coordinates system

[salman]

Hi all

 

How to check if an autocad drawing (from unknown source) is in WGS84 coordinates system or Clark.

 

Thanks.

[jwkilgore]

First, how much do you trust a drawing from an unknown source?

 

One option, if you have enough information, is to ignore the initial coordinate system. Just use relative distances between objects on the drawing that can be found in the field. Specifically, establish two baselines (N-S and E-W) at set distances from fixed objects (field survey marker, fire hydrant, center of manhole cover, etc.) Set coordinates on the drawing based on the intersection of the baselines as (1000,1000) or similar.

 

If you absolutely need "real" coordinates (state plane, etc.), then next try scaling the drawing to points from another source. Specifically, if you can establish the actual coordinates of two (preferably more) points where you have a corresponding point in the unknown drawing, then plot those points. Rotate and scale the unknown drawing so the points line up. The unknown drawing is now in whatever coordinate system you want it to be in.

 

Another option is to try importing the drawing into another drawing with a known coordinate system. Then check to see if it's in the right place. If you don't have another drawing with data close enough, then try converting it to something else that can be compared. Specifically, you need to overlay your drawing on top of something that is known. I think Map (comes as part of Civil 3D) can convert into other formats.

 

I you don't have access to Map, then you can download a trial version of ExpertGPS (this is the second time I've plugged this software in two days, but I SWEAR I'm not affiliated with the company... it's just coincidence, plus I like the software).

 

Save your file as a DXF. If there are lots of text entities, blocks, or items with lots of detail you might want to strip those out first. Lots of detail just slows down the process and you can't see it on a map anyway. In ExpertGPS, add the coordinate system you think the file is saved as (go to menu Edit -> Preferences -> "My Coordinate Formats" tab -> click "Add Format"). Then go to menu File -> Import Data. Set the View menu to either USGS Topo map or Aerial map and see if the lines are in the correct place. You can also go to menu File -> Export Data on map and save as Google Earth KML for viewing there, but you should realize that Google Earth imagery can easily be off by over a hundred feet in some locations (in other locations it's dead-on).

 

Good luck!