Creating custom coordinate system (with datum parameters TOWGS)

[Kevin Malone]

I'm tying to create custom Coordinate System in C3D 2012 with datum parameters (Dx, Dy, Dz, Rx, Ry, Rz and scale), but during the creation process (thru MAPCSCREATE function) I don't get anywhere to input this values. As you can see on screenshot, TOWGS parameters are all "0".

 

 

 

Only way to form some kind of datum with parameters is creating Geodetic Transformation, but I can't link/import that in my CS definition

 

I would appreciate any help

[Pointdump]

Kevin,

 

A little more information please. In what coordinate system are you starting with and what and why are you changing it to?

 

Dave

[Kevin Malone]

Hello David,

 

Thank you for replying. I'm not sure I understand your question. Why would I have to start from something, and why would I change it to something?

 

I'm expecting to create new Coordinate System by defining:

 

1) Ellipsoid (Bessel 1841)

2) Datum (Dx, Dy, Dz, Rx, Ry, Rz, scale)

3) Projection (Transverse Mercator:central meridian: 18º , false easting: 6500000, scale reduction: 0.9999)

 

I'm able to define all, except Datum parametres.

 

My projection from QGIS (what I'm trying to accomplish in C3D):

 

+proj=tmerc +lat_0=0 +lon_0=18 +k=0.9999 +x_0=6500000 +y_0=0 +ellps=bessel +towgs84=582.423611,193.160015,418.65025,5.94041362,-0.17569994,-12.99256834,3.46772346
 +units=m

[Pointdump]

Kevin,

 

"Why would I have to start from something, and why would I change it to something?"

 

I guess what I'm asking is for specifics on what you are doing and why you need to create a unique coordinate system. Are you incorporating GIS data in your drawing? Survey points? To whom are you passing your drawing to, and will they be able to re-create your unique coordinate system?

 

C3D has a fairly rich library of Ellipsoids, Datums, and Projections. I'd find a coordinate system close to what you want and duplicate, then edit.

 

Dave

[Kevin Malone]

Dave,

 

I'm surveyor and I'm currently working on GIS, so yes - I do work with GIS data and survey points. Unfortunately none of current CS's in C3D library does not fit my needs entirely. Closest CS would be EPSG:31276 MGI\BALKANS ZONE 6. This is CS with "global" parameters, and they differ from my local parameters (0.7-2.3m on control points). Local parameters are calculated for city area.

 

I have earlier tried to duplicate and then edit duplicated CS, but again - I can only edit ellipsoid and projection.

[Pointdump]

Kevin,

 

"Local parameters are calculated for city area."

 

OK, I think I see now. You want ground distances and Geodetic North to match AutoCAD "top of page", right?

 

"I'm able to define all, except Datum parametres."

 

It's amazing how easy it is to do that in the free QGIS, and so unfathomable in expensive C3D. Unfortunately, I don't know how to do that either. But maybe a Custom Low Distortion Projection would serve you. You can keep the WGS84 Datum (or whatever CS you use in your GPS receiver) and just define a new Origin Longitude, Origin Latitude, False Easting, False Northing, and Scale Reduction. For scale you'd need the Combined Factor, Scale Factor X Elevation Factor.

 

Dave

[Kevin Malone]

Dave,

 

Thank you for clarification. Now, at least I know it's not possible (still can't understand why).

[Pointdump]

Kevin,

 

OK, I did some digging, because I sure didn't know how to create a new Datum Definition with a 7-Parameter Transformation. I figured out that it's a 2-step process. But it seems doable.

 

Here's what the Help Section says:

1. Use >> to name your new Datum and select an Ellipsoid.

2. Use >> to start the assistant to define the geodetic transformation.

 

Dave