Define local coordinate system - what will the "map scale" actually do?

[john_walkerDZYZ6]

I've defined a local coordinate system using the MAPCSCREATE command.

On step 3a I chose "Create a new coordinate system from an existing coordinate system" and selected the correct UTM zone etc. that the local system was based on.

On step 3c I entered the combined scale factor in the "Map (paper) scale" field.

My drawing settings are set to the new local coordinate system.

As far as I can tell all of that is correct. My question is what will the "map scale" actually do? I tried an experiment xref attaching another file that has its coordinate system set to UTM (the same zone etc as mentioned in step 3a). When I checked the endpoints of some of the AutoCAD lines the coordinates are the same, meaning it did not scale the xref. So I am trying to understand when the map scale actually does something (like Map Query?).

I need to understand this better in order to frame my second question.

Thank you

John

[Pointdump]

Hi John,
Welcome to the Autodesk forums.
"On step 3c I entered the combined scale factor in the 'Map (paper) scale' field."
No. I can't think of any reason that should be anything other than 1.
UTM coordinate systems have a locked-in scale factor of 0.9996 at the central meridian. In your custom Transverse Mercator projection you'll put the scale factor along the central meridian under "Scale Reduction".
Please post the parameters of your custom coordinate system.
Attached please find an excellent overview on custom coordinate systems by a much-missed Sinc.
Dave

[john_walkerDZYZ6]

Thanks Dave

I read that PowerPoint in detail yesterday and am reasonably familiar with geodesy. My problem is coding this in C3D.

Here are the details of the PROJECT Coordinate System:

The PROJECT Coordinate System is a ground level system based on an average project elevation of 50 meters.
To convert from the PROJECT Coordinate System to UTM:
• Multiply the northing and easting by the combined scale factor 0.9995992191 at the project origin (0,0).
• PROJECT Coordinate System Notes:
• The horizontal datum is NAD83(CSRS) 4.0.0.BC.1.MVRD.
• Elevations are based on geodetic control monument “blah blah” HPN (high precision network monument) with an elevation of 45.492m. The Vertical Datum is CVD28GVRD2018
• The combined scale factor is a product of a UTM factor and an elevation factor. The UTM factor is based on an easting of 518150m which is approximately the midpoint of the whole PROJECT.
• The elevation factor is based on an average project elevation of 50m.
• For precise survey layout and measurements, measured horizontal distances must be reduced to the ground surface at the average project elevation of 50m.

Can I still define it based on UTM Zone 10 and go from there? I am unclear exactly how C3D handles this.

Thanks

John

[Pointdump]

John,
MAPCSLIBRARY has no way to modify a UTM system. There are, however, about 7 variants of the Transverse Mercator projection.
Combined Scale Factor only applies to one point. It is not the same everywhere. And the Combined Scale Factor for a point in a UTM projection will NOT be the same in a modified system.
With what you've got, I think the Transformation Tab would be easiest. Next easiest would be a custom affine projection. For that you'd need at least 3 tie points with both UTM and Local coordinates.
Dave

[Pointdump]

John,
Another good way, from Alan Gilbert, is >>>Here<<<.
Dave

[ChicagoLooper]

Hi @john_walkerDZYZ6 

OK, so you want to convert Ground-to-Grid (and vice versa).

If you have the combined scale factor, then I would Alan Gilbert's method (see link provided by @Pointdump Post #5) before going through all that mumbo-jumbo stuff you described in Post #1. Be sure to give this new coordinate system a NEW name to differentiate the new from the old CS. Don't get me wrong, that mumbo-jumbo isn't bad, since you're using an existing coordinate system to begin with, it's just more efficient to use Alan Gilbert's method.

NOTE:

the method shown in the video works for State Plane referenced to Transverse Mercator coordinate system. If your assigned coordinate system uses a Lambert Conformal Conic then you'll need a different procedure.

[lim.wendy]

Hi John,

Welcome to the Civil 3D forum! 

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