Using Post code Sector GeoJson from TableauMapping.bi

Ok, so this time I'm using https://www.tableaumapping.bi/ properly. I connect using the web connector from tableau Public to https://www.tableaumapping.bi/wdc and chose the Post code Sector table. I then blend in my own data with post code addresses. I use a calculated field to generate the sector from the full post code in my data:

left([postcode],len([postcode])-2) 

and voila! a much more detailed map than the one I got before using only the first half of the postcode.

Splitting the prefix out of a british postcode with no spaces

I came across a dataset this week that had postcodes in this format

Tableau only understands the first half of the postcode, but how do we split it out? Wikipedia as always has a fairly comprehensive description: The outward code (i.e first half) can be from 2 to four characters, but the inward code (second half) is always three characters. Therefore we can isolate the outward code with a calculation:

left([Post Code],len([Post Code])-3)

This now gives the outward code alone in a field that can be given a post code geographic role and  used with the filled map mark type.

And if you do spatial analysis, that little anomaly on the river Thames would have caught your attention. Lets add place names and streets in the map layers and zoom in:

Layering filled maps and polygons on map (2018.1 update)

I wrote about this technique more than a year ago and it was one of my more popular posts, thanks to a tweet by zen master Chris Love. It is time to update the particular example as version 2018.1 finally brings generated Latitude and Longitude closer to Latitude and Longitude already in the dataset.

We start by UNIONing our two sources, the polygon source having latitude and longitude, the mark source having a field that can be interpreted as a filled map.

 We then use the new trick of putting generated latitude and longitude on the axis, with source latitude longitude in the level of detail for the polygon layer. In the past I would import the filled map source, generate the latitudes longitudes, export crosstab to excel, save as csv, and do a union with the polygon csv. No longer necessary as of 2018.1.

Now all that is required is dual axis to overlay the polygon over the filled map.

Layering marks and polygons on map

This is a trick that has become much simpler to perform since the introduction of union in Tableau 9.3.

We start with two data files, one with the vertices of our polygons and another with the locations where we want the marks. We create a union of those two when we create our tableau data source.

the wildcard union is particularly handy for multiple files so keep it in mind, in this case we don't really need it. What we then need to do is select the Latitude and the lat columns, right click and select 'Merge mismatched fields', likewise for the longitudes.

Then we can create our map with these merged latitudes and longitudes, but we really want to create two maps, one for each layer. Here's how to create the marks map:

Beware of the averaged coordinates, if you don't put all the dimensions in the level of detail you might not get a mark for each row in your dataset! And here's how to create the polygon:

Now we need to select dual axis and right click and hide the 'Null' location.This will give us the desired two layer map.

As it happens, my marks are the centroids of post-codes. So we can tell tableau that through the geographic role of the location field, and select filled maps as the type of mark to get the postcode polygon instead of the dot at the centroid. Note that the (generated) Latitude and Longitude is no good for this as it is not visible when editing the source and cannot be merged with the mismatched latitude longitude of the polygon source after the union, they can't even be used in calculated fields which could be another way round (the pre-9.3 way of doing things). So an original text only source might have to be imported into tableau and the generated coordinates will have to be copied to a new source to use for a union.