The last ten minutes before a map goes live are where most of the damage happens. A colour ramp gets flipped. A legend entry drifts out of alignment. A boundary layer downloaded three years ago quietly survives into the final export. None of it looks wrong at 100 per cent zoom on your own laptop. All of it looks wrong to the person who knows the place.
Maps differ from most published work in one awkward respect: their errors are plausible. A misplaced river still looks like a river. That is why a fixed routine beats a careful eye, and why the checks below are worth running in the same order every time.
Know your sources
Before you touch the styling, write down where each layer came from, when you downloaded it and under what licence. Four minutes of note-taking now saves a awkward email in eight months' time, when someone asks why a boundary is wrong and nobody remembers which file it came from.
Then check the dull technical things. Do the coordinates make sense — are those shapes actually where you think they are, or did a layer arrive in a different coordinate reference system and land somewhere off the coast of Ireland? Is the geometry valid, or are there self-intersecting polygons that render fine in one tool and vanish in another? Are the units what you assume? A distance field recorded in feet and plotted as metres produces a map that is wrong by a factor of three and looks entirely convincing.
Watch for version drift as well. Administrative boundaries, school catchments, postcode areas and flood zones all get revised. If your basemap and your data layer disagree about where a district boundary runs, readers will spot the mismatch long before they engage with your analysis.
Projection and scale
Every flat map lies about something — area, distance, shape or direction. Decide which compromise you can live with, then say so in the caption or notes. Web Mercator is perfectly reasonable for a street-level web map and badly misleading for a world choropleth, because it inflates high latitudes and shrinks the tropics. A national grid projection will serve a UK-wide thematic map far better than anything global.
Then test the furniture. Measure a distance you know on screen against the scale bar; a scale bar is only strictly accurate along the line where it was calculated, so a single bar on a continental map is a rough guide at best. If the map is rotated to fit a page, the north arrow has to rotate with it. If there is no north arrow, make sure the orientation is obvious from the labels instead.
Labels, legend and layout
Typography does more for map legibility than any other single decision. Give place names a light halo or a subtle outline so they survive busy terrain, keep case consistent, and spell out an abbreviation the first time it appears. Nothing undermines a map faster than a label reading Unnamed: 47 or a value of NULL sitting in the middle of a city.
Check the legend against the map itself, symbol by symbol. Same number of classes, same colours, same wording. It is surprisingly common to revise a classification and forget the legend, leaving six shades on the map and five keys in the corner. Finally, confirm the title, subtitle and date are correct and specific — "Population change, 2011–2021" tells a reader far more than "Map 3".
Colour and accessibility
Colour is where good maps quietly fail. A few checks worth building into your export routine:
- Convert the map to greyscale. If two classes become indistinguishable, the palette relies on hue alone and needs reworking.
- Vary lightness as well as hue across a sequential ramp, and state the direction in the legend — does darker mean more, or less?
- Never encode a binary distinction in red and green alone. Add a pattern, a shape or a label.
- Simulate colour-vision deficiency and look again. Roughly one in twelve men has some form of it, and they read maps too.
- Check text contrast against its background, particularly white labels over pale satellite imagery.
- For interactive maps, provide alternative text and a data table, and make sure the map is usable by keyboard.
If the map will be printed, check that the darkest class is not solid black — detail disappears into it — and that fine hairlines survive at final output size.
Attribution and licensing
Credit belongs on the map, not buried in a separate document. Most licences specify an exact wording, and some require the credit to be visible rather than hidden behind an information button. Where you have simplified, generalised or otherwise derived data, say so. Avoid layouts that imply an organisation endorses your work simply because you used its data, and keep a copy of each licence alongside the project files so the terms are still there when the map is updated.
Licence conditions vary, and some carry restrictions on commercial use or redistribution. If the terms are unclear, or the stakes are high, take proper legal advice rather than guessing.
The ten-point checklist
- Every layer has a named source, a date and a licence, recorded where you can find it again.
- All layers share one coordinate reference system, verified rather than assumed.
- The projection suits the purpose, and its limitations are stated.
- The scale bar and north arrow are correct for the map's orientation and extent.
- Labels are legible, non-overlapping, consistently styled and free of raw field names.
- The legend matches the map exactly.
- The palette still works in greyscale and under colour-vision simulation.
- Text has enough contrast to read comfortably.
- Attribution appears on the map, in the wording the licence requires.
- Someone else has looked at it, on a different screen and at printed size.
Ten minutes well spent
Two habits matter more than the list itself. First, run it in the same order every time, so a check never depends on remembering to do it. Second, keep the finished export, the project file and a note of the data vintage together in one dated folder — when a boundary changes or a source is revised, you will want to know exactly what the published version contained.
And when a check fails, either fix it or label it. A short note reading "boundaries as at March 2024" is honest and useful. A silent error is neither.
Photo: Jakub Zerdzicki / Pexels
