Wait just a few weeks (or maybe a month) and Tableau will release 10.0, and you will be able to do this easily within minutes instead of hours. Ten has a new feature where it can take known geographies (counties, which is what DMAs are based on) and combine them into a single shape. All without doing any of the painful (and limiting) process of using custom shapes.
Stall, stall, stall! Your solution is about to be released. Join the 10 Beta and you'll see what I'm talking about. (This is by far my favorite new 10.0 feature!)
I appreciate the reply, but I'm not sure that's an option for us. Even if Tableau 10.0 dropped, I don't believe we would be upgrading in the time frame we are looking to solve this problem.
I have already assumed I will need to do custom shape boundary manipulation outside of Tableau. I was hoping to get a lead on which process/software would be best to go about this work.
I am not seeing DMA's in 10.0.
Can you show how to group the counties into DMAs?
look at Mark's csv file. He used the lat/long coordinates to group into the DMA boundaries using custom polygons. DMA is not a default geography.
I did that, it worked well, his file isn't perfect though. I had to make a few tweaks. I'd like to know if 10.0 had included some way of reporting by DMA. That's what I read above, but seems to be missing.