January 25, 2008

Georeferencing and the Road to Oklahoma

Basketball season is in full swing with multiple games most weekends. This year we’re fielding two teams a JV and a Varsity team. Like last year our varsity team still includes all underclassmen one 11th grader, three 9th graders and an 8th grader. In the first round of league games the girls remain undefeated. This year the girls are attending both the California state tournament and the National tournament in Oklahoma as an 18 and under team (last year they won the California tournament as a 14 and under team).

Several different divisions will be hosted at the National homeschool event. Our girls will be competing in the 18 and under all-comers group against various teams from around the country. There is a higher division we will not play, seeded from regional tournaments which field teams more at the AAU club level of play. Regardless we will be stretching out from our local athletics organization to a bigger collective enterprise. We will likely participate in contests against teams that out match us in age, skill, speed, strength and experience. This is nothing new for the girls; it has been true for these girls all the years I have coached so far, albeit now they will set their sights on a higher level of play. There they will be integrated into the bigger picture: some of the athletes competing in Oklahoma will go on to play basketball at major universities.

Often a CAD file is drawn in a local coordinate system. When the drawing depicts spatial information it can be drawn accurately without having to be tied into a global coordinate system. However there may come a time when that drawing needs to participate at a higher level and be brought together in a mapping context with data from many different sources. Tools in ArcMap can quickly reposition a CAD file and integrate it into the bigger picture without modifying the CAD file.

Before I start I’ll need to know where two points on the CAD file should match up to two points in the map. I use the Georeferencing toolbar in ArcMap. There are several different tools on the toolbar to support different workflows. Here is how I like to use it. First I load the CAD drawing and make sure one of the CAD feature layers are listed in the drop down of the georeferencing toolbar. (Doesn’t matter which CAD layer POINT, POLYLINE, ANNOTATION, POLYGON, etc...) Next I zoom to roughly the place on the map were the CAD drawing is to end up. I don’t bother zooming out to the position of the CAD file, instead I use the fit to display tool on the menu to quickly get the CAD file in the map frame so that I can pick control points. For accurate placement I will turn on my ArcMap snapping so I can pick control points exactly based on existing geometry. If the CAD file needs to be rotated I might use the rotate tool to get the drawing closer to its final position… then only if it makes it easier to pick the points. I sometimes use the interactive scaling tool for the same reason. Most times I just fit to display and pick the four control points and I’m done. I select the update georeferencing option and the tool creates a .WLD file for me that will be read from now on to put my CAD file in the right place.

Now the coordinates are always adjusted into this position. At this time I can choose a coordinate system for my CAD drawing so that if I ever need project its coordinates along with the map or during some geoprocessing operation it will work.


Blogger Walker said...

Your blog has been a godsend to me for my current project which entails bringing cad dwg files from various sources into an esri project, so maps and figures can be created for my client's reports.

I had georeferenced raster files before, but this was my first time for these dwg files. At first I was flummoxed when I went to create my third control point, but reading back in your post (and after finding another help thread at the esri site) I realized the "four" points to which you refer at the end of this one are actually the two pairs of control points. So ArcView wants two and only two points for georeferencing dwg files.

My problem is that I find myself unable to do any editing of the dwg files, and since I want to package a neat file geodatabase anyway for this project, I used the "Feature Class to Geodatabase (multiple)" conversion tool to move each dwg file into its own feature dataset. There I can also define the projection to use to match the local county's shape files - though at the scale I'm working at, I doubt the projection is a big deal.

The tool worked, and I find that I can now edit the feature classes thus converted, but I've lost the earlier georeferencing, which is no longer available for a feature dataset. The odd thing is that the objects are in the position where they were after "Fit to Display" but before "Update Georeferencing".

I suspect my sequencing of steps is not correct.

Also, though I have a reasonably good theoretical understanding of projections, I really don't quite get how projection information is carried around, attached to files or frames, or why sometimes newly added data flies off into never-never land, and other times gets pretty close to where I want it in spite of coming in "unprojected." I think some dwg files appear to contain some sort of lat/lon info, and others don't.

5:59 PM  
Blogger Don Kuehne said...

One Georeferences a CAD file when the coordinates in the CAD file can not be identified as a nominal coordinate system, and need to be offset, scaled and rotated into a known nominal coordinate system (like State Plane etc...). The result of georeferencing is a .WLD file that defines the offset. A .PRJ file would also be required to define the coordinate system the world file was offsetting to.

When any GIS data set is copied, including a CAD data set, the coordinate system is used to project the data to and from data frames or target geodatabase feature classes. In order for the process to work, both the target and the source must have a defined coordinate system. If the source is undefined it can still work if the actual CAD coordinates are in the right place, but more likely something weird will happen.

Best to define the .PRJ file for the CAD file which would be the same as the data set you are using in the ArcMap data frame for georeferencing and then actually perform the georeferencing task to create the world file and from there the full spatial reference should result in a GIS dataset that understands where it is and where it is going.



4:17 PM  
Anonymous Anonymous said...

I am having trouble georeferencing the dwg I bring in since I can't select the dwg when I start the Editor. I also tried converting the drawing to a shape file and trying to follow your same instructions, but when I do that I can't use "Fit to Display". Any suggestions? Also where is the scale tool found? The buildings came in much smaller that the ortho photo I have beneath my map.

2:51 PM  
Blogger Don Kuehne said...

Just to be sure you should double check you are using the "georeferencing" toolbar and not the "spatial adjustment" toolbar.

3:00 PM  

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