June 10, 2009

Coordinate Systems in ArcGIS for AutoCAD; Is it morning already?

Traveling from the West Coast to the East Coast has its challenges for business. On my last trip I shared a hotel room with an associate at a large conference; My colleague, tired after a long day, was looking to get to sleep at 10:00 pm, which was 7:00pm my time. My 5 year old back home, wasn’t going to bed for another hour. My colleague then announces he would be getting up at 5:00 am (booyah!) and was asking if he would have a conflict with me getting ready in the morning. Quick calculation in my head… that’s 2:00am!

The Earth is round, so even though we may have an accurate measurement that same measurement has a geographic relevance for a particular place. The watches on both of our wrists were accurately measuring the time in our zone. Trouble was I was out of my zone.

For many CAD related efforts I may be entering coordinates accurately in my zone, be it state plane or UTM or the alike. However, without some means to identify which zone I am in, or more basically what coordinate system my coordinates are depicting, when I am outside that zone things don’t line up.

In ArcGIS for AutoCAD you can identify which coordinate system you are in so that map services from other coordinates systems will automatically line up in the right locations. Likewise when the drawing is added to ArcGIS software it will also line up. By default ArcGIS for AutoCAD will assume that if you are adding a map service within an AutoCAD file without a coordinate system defined you will want your drawing to have same coordinate system as the map service. This may not be true; therefore you will want to select your coordinate system from a list of some 4000 coordinates systems that ship with ArcGIS for AutoCAD first. If your AutoCAD file has a coordinate system definition in an external .PRJ file, ArcGIS for AutoCAD can import that .PRJ file and store it inside the CAD file according to the Mapping Specification for Drawings.

For Microstation files and AutoCAD files not used in ArcGIS for AutoCAD you would copy a .PRJ file of the appropriate coordinates system in the same directory old-school.


Blogger jcarwile said...

When I add a map using the ArcGIS for AutoCAD toolbar, the map is just dark gray. I'm using AutoCAD Map 3d 2009. Any idea why?

11:13 AM  
Blogger Don Kuehne said...

If the map service is being projected and you are zoomed in far there could be some extreme scaling going on. I'd suggest trying ArcGIS Online maps in a new drawing to get something you know should work.

3:16 PM  
Blogger Sheldon said...

So I have AutoCAD .dwg map file floating in space arbitrarily. Is there a way that I can project it into a UTM coordinate system and georeference it to line up in the correct geographic space? Any hints?

1:16 PM  
Blogger Don Kuehne said...

If you are an ArcMap user you must first determine if the coordinates in the file are in UTM or not. If they are but ArcGIS doesn't recognize them you can add a projection file to the data with ArcCatalog (see the help.) Once in ArcGIS you can fit the data to existing GIS data if you know what at least two coordinates in the drawing file are supposed to be. Using the ArcGIS georeferencing toolbar in ArcMap you can interactively postion the drawing to fit the map. Using the snapping capability of ArcMap you can select two point on the drawing and their actual position on the map.

If you are an AutoCAD user. You can get the free ArcGIS for AutoCAD application and add the appropriate UTM zone coordinate system to your drawing using the ASSIGN Coordinate system tool and then use standard AutoCAD MOVE,SCALE and ROTATE tools to position the drawing correctly.

You can also read some of my old posts on the topic by going to the archive listed in the column to your right on this blog. Under the title "This Blog's Master Index".

1:39 PM  
Blogger Jerry Hood said...

Hi Don,

I recently downloaded ArcGIS for autocad to use in my work as a CAD tech. My background is GIS but I am now working a job specific to AutoCAD. Our dwg files are set to UTM8310IF(NAD 83 UTM Zone 10 International Feet) and my data will not line up correctly with the ESRI base maps. If I ignore the set coordinate system option, I do get a grey box where my data is. However, it never really finishes drawing the map. If choose ESRI's UTM NAD83 10N (which is the closest to my dwg files) the base mapsare pretty far out of alignment. Any suggestions?

10:10 AM  
Blogger Don Kuehne said...

The coordinate system you select defines what your intent of the coordinate system in model space should be. Setting the coordinate system in ArcGIS for AutoCAD is a "contract" with ArcGIS that you promise that your data will be drawn in the correct coordinate space. If you agree to drawn in AutoCAD according to that coordinate system then the entire ArcGIS System will properly position your data with other coordinate systems, and it will provide you with data from other coordinate systems in your selected coordinate system. Your data is either drawn in the correct location or it isn't according to your own definition of the coordinate system. If you need to make adjustements to your drawing to move scale and rotate your data I find the AutoCAD ALIGN command is the most useful. I have created a sample AutoLISP routine that assists in this Alignment process using ArcGIS Web Services as a way to adjust your data so ArcGIS Web Services. It basically allows you to ALIGN your drawing while it keeps ArcGIS web services in place to give you something you know is correct to snap to. These bonus tools are found here: http://www.arcgis.com/home/item.html?id=103f4d42de804f4ab42ff498026d9ddc

There is also a video that demonstrates the tool in use here: https://www.youtube.com/watch?v=V3VjP1XRoyo&list=PLHTg8zWG0OunfoXfgWWSTvU0UTCFs6ebl&index=2

Hope that helps.

7:43 AM  

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