C3D .prj file for GIS

Full disclosure: I know nothing about C3D and little about ArcGIS. I do however know that a transformation is not a projection. A transformation is like an answer key that takes you from one system to another (move this much, rotate this much, scale this much). A projection takes a geographic position and relates it to a flat grid. Likely your evil (yes evil) modified state plane coordinates are being transformed by the C3D software (translate this much, rotate this much, scale this much). What your GIS guy is asking for is a relation from your evil (yes evil) coordinates to geographic (or ECEF - I don't know what the internal ArcGIS database is).

It is possible to develop an actual projection that accommodates your evil (yes evil) modified state plane coordinates. It involves taking the State Plane parameters and modifying the scale factor and origin to hit your site coordinates. It's an iterative process (trial and error). The one time I did this was with a Lambert two parallel system, which required creating a Lambert single parallel.

Yeah. Pretty basic stuff there. I'm looking for software-specific routines. I'm not confused about the geodetic surveying aspects of this. That's the part I understand.

[sarcasm]yeah. basic.
[/sarcasm]

Your transformation is like a localization. It's not a projection. They don't equate. The GIS guy is asking for a projection. What's the color blue taste like?

Is it possible to just perform a 2D scale of the CAD file to the real SPCS?

This is what I have done. Take you modified SPC data and move it back to SPC. Then give them you cad file and have them bring it into GIS (ESRI can import autocad). The bigger issue that I think you will have is with your C3D objects.

Obviously, the point is to perform a 2D scale of the CAD file to the real SPCS, which is a coordinate system with known parameters, known projection type, and known datum. But the overall object is to be able to automate the process. We have millions of dollars worth of legacy work which may slowly make its way into a GIS. We want to be able to automate this process as much as possible. Like I said, a software specific solution for dozens of projects each having its own bastardized system.

For what it's worth, I would love it if these projects had originally been done on low distortion projections and could be directly related to NAD83(epoch), but that part is out of my hands.

So what we want to do is describe the scale factor and scaling point of origin of our projects and also specify the SPCS from which they were bastardized in an automated way; in a way that ArcGIS will accept without us having to go to each CAD file and scale it back to true state plane.

I'm fixing someone else's mess here, not creating a new one...

And to clarify: in C3D, the "Transformation" tab allows input of the scale factor and scaling origin point, while the "Units and Zone" tab is where you specify the coordinate system and zone from which your points were scaled.

> And to clarify: in C3D, the "Transformation" tab allows input of the scale factor and scaling origin point, while the "Units and Zone" tab is where you specify the coordinate system and zone from which your points were scaled.

Not the same as the origin and scale factor of a projection. Two different animals.

I think the best way to do what you are wanting to do is to create a custom projection that mimics the modified SPC. If your SPC is Transverse Mercator, it shouldn't be too hard. Like I said, Lambert gets trickier.

I'd like to play with it if you don't mind. My email address is

shawnbillings at cablelynx dot com

send me the some points in true SPC, modified SPC and the SPC zone you're working in. I'll see if I can walk you through one.

I appreciate the offer, but we don't want to go down the road of having to define dozens of custom projections. I'm well aware of projection and coordinate system parameters. SPCS zones are all canned and predefined in C3D, so I don't have to mess with those. (The scale factor and origin point I referred to just mean which 2D point did you scale around--not a reference to "Scale Factor at Origin", "Latitude of Origin", False N/E, etc., for SPCS definition).

The object is truly to
First: Scale the Modified Coordinates back to State Plane
Second: Define which zone that is.

It seems like both of these things could probably be described in a single file format that ArcGIS could read. So we could just send this file to accompany a .dwg file and the GIS professional could rock and roll. Maybe that's too much to hope for, but we really don't want to be going back and scaling a bunch of projects; we would prefer instead to describe that process in the correct file type so that ArcGIS could do the work...

If you want to keep the coordinates in the modified State Plane system, then you would need to define a custom coordinate system for ArcGIS to use. Normally, given a combined scale factor (CSF) or something similar, you take the original SPCS definition, multiple or divide (as appropriate) the CSF with the false easting and false northing values, plus the SF if transverse Mercator OR add the CSF as a new parameter to a Lambert conformal conic definition.

For an easy example, let's say I have a UTM zone and a CSF that converts to ground of 0.9999385. The modified parameters would be:

false easting: 499969.25 m
false northing: 0 m
scale factor: 0.9995385246

Melita

Thank you for the tips! That informs my approach.