Importing dwg files

[francescarusso]

Hi,

I got houndreds of dwg files and DTM from a Geophysical survey for a submarine cable route (They are from alignment sheet). When I try to import them into ArcGIS (Desktop 10.2) it shows the message about the "Unknown Spatial Reference".

I have an .mxd with the Coordinates and projection already set up for that project.

Does anyone know why if I ignore this message  sometimes it is drawing the dwg in the right place and sometimes not?

If I try to convert them in shp in the first place and I try to give them the same  coordinate system and projection from the data frame, it doesn't really work for all of them. Some of them are projected in the right place and some not.

Please can someone advise on the best practice to import this data into my GIS?

Thank you very much in advance.

F.

[DanPatterson_Retired]

the best solution is to try to track down the information on the coordinates that were used for each file.  If they load fine, then you got luck that the coordinate system matches and you should define it accordingly.  If the file doesn't match it is in a different coordinate system and you need to find out what it is, so it can be defined accordingly.  You can match unknown coordinates to know coordinates like GCS WGS84 or a file that is in a  commonly  used projection.

[francescarusso]

Hi Dan,

thanks a lot for your reply. As I have also the PDFs of these aligment charts I had a look to the chart details. in the legend there are "Survey details": WGS84 Grid North displayed in chart and Projection :UTM Zone 30N.

My question is: Do the dwg files related to this pdfs have Spatial reference already?

If not, why when I try to import into my xmd which is set up with WGS84 UTM 30N not all of them are displayed in the right place?

Why even if I convert the dwg into shp and give the projection some of them are not displaying in the right place?

If the dwg files  come all from the same survey why the do not behave in the same way?

Thanks for help.

F.

[DanPatterson_Retired]

The best way to assess whether a file has a projected coordinate system is to 

• open a new dataframe
• do NOT set its coordinate system
• add 1 file and only 1
• look to the bottom right of the screen... what do the numbers look like? 
  • If they are in the range -180 to 180 EW and 90 to -90 NS then the data are probably geographic aka in decimal degrees
  • If they are relatively large numbers, then they are projected
• If they are geographic, they can be defined as such ... perhaps a GCS WGS84 (like from data derived from a gps in its natural coordinate system ... perhaps GCS NAD83 (or NAD27) if you are in North America
• If they are projected... guess what you think it might be
  • add a known projected  file to a new dataframe.... delete the other one... data with a known coordinate system will set the coordinate system of the dataframe without question
  • Add the unknown file to it... if it matches, then the unknown file, just has to have the Define Projection tool used since you have a match

Works for me... others have different workflows... the trick is new dataframe, one file, one estimate, delete, repeat to try again

PS

If you are a visual learner... /blogs/dan_patterson/2016/09/07/define-projection-vs-project-a-visual-guide with a somewhat unique perspective on the whole issue

[francescarusso]

Hi Dan,

Thanks. I did what you explained to.

When I imported in anew data frame the dwg as first thing I had the message that the dwg has Unknown spatial reference. I ignored it. The number on the bottom right is a large number followed by "Unknown Units"

This has raised another issue I always struggle with:

I added the Known shapefile which has Proj coordinate system WGS84 UTM 30 N. No messages appeared but on the bottom right is still large number followed by "Unknown Units". In fact if I check the data frame properties  it says under "Current coordinate system" : No Coordinate system. Why the data frame didn't pick all the information from the known shapefile? Do you haver a suggestion for this too?

A part from that it looks like I have to define the projection for my dwg files .Am I correct?

Thanks

F

[NeilAyres]

Perhaps have a look at the properties of several of the CAD files in ArcCatalog.

Select the CAD and right click to get the properties page. Look at the extents.

If, as Dan says, the values are quite small : -180 < X < 180 and -90 < Y < 90, you are looking at GCS (a grographic coordinate system). You already know that some, or maybe all, are actually in UTM30N. Have a look at some of the ones that don't plot in the correct place.

CAD very rarely has a coordinate system defined. I don't think I have ever seen one that came, ready to go.

You can just define the ones that look like UTM30N using the same properties page described above.

Do you have any more metadata from the originators of this CAD stuff?

[DanPatterson_Retired]

Youdon't add the known shapefile to the data frame with the cad file... you open a new data frame then add it... the coordinate system will be set IF the shapefile has a defined coordinate system (check for a *.prj file using file explorer).  If the shapefile doesn't have a defined coordinate system, then it won't help.

You need something to compare it to.  It must be known

[francescarusso]

Thanks I got it.

As suspected from both of you the dwg files have no their own coordinates or projection. So I am going to define them.

Even after I defined the coord system, if I right click in the TOC on the dwg which I have just added (With defined coord system) the table of attribute looks empty. It is weird to me and I do not understand how it is possible that some of them are fine and some not.

I have also tried to export the dwg into shapefile. Is it a good practice to do something like that?

Thanks a lot for your help

F.

[francescarusso]

Sorry I have to explain myself better.

Each dwg file I imported in GIS is combined of

- Annotation

- point

- Polyline

-Multipatch

-Polygon

Once I have defined the coordinates it happen that if I right click on Annotation (the table of attribute is full), but if I right click on Point the table of attribute is empty. I do not know weather all the elements above  must have something in the TOA  or they can be also empty. Sorry it is my first time working with dwg file and I do not use the GIS very often (you can tell!)

Thanks a lot

F

[NeilAyres]

Well, the definition of the coordinate system has got nothing to do with the contents of the data itself.

They are 2 entirely separate issues.

So once you have the coord sys issues sorted, then deal with the content.

If you open up the properties page of one of the offending CAD files, look at the details page. Are the layers visible?