Posts Tagged ‘autocad’

Defining the projection of AutoCAD DXF files

Problem: Your AutoCAD data isn’t lining up correctly with other layers in ArcMap

Most of the time, ArcMap “just works” invisibly behind the scenes to line up everything correctly. It doesn’t matter if the datasets are in different coordinate systems, and sometimes it doesn’t even matter if the coordinate systems are defined. This can make the times when something goes wrong seem like mysterious aberrations, hard to troubleshoot. But there really are only two reasons why it works and why it doesn’t.

Datasets will line up correctly if

  1. They are in the same coordinate system, even if one or more datasets is <undefined>
  2. They are in different coordinate systems, but all coordinate systems are correctly defined. ArcMap will do on-the-fly projection.

Datasets will not line up correctly if

  1. They are in different coordinate systems and one or more datasets are <undefined>
  2. One or more coordinate systems are incorrectly defined

If you are suffering from problem #1 or #2, the fix is running ArcToolbox’s “Define Projection” on any undefined or incorrectly defined datasets. That is, if you are working with typical ArcMap data like shapefiles and feature classes. The fix is different for AutoCAD data.

The City of Bloomington, Indiana, offers their GIS data in both shape and dxf format, so we can experiment. In this example, I have projected a road shapefile into GCS WGS84, but I have left the corresponding dxf road layer alone. The dxf is now in a different coordinate system than the shapefile, and that coordinate system is not defined. Hence, the two layers which should be exact duplicates are showing up worlds apart.

They will be brought back together if ArcMap knows what the coordinate system of the dxf is. But, as I said above, this shouldn’t be done like normal with ArcToolbox. Instead, it is better done in ArcCatalog. If you right click on the roads_city.dxf layer in ArcCatalog to bring up its Properties, you will see that the coordinate system is Unknown. Pressing the Edit… button will allow you to select a coordinate system. The metadata on the City of Bloomington website says this data is in NAD 1983 StatePlane Indiana West FIPS 1302.

Note: Be sure to edit the projection on roads_city.dxf itself and not the individual point, polyline, polygon components. The new projection will carry over into the entire collection.

Selecting this coordinate system will prompt you to save a .prj file. Be sure it is in the same directory with the same name as the dxf.

This is what is inside the .prj file if you look at it with Notepad:

PROJCS["NAD_1983_StatePlane_Indiana_West_FIPS_1302",GEOGCS["GCS_North_American_1983",
DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],
PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],
PARAMETER["False_Easting",900000.0],PARAMETER["False_Northing",250000.0],
PARAMETER["Central_Meridian",-87.08333333333333],PARAMETER["Scale_Factor",0.9999666666666667],
PARAMETER["Latitude_Of_Origin",37.5],UNIT["Meter",1.0]]

Note: Shapefiles have these too! If you delete them, the projection goes to Unknown. For some reason, ArcToolbox’s “Define Projection” tool creates these .prj files for shapefiles, but it doesn’t do so reliably for AutoCAD data. In some instances it only creates an .xml file for the AutoCAD data, which isn’t enough to tell ArcMap the projection. This problem appears to be fixed with 9.3, but 9.2 users will have more luck the ArcCatalog way.

All users will have better luck getting projection assignments to “take” if they make sure that:

  1. The dataset is not concurrently open in ArcMap
  2. The folder that holds the dataset is not set to “read-only”
  3. There are no spaces in the file name or path

Now, if you are successful, when you add the road dxf to an ArcMap session with your road shapefile, you will get a message asking you what geographic transformation to use. This means ArcMap has recognized the projection.

Pick a transformation and you’re good to go.

Now, what do you do if you don’t have metadata like the good City of Bloomington provides, so you don’t know the coordinate system of your CAD data? You have two options:

The easy way: Georeference it just like an image! This ability was added in ArcGIS 9.2. You can see below that I’m able to select the dxf in the georeferencing toolbar.

There’s some minor differences, so read here if you want the details or if you’ve never learned to georeference images:

The hard way: If you’ve got an older version than 9.2, you will have to create a world file from coordinates at the corners. It’s an arduous process but it will work. Instructions here:

I have done this successfully with CAD maps that have rectangle frames around them, making it easy to find the corners. I can see it being very difficult otherwise. If the world file will not work for you, there’s one other option.

The other hard way: Spatial Adjustment. In other words: “Georefrencing for vectors.” It works for shapefiles without attainable coordinate systems as well. I will be covering it in a later entry.

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ArcCatalog Comments 1 Sep 14th, 2009