I'm trying to georeference CAD to a building footprint in ArcMap 10 (although I've had this same problem in earlier versions). The problem is, it won't let me use more than two control point links, so I have to update the display before adding two more. Every time I update the display, the CAD is stretched and placed properly where the control points were, but the other ends of the CAD are off. Is this simply something I'll have to live with? Or is there a way I can use more control points, or possibly convert the CAD to a less restricted file format?
ArcMap Georeferencing – How to Use Multiple Control Point Links
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Related Solutions
For future reference, it helps to add as much relevant information to your question as possible in order to obtain the best answer. In this case, something that would be useful is to say what projection you think your CAD file should be in, and possibly give a screenshot of the coordinates you are seeing when you have loaded the CAD file into ArcGIS.
You have a few options to ensure that your CAD data will line up with other data you may add. Both of these involve defining a projection for your CAD drawing, without editing the drawing itself.
- If you know the features in the CAD drawing are already in a projected coordinate system, you may simply define the projection for the dataset. Here is the Help document describing that process: Defining a coordinate system for a CAD dataset.
- If you know the CAD features are not in a projected coordinate system, but want to define a projection for the drawing anyway, thus enabling it to be projected on-the-fly in ArcGIS, then here is the procedure to Create a custom projection file in ArcMap to align CAD data.
- If instead of creating a custom projection for your CAD data, but instead want to georeference your CAD file, which creates a
.wldfile with parameters to shift, scale and rotate your drawing, you will want to use the following help file: Georeferencing a CAD dataset
The reason that I have focused on locating the CAD file with reference to your other data as opposed to converting data from CAD to a different GIS format is the time savings involved. It is a lot faster to define the projection, or georeference your CAD dataset once, knowing that any data you convert out of that drawing will then carry along that projection information, rather than leaving the CAD file as it stands, then having to define the spatial reference separately of each dataset you export from the CAD file.
To complete the process of converting to a shapefile, you would then do the following:
Use the various tools for importing CAD data to convert features from your CAD drawing to a shapefile or any other desired GIS format. This help file contains links to the various options: Importing CAD data
At the time you perform the import, you will have the option of keeping the data in the coordinate system of your CAD file, or using the coordinate system of the Data frame, or the Feature Dataset you are importing the data into.
Knowing the position of the camera, relative to the ground, and assuming the ground to be flat, it should be possible to create target control points at each of the four corners of the image. Then pass those control points to the Warp tool. The source control points would be found by examining the unprojected extent of the image.
To use a higher order transformation, I think you'll need additional control points at interior points, thereby adjusting the scale of the image in the middle more than near the edges.
Warp is useful when the raster requires a systematic geometric correction that can be modeled with a polynomial.
Update
If you just want a quick and dirty approach, then you might be better off building a world file for each photo, by adding an offset to the mid point (assuming that's where your GPS point falls) to describe the corner of the photo, and a pixel size in the linear units of whatever projection you're using.
Best Answer
I believe you are indeed limited to two control points with CAD data. Here is a possible work around. Convert the CAD data to a shapefile (or geodatabase), then use the spatial adjustment tools to georeference the vector data.