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Polygon Files Structures
Posted November 2nd, 2007 by Ricky Coppola
in Maps & Links
I'm interested in the use of Polygons. I work a lot with maps and is relatively easy to export from shape (ARCVIEW) to any format. Can you provide me with details of the file structure.
Thanks
Comments
Instead of explaining it all, I've attached an example. This has the datasource and several examples in a packaged workbook. Note the importance of having all the points in the polygon and the order that those points are supposed to be connected.
Marc,
I would like to do domething similar, but instead show a map of New York state, with polygon shapes for each zipcode. Can you share what you used as your source of the lat/long numbers so I can recreate on a state level?
Dan,
The Packaged Workbook that Marc attached can be unpackaged to reveal both the data and background images used in the workbook.
To unpackage the file:
1. Right-click the .twbx file in Windows explorer.
2. Select Unpackage.
The resulting folder has a spreadsheet in it called election.xls, which contains an example of how the latitude and longitude values were structured.
However, for convenience, I've attached the MS Excel file below.
Erin,
I do follow as to how the underlying data forms the shape of each state (and how to observe the underlying data). But what I was hoping to do was to recreate this for a single state (New York), and to setup a file that has the lat/long for the counties withing the state.
Thus my question is not how it was done, but rather the SOURCE of the lat/long points that are used. I'm trying to identify where one would get the lat/long values needed to accomplish what I would like to do.
One source for Zip code boundary files is the US census, http://www.census.gov/geo/www/cob/zt_metadata.html
These are actually ZCTA (Zip Code Tabulation Area) boundaries, as Zip codes are delivery routes and do not always correspond exactly to a polygonal area. See the Census web site for more discussion of the differences.
The census boundary files may be excessively detailed, causing Tableau to spend a long time rendering them.