Transform surface from one coordinate system to another

[Anonymous]

I have a TIN surface where the points on that surface are defined in the British National Grid coordinate system (although the exact system is arbitrary), and I need to transform these into a local site grid for a construction site.

I have the transformation parameters, and the equations to get a points from the British National grid, to the site grid. But how do I apply this transform to the whole surface in one go?

[Anonymous]

Can you ALIGN the surface definition components (possibly extracting them first using surface utilities) from the old coordinates to the new, then use those components to re-define the surface in its new location? If the local site is a known coordinate system you could use a Map3D query to do it also, but ultimately you'd want local control points to align it onto.

[jefflambert9091]

This is a little out of the box, but you could export your original surface as an xml and then insert it back in your drawing. Then select your  inserted surface, move and rotate it just like any object. 

Once you do, you can see it in the surface definition under edits. 

Gets away from all the transformation confusion. 

I edited to add the attachment

[kcobabe]

Do you know how to build a transformation in the second tab of the settings window?  This often is the easiest way to do it.  Make sure you set it up by a known point though.

[Pointdump]

Andrew,

Since C3D will only transform dumb AutoCAD Linework, your best bet might be to extract the Surface Points to use to rebuild your surface in a drawing with the desired Coordinate System. To extract Surface Points, you can use either Surface Points to CSV Report or @BrianHailey 's method >>>Here<<<.

Dave

[BrianHailey]

LandXML SHOULD be the way to do this. When you export to a LandXML file, the coordinate system information for the drawing is included the file.

Unfortunately, when importing back into a drawing, it ignores the coordinate system information.

The approach I would take is similar to what @Pointdump recommends but, in my opinion, you need more than just the points. Extract the points and triangles into a drawing, assign the coordinate system to that drawing that the original surface used, create a new drawing and assign your new coordinate system to it, use the Map 3D Query tools to query the original objects into the new drawing (this does the coordinate transformation), and finally create a new surface from that data.

I'm hoping someone will come along and say, "That's ridiculous Brian! Just do this instead!" and I will be ever grateful if that happens!

[Anonymous]

He's transforming to a local site system, so a Map query will only work if that system is defined in the coordinate system library. But, getting the surface in via LandXML and then graphically moving it onto the local site using common points with known XYZ coords, then updating the definition, would work, if it's a 5000/5000 or some assumed coordinate system.

I never quite trust a Map query for transformations. I mean, it works? But here in WI we have old state plane coordinate systems with grid to ground factors involved, and then we have modern county-wide systems in use today that are projected so close to the actual ground surface that there is no grid to ground scaling. If I query in an old NAD27 drawing to a county coordinate drawing it's usually off by a few tenths to a few feet depending on the size of the project. It really depends on what is being transformed - an existing ground surface I wouldn't worry too much about as long as I checked. but any proposed (for staking or GPS modeling), property boundary, PLLS or control work I'll use other conversion apps (the state cartographer provides one) and manually force the old onto the new. It's imprecise and begs to be remeasured if you really want to be sure.

[autoMick]

As @Anonymous says, this won't work unless you have a defined coordinate system for both ends, but I'll share what I do...

In the original drawing (with its original coordinate system), display triangles in the surface, then extract objects so that I have a heap of 3D faces. Save and close.

In second drawing (with your preferred coordinate system), do the map query method to get those 3d faces, and add them to a new surface (maintaining edges).

A better option would be if Autodesk implemented my idea: https://forums.autodesk.com/t5/civil-3d-ideas/account-for-coordinate-system-differences-when-data-re...

Cheers

Mick