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I would like to produce a map of "points near a feature" in Tableau. For example, I would like to plot "rapid transit stops within 2 miles of the charles river". I know how to do this using something like QGIS; I'm trying to figure out if this is even possible with Tableau.

The first step, ignoring the "near a feature" aspect, is simply to get the two sets of data to display on the same map at the same time, and I've been unable to make that happen. A major stumbling point for me is that Tableau appears to want to join all the data sources, but there is no clear join column in the absence of GIS functions like st_distance or something similar.

The examples I've seen so far (e.g., this one) for what Tableau calls "dual axis maps" seem to be either trivial (just a couple of rows of data) or to involve data that is tightly linked and has an obvious join column.

Am I chasing after the impossible here?

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The approach in the link you provide is for a different scenario. There Tableau is using built in geocoding rather than spatial data (a secret left join on your data to spatial data held in a firebird database in the install directory)

In the current release of Tableau you have to have all spatial data in a single table. So for your first part "how do i see them all together" you will need a UNION ALL. you can just get Tableau to do this for you though in the interface by dragging one table half way over the other in the data source window. If you press convert to custom SQL then you will see how Tableau has written the SQL for you.

You will need that to export the data out of PostGIS to a spatial file (TAB/SHP) as the current release of Tableau you have wont connect directly to geography columns. Now add that TAB/MIF to Tableau and create a dual axis map as per the instructions before.

You can go a step further and edit that SQL to only show points within stdistance of features but I don't know how to write that part yet....

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