# Map layers Multiple data sources - Tableau 2021.4 - I mapped every road & bus stop in Central London

> This is content from just-tim, the data-and-analytics channel by Tim Ngwena (formerly 'Tableau Tim'). Tim has 12+ years of hands-on BI experience and covers Tableau most of all, plus Power BI, Looker, Hex, SQL and data modelling, the analytics industry, and the craft of doing the job — always tool-agnostic and honest about the trade-offs.

- **Author:** Tim Ngwena (just-tim, https://just-tim.com/about)
- **Published:** 2021-12-06
- **Format:** Video · 18 min watch · transcript available
- **Topics:** Data visualisation, Data prep
- **Tools:** Alteryx (input output, spatial); Tableau (extensions, makepoint, maps)
- **Canonical:** https://just-tim.com/posts/map-layers-multiple-data-sources-tableau-2021-4-i-mapped-every-road-bus-stop-in-central-london
- **Watch:** https://www.youtube.com/watch?v=O_LvTD2UyYg

I demonstrate Tableau 2021.4's new ability to use multiple data sources across map layers, removing the need to merge spatial datasets via relationships or external prep. Using London bus stop data and Ordnance Survey road data, I build a map layering bus stops over the road network, and show how I used Alteryx to convert Eastings/Northings into points when Tableau's MAKEPOINT() choked on the large CSV.

## Key takeaways

- In Tableau 2021.4 you can layer spatial data from two separate sources directly, without relationships, unions or prior data modelling
- UK Eastings/Northings map to longitude/latitude respectively, and MAKEPOINT() needs the coordinate system code (EPSG:27700 for British National Grid), which you can look up on the EPSG website
- Running MAKEPOINT() on a very large CSV cripples performance, so pre-processing with Alteryx's Create Points tool and writing out a hyper extract is far faster
- Adding the road number or stop name to the Detail shelf makes individual roads and points selectable rather than aggregated into one mark
- Use Map Layers controls to lock a background layer so hovering over it doesn't activate selection, keeping context layers passive beneath your main data

## Chapters

- 0:00 New multi-source map layers feature
- 0:52 The data: OS roads and TfL bus stops
- 1:59 MAKEPOINT and the Eastings/Northings problem
- 6:26 Performance issues on the large CSV
- 6:41 Pre-processing bus data in Alteryx
- 8:06 Connecting the hyper extract in Tableau
- 8:51 Adding the road shapefile layer
- 10:40 Filtering roads to A and B roads
- 11:40 Layering bus stops over roads
- 16:17 Styling and locking background layers

Watch the full video, read the transcript and use chapter deep-links on the page: https://just-tim.com/posts/map-layers-multiple-data-sources-tableau-2021-4-i-mapped-every-road-bus-stop-in-central-london

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just-tim — Data and analytics, with a point of view. · https://www.youtube.com/channel/UC7HYxRWmaNlJux-X7rNLZyw · https://twitter.com/TableauTim · https://www.linkedin.com/in/timngwena