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Save Georeferenced DWG in AutoCAD

Anonymous

Save Georeferenced DWG in AutoCAD

Anonymous
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Hello, very new to using Autocad so apologies if this question has been answered many times before.

 

I have been supplied with a DWG file which appears to have no georeferenced data. I have manually attempted to do this myself using the insert> set location routine. This appears to have worked well and the site layout overlays well to the imported bing map.

 

What I was hoping to happen from here is saving the DWG would store the geolocation/co-ordinate data within it. But appears not. I am trying to open this DWG in GlobalMapper software and it appears to projection data is stored within it.

 

Do I need to export the files in an alternative format? Any advise would be appreciated.

 

Regards, Steve

 

@Anonymous

john.vellek has edited your subject line for clarity: Save Georeferenced DWG

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Michiel.Valcke
Advisor
Advisor

Hello,

Are you using Autocad MAP 3D or just regular Autocad?

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Anonymous
No aplicable

Hi, just regular AutoCad.

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john.vellek
Alumni
Alumni

Hi @Anonymous,

 

I see that you are visiting as a new member to the AutoCAD forum. Welcome to the Autodesk Community!

 

 When I use Geo (Geographic Location) and set this in my drawing I can view my aerial map just fine. I save my drawing and then reopen and this location is retained.

 

If you reopen your drawing in AutoCAD, does it retain the information there? Is is just the other application that seems to be having this issue?

 

 

Please select the Accept as Solution button if my post solves your issue or answers your question.


John Vellek


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ChicagoLooper
Mentor
Mentor

Regardless of what you've read, heard or are told by others, 'plain vanilla Acad' cannot save a your drawing with a georeference. This is because plain vanilla Acad doesn't have the capability to georefeference the dwg like Civil 3D or Map 3D. This, however, doesn't necessarily mean you can't 'fake-out' Global Mapper into thinking you are, indeed, using a 'known datum.'

If both of the following conditions are present in your Cad dwg:
1.Your x and y coordinate values coincide (or are identical) to a projected coordinate system that uses either feet or meters (as opposed to a geographic-coordinate-system-which-use-degrees) such as State Plane or UTM, then you can do it. If your drawing x's and y's are NOT coincidental, then it means your drawing's line work has been arbitrarily a) positioned, b) rotated and c) scaled in modelspace, and in cases such as these, you can't do it. Just because you can open the dwg or dxf in GM doesn't mean GM will magically provide a georeference.

2.You configure or assign the correct State Plane or UTM Zone to Global Mapper when opening the dwg or dxf in GM. For example when opening a dwg/dxf that is NOT georeferenced in Acad, the GM configuration looks like this:

 

Glo Map configuration=>then Projection from panel on left.Glo Map configuration=>then Projection from panel on left.

If you are trying to establish a georeference to an un-referenced dwg, then you cannot simply force Global Mapper create a georeference for you because your dwg didn't provide the geospatial parameters which GM needs to do so. 

 

2-This is the dxf in Vanilla Acad.2-This is the dxf in Vanilla Acad.3-This is dxf 'opened' in GM. (Imagery added usnig GM.)3-This is dxf 'opened' in GM. (Imagery added usnig GM.)

In 2 and 3 above, you can see the coordinates of that the blue point in vanilla Acad has transferred over to Global Mapper. If the coordinates in Acad were NOT correct, then they wouldn't be correct in GM either.  The blue point is NOT arbitrary, it has been placed at those coordinates on purpose. On purpose=coordinate geometry 'know-how.'


NOTE: Earlier, in the configuration=>projection dialog of GM, the appropriate State Plane and Zone, which happens to be Texas State Plane, Central zone, was established. (Again, knowing the correct St Pln and zone='know-how.' ) If it wasn't established in GM then the Cad line work wouldn't be correct because GM would've positioned the line work arbitrarily.

 

The key to this entire exercise is the Blue Point, or some other reference point, MUST have Coordinates that coincide, or are equal to State Plane, so that Global Mapper will treat those x and y numbers as State Plane. By treating those x/y's as State Plane you have effectively 'faked out' GM into thinking your drawing is georef'd even though your program, vanilla Acad, doesn't have the capability to GeoRef in the first place. 

 

If you cannot get the blue dot accurately placed in Vanilla Cad, or you don't know how to place it even though you have the coordinates, then, sorry, you are stuck with a non-georef'd dwg.

Chicagolooper

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Anonymous
No aplicable

Hi, Thanks for the detailed response ChicagoLooper,

 

It appears I may have problems getting the 'blue dot' placed correctly. 

 

I do believe the model has been scaled correctly, i.e. 1 meter in the DWG is 1 meter on the British Nation Grid reference (I'm in UK). I'm led to believe this because using the Insert> Location> Set Location> From Map procedure, using the above (BNG) projection, setting a reference point on both map and DWG, the whole drawing overlays accurately over the map. And I can see there is imported drawings of the surrounding site for which the model has been based.

 

What I think my expectation from there was that the DWG co-ordinates would be reassigned as the projected co-ordinates. Which is obviously not the case now.

 

Let's say I picked a reference point on the drawing, say corner of a building and the local coordinates were x-100/y-100. Then found reference point on the projected coordinates, let's say x-1234/y-1234. Is there a tool in vanilla AutoCad where I could offset the whole drawing, i.e. move whole model on both axis by 1134? 

 

Alternatively, would this be a lot easier if I just used AutoCad Map 3D?

 

Thanks.

 

 

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ChicagoLooper
Mentor
Mentor

First and foremost, you must determine what projection (coordinate system) in the UK you want to use for the final map. In other words, how would you determine the red highlighted 'reference' point or what datum is that x and y referenced to? If you know that point then, of course you can do it, but it would be better with two points, not just one. Two points will account for any rotational effects.  

 

<Let's say I picked a reference point on the drawing, say corner of a building and the local coordinates were x-100/y-100. Then found reference point on the projected coordinates, let's say x-1234/y-1234. Is there a tool in vanilla AutoCad where I could offset the whole drawing, i.e. move whole model on both axis by 1134? >

 

Second, that reference point must be in meters, not latitude and longitude. If it is in lat/long, you'll need to perform an extra step and 'look up' the corresponding x/y values in meters (many free online tools will easily do this).

 

Third, I'm a little rusty (maybe not a little but a lot) when selecting UK datum, so if you set the projection in Global Mapper to British Grid, would you then know what datum to select? There are many. (Alternatively, you can use UTM.)

What datum to choose?What datum to choose?

Your objective now is to determine the location of that building corner in the real world using a known coordinate system. If all you have is a 'local coordinate system' sometimes referred to as a site-specific coordinate system, then you are relegated to simply 'eyeballing' the location of the building and moving your all of your line work manually to the corner (think cut-and-paste).

 

Chicagolooper

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