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Home - General / All posts - Assigning projections to and using CAD files
Ian
205 post(s)
#12-Jul-18 05:23

I have been supplied some DXF files from a CAD program. They use Mount Eden 2000 datum. How do I assign a projection or what is the equivalent projection for this? I have tried in M8 and M9. I have never used a CAD program so I'm not sure how they work or their capabilities when it comes to assigning projections. Any suggestions would be gratefully received.

tjhb
8,093 post(s)
#12-Jul-18 06:04

See official LINZ specifications here.

Each of the meridional circuits (including Mount Eden 2000) defines an official local projection in terms of datum NZGD2000. They are used for survey purposes.

For use in Manifold, you need to define a custom coordinate system. I have pre-prepared XML files for Manifold 8 (just ask). For Manifold 9, the custom parameters can be entered in the usual dialogs, then the resulting JSON string can easily be saved for re-use.

Happy to help with this.

Ian
205 post(s)
#12-Jul-18 06:11

Think I must be getting old! We have done this before and I have the Mt Eden file. Can't remember how I use it though

tjhb
8,093 post(s)
#12-Jul-18 08:45

See Custom Coordinate System Presets. Talk tomorrow.

drtees27 post(s)
#12-Jul-18 18:46

I have had a lot of experience trying to get CAD files correctly registered in Manifold 8. In Washington State, a CAD file could be projected to Washington State Plane North or State Plane South. The datum can be NAD83 or NAD27. Units of measure can be in feet or meters (typically feet because we Americans are mired in the illogical Imperial units of measure). It all depends on where in the state the survey was conducted and how the base monument was located by the local jurisdiction. Sometimes (fortunately not often), a survey could be in Universal Transverse Mercator Zone 10N. I will go through the most likely projections until I come across one that is reasonably correct based on the Lat-Long on the status bar. It would be nice if the surveyor would provide projection information on their files. Mostly, they don't.

Things get really interesting when the survey is based on pseudo-coordinates. Surveyors will often get a CAD drawing out that is dimensionally correct, but not projected. The CAD files can be projected at a later date if needed. When I get a file using pseudo-coordinates, I end up having to manually georegister each layer. I will register one CAD layer to a GIS drawing layer that contains the same information (such as a parcel boundary). Then, I will use the newly registered CAD layer to register all remaining layers. If I were facile in writing scripts, the process could be automated to save me time.

steveFitz

205 post(s)
#12-Jul-18 23:31

Then, I will use the newly registered CAD layer to register all remaining layers

You can import CAD into M9 with all layers in a single drawing (a check box option) which would make it much easier to manually georegister & georeference or, in M8 you can use the add-in dwg Tools to flatten all layers to a single drawing. In both cases the layer names are retained in a table field so you can then separate the layers into individual drawings based on that field.

Steve

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