What is the process behind changing local coordinates into global coordinates?

[KaitlynAbrahamson]

I am working on a zoning project for a small town in Montana and the layers I made into shapefiles from DGN files I have are in an unknown local coordinate system.  I want to change them to a global coordinate system (e.g. WGS 1984 or State Plane Montana), but I have no idea how to start.  Every time I define the projection, my layers are VERY disproportionate and in a totally wrong location.  How to I go about changing local coordinates?

[SimonKettle]

This blog might answer some of those questions...

https://community.esri.com/groups/coordinate-reference-systems/blog/2018/12/12/checklist-adding-coor...

Otherwise can you expand on what you mean by local coordinates? e.g. how were these collected, have you tried to use the define project tool to assign a local CRS to the coordinate values and from there the project tool to transform/convert to a global CRS?

[NeilAyres]

Unfortunately, you cannot go from an unknown to a known.

You must find out more about the real coordinate system from those CAD files, then define your shapefiles appropriately. Then you can re-project, change the base datum etc etc.

[MelitaKennedy]

Sorry, Neil Ayres‌  you can go from an unknown to a known. If you have known points or can acquire known points in both systems, you can take a look at georeferencing the DGN files to a 'known' coordinate system. 

Another possibility is trying to determine a custom coordinate system based on Local or RSO (rectified skew orthomorphic) but that's a lot of repetitive work to fine down the parameter values. Can you post any sample coordinate values where you do have info in 2 systems or a location (like an address) for a local coordinate pair?

If you send me an email, I can send you a document that discusses local/CAD data and how to find or define its coordinate system.

Melita

[NeilAyres]

Melita,

well yes, you can "georeference" the CAD, much like you would with an image.

But, I think, first choice would be to get to the truth. Especially regarding the underlying datum.

If that step proves to be impossible, then, yes, go ahead.

I have Margaret Maher's book thank you.

Also, done lots of work fiddling with RSO etc etc.

Cheers, Neil

[KaitlynAbrahamson]

The files I recieve are Microstation (DGN files) drawings either from plat maps or in local coordinates.  Attached is an example.

CityLimitsDGNEX.zip - Box 

[MelitaKennedy]

Hi Kaitlyn, 

The coordinates look like they're in a state plane system, but I've tried NAD27 Montana North, Central, and South plus NAD83 in meters and feet and none obvious match the data. Can you tell which city it is? You can email if you want, mkennedy at esri dot com.

Melita

[KaitlynAbrahamson]

This example is actually in North Dakota.  Bowman is the name of the town.

[MelitaKennedy]

Hi Kaitlyn, 

The data's using feet (international), and it's very close to a State Plane North Dakota South zone like 2266 (NAD 1983) which uses NAD 1983. It may be on a latter realization of NAD83 like HARN or 2011. There may be something else that has modified the state plane zone, because particularly on the west side, the city boundary is offset versus the airport so there could be a rotation applied. As Eric stated, it's best to go back to the data provider and ask about the coordinate system and whether any combined scale factor or grid-to-ground adjustment or rotation/north angle was applied.

Melita

[EricCollinsOVV]

You need to go back to whomever created the DGN files and get them to supply you with the information you require. You should not be guessing at this. They either used a proper projection or they used a local system. Hope that it is a proper projection. If they picked 0,0 in the SW corner, I would get them to provide transformation parameters to shift to a real world projection (assuming you know MicroStation). Better yet, get them to translate, rotate and scale the DGN files to a real-world projection and then give the updated files back to you. 

It is important to have proper control over things like this as a client and set standards at the outset or you will be continually pulling your hair out.

As a last resort, do some georeferencing. Not a good idea though assuming the people who gave the DGN files got paid for their work.