accuracy, although your bearings are likely to change.
One of the other problems you can run into is that a typical "coordinate system" is usually created on Elevation = Sea Level, and if you are not at sea level, your distances will also be impacted.... For the most part, ALTA surveys should have GROUND distances, not grid.
But in the simplest case, I agree that holding the coordinate system of the ALTA may be the way to go. Transforming a topo to a different coordinate system usually results in errors that are insignificant for such a thing. But of course, the other factor is that C3D is VERY BAD at transforming data, and Map can't even see C3D entities, so you can run into all kinds of problems. And that's not even getting into the ramifications of the "Transformation Tab", which Map also ignores.