Only where …: mute the hexes that miss your conditions
About 2 330 000 Londoners on the map live in a hex with no pharmacy within a 10-minute walk. Here is the control that finds them - Only where … (a boolean mask) - and how to read the muted map it draws.
Who lives with no pharmacy within a 10-minute walk?
In London, about 2 330 000 of the 10 800 000 residents on the map live in a hex - one hexagon, about 350 m across - with no pharmacy within a 10-minute walk. That is 18 640 of the 31 627 hexes measured. The map that finds them is an ordinary map of residents with one condition added: only where no pharmacy is within a 10-minute walk.
That condition is Only where …. It works on Amount, Variety, Fair share and Missing: you keep the map you are on, add up to three conditions, and every hex that misses one is muted - still on the map, drawn in a faint gray, never removed. GIS tools call this a filter, or a boolean mask: one yes or no per hex, laid over the map.
Only where …the field's term: Boolean mask
Residents per hex, only where no pharmacy is within a 10-minute walk. Almost all of central London is muted - a pharmacy is within reach nearly everywhere there. The hexes left in color are where residents are out of reach, and the brighter the color, the more people live in them.
The idea, in one sentence
Draw the map as usual, then mute every hex that misses your conditions. The hexes that match are colored by their own values, with the legend worked out over them alone; the rest stay visible, so you still see where everything is.
Matches - drawn as usual
Every condition holds, so the hex is colored by its value, as the map always colors it.
Misses one - muted
At least one condition fails. The hex stays on the map in a faint gray of its own, apart from the no-data color.
Unknown - never matches
If a hex has no data for a condition's measure, the hex does not match. An unknown is never read as a zero.
Where to click
The fastest way in is the ready-made question. Open Questions & guides and pick Out of reach: pharmacies.
Out of reach: pharmacies in the Question Library. It opens the Amount map of residents with the condition already set.
To build the same map by hand - or the same idea for any other kind of place - open the panel of any map of one measure and use the Only where … row under the measure.
Where to click
Map type
Amount
Total population
Only where …
Add a condition
Pharmacy
Has none
Look at: Within a 10-minute walk
The Amount panel with one condition. The map's own measure stays Total population; the condition is a second measure, tested on its own.One condition: a measure, how it is tested, and where it is read - in the hex, or within a 10-minute walk. The ✕ removes it.
Each condition is tested one of six ways:
Has none - nothing of it is there. It reads the plain count, so a hex has none only when there is truly nothing.
At least, At most, Exactly - a number, in the measure's own units (Total population at least 2,000).
In the top %, In the bottom % - a share of London's hexes (Café in the top 10% of hexes). The cut is always taken over the whole city, so drawing an area never changes which hexes match.
Up to three conditions can be set, and a hex must meet all of them. On Amount and Variety a Range on the map's own measure takes one of those three places. Within a 10-minute walk means the places you can reach on foot within 10 minutes, along streets and paths - nearer ones counting more (full weight under 3.5 min, 70% to 7 min, 40% to 10 min). For Has none that simply means: none can be reached.
Reading the map
While a condition is set, a chip on the map says so, with the server's own count of what matches. Its ✕, Show every hex again, clears every condition in one tap - whether you set it, a link carried it or a question did.
The chip names the condition and counts what matches: 18 640 of 31 627 hexes, holding about 2 330 000 of the 10 800 000 residents on the map.
The legend gains one row for the muted hexes, so you can tell them apart from hexes with no data. The legend's colors, the spread, the advice under the legend and Standouts all read only the hexes that match.
Two gray rows: no data, and Misses your Only where … conditions. A muted hex is not missing data - it just fails a condition.
Hover a hex and the card gives its value as usual. A muted hex adds one line: “Muted: this hex misses your Only where … conditions.”
A matching hex near Swanley, on the south-east edge of the map: 132 residents with no pharmacy within a 10-minute walk. A matching hex's card is just its value; only a muted one adds the Muted line.
The math, honestly
Each condition is one yes-or-no test per hex; a hex matches when every test says yes. The numbers below are illustrative - the shape is exactly what the app runs.
Three London hexes under 'no Pharmacy within a 10-minute walk' (illustrative)
Hex A · 2 pharmacies within a 10-minute walkmisses the condition
muted
Hex B · none within a 10-minute walk, 1 450 residents
matches
Hex C · none within a 10-minute walk, 900 residents
matches
Residents out of reach
1 450 + 900 = about 2 350
The chip's sentence is this sum over every matching hex in London. Resident figures are estimates for each hex (ONS Census 2021), so the sentence says 'about' and rounds them; places are counted, so a total of places is exact.
Three things deliberately do not change when you add a condition:
The average compared with. On Fair share, a condition narrows what you look at, never what it is compared with: the hexes that match are still compared with London's average (or the area you selected). Missing's color classes are still set from the whole comparison too.
The top and bottom cuts. “In the top 10% of hexes” is the top 10% of London, worked out once for the city.
The hexes themselves. Nothing is deleted. Clear the chip and the full map is back, exactly as it was.
Why Clusters says no
Clusters is the one map Only where … does not work on. Its test asks whether a hex and the hexes around it are busier or quieter than London as a whole makes likely - so every verdict depends on the whole city, and there is nothing a condition could narrow. Switch to Clusters with a condition set and the panel says so: “Showing every hex instead: a cluster test reads the whole city.”
What Out of reach does not tell you
No pharmacy within a 10-minute walk is not no pharmacy at all: a bus ride or a short drive may still reach one. The places are those Overture Maps lists (release 2026-08-19), so a pharmacy missing from it is missing here too, and opening hours are not known. Residents come from ONS Census 2021 and are an estimate for each hex. Treat the map as decision support, not a verdict on any one street.
Only where … draws a map only where up to three conditions hold - the field calls it a filter, or a boolean mask.
A hex that misses one is muted, never removed; an unknown value never matches.
The average compared with is unchanged, and top or bottom % cuts are always London's.
Out of reach: pharmacies is residents, only where no pharmacy is within a 10-minute walk.
The chip counts what matches; its ✕, Show every hex again, clears every condition.
Clusters is the exception: its test reads the whole city, so conditions cannot narrow it.
Quick questions
What does Only where … do?
It draws a map of one measure only where up to three conditions hold, such as no pharmacy within a 10-minute walk. Every hex that misses a condition is muted - drawn in a faint gray, never removed.
Which conditions can I set?
Each condition is a measure tested as Has none, At least, At most, Exactly, In the top % or In the bottom %, read in the hex or within a 10-minute walk. A hex must meet all of them, and a hex with no data for a condition never matches.
Does a condition change the average the map compares with?
No. On Fair share the matching hexes are still compared with London's average (or the area you selected), and top or bottom % cuts are always taken over the whole city.
What is Out of reach: pharmacies?
A ready-made question in the Question Library: the Amount map of residents, only where no pharmacy is within a 10-minute walk. In London its chip reads 18 640 of 31 627 hexes match, holding about 2 330 000 of the 10 800 000 on the map.
Why can't I use Only where … on Clusters?
The Clusters test reads the whole city, so there is nothing for a condition to narrow. Switching to Clusters shows every hex instead and says so in the panel.
See it on a real map
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