Five Common Map Projection Mistakes and How to Avoid Them

Every flat map tells a small lie. The trick is choosing which lie you can live with. A projection is not a neutral window on the world; it is a set of compromises that decides what stays true and what stretches. Most mapping problems do not come from complicated maths. They come from habits: reaching for the same projection, ignoring the difference between a basemap and a data layer, or assuming that a coordinate system that looks right on screen is right on the ground. Here are five mistakes that show up again and again in UK mapping work, and the fixes that take minutes rather than days.

1. Treating Web Mercator as the default

Web Mercator (EPSG:3857) is the quiet workhorse of online maps. It is conformal, so local shapes and angles look right at street level, and it tiles neatly. That makes it excellent for a slippy basemap. It is not a good choice for measuring area or comparing regions across the UK.

Because Mercator stretches distance away from the equator, northern latitudes are inflated. On a UK map, Scotland can look disproportionately large next to southern England. A choropleth of population density, agricultural land or woodland cover built on Web Mercator will mislead, even if every polygon is drawn perfectly.

Fix: Keep Web Mercator for the basemap if you need one, but put thematic data in an equal-area projection. Or choose a projected coordinate system suited to the UK and use it for the whole map. If you must publish on Web Mercator, avoid area-based comparisons and say clearly that the map is for location, not measurement.

2. Choosing a projection before deciding what the map must preserve

No projection preserves area, shape, distance and direction at once. Every one favours something. The mistake is picking a familiar name before asking what the map is for.

• Area: population density, land cover, flood extent and election turnout per square kilometre need an equal-area projection.
• Shape and local angles: topographic plans, property boundaries and navigation charts usually need a conformal projection.
• Distance from one point: a catchment or coverage map centred on a city can use an azimuthal equidistant projection centred on that place.
• Direction: wind, flight or radio maps may need a projection that preserves bearings from a chosen point, not a world map stretched to fit a screen.

Write one sentence before you open your GIS: “This map must compare [area/distance/shape/direction] across [extent].” That sentence will eliminate most unsuitable projections immediately. A map of UK parliamentary constituencies, for example, is usually about area and pattern, not local shape, so an equal-area projection is often more honest than a conformal one. A map of footpaths for walking, by contrast, cares more about shape and local angle.

3. Using a global projection for a UK-scale map

World projections are designed for the whole world. They are rarely the best choice for a map of Britain, a county or a city. At UK scale, you can use a projection with very low distortion across the area you care about.

For Great Britain, the British National Grid (EPSG:27700) is a sensible default. It is a Transverse Mercator projection with a central meridian and scale factor chosen to keep local distortion small. It works well for detailed mapping, grid references and OS-style products. It is not designed for Northern Ireland. For Northern Ireland and the Republic of Ireland, use the Irish Transverse Mercator (EPSG:2157) or the Irish Grid (EPSG:29903), depending on the data you have.

For UK-wide thematic maps, especially ones that compare areas, consider an equal-area projection such as Albers Equal Area or Lambert Azimuthal Equal Area, centred on the UK. You can define standard parallels close to the north and south of your mapping area. If your map includes marine areas or crosses several UTM zones, ETRS89 / UTM zone 30N (EPSG:25830) or WGS 84 / UTM zone 30N (EPSG:32630) can be a practical compromise, but check the distortion at the edges.

Fix: Match the projection to the extent. Do not use a world map to show a country, and do not use a country grid to show a continent.

4. Mixing datums, projections and coordinate systems

A surprising number of mapping errors happen before anyone styles a single layer. A GPS point arrives in WGS 84, a boundary file is in British National Grid, and the basemap is in Web Mercator. Load them together without transformation and features can shift by a noticeable distance — enough to put a point on the wrong side of a field boundary or a postcode on the wrong street.

On-the-fly reprojection is convenient, but it is not a substitute for correct transformation. It can display layers together while leaving underlying coordinates in different systems. If you then measure, clip or join those layers, errors follow.

Fix: Set the project coordinate system first. Then transform every layer into it using the appropriate transformation. In the UK, OSGB36 to ETRS89/WGS 84 transformations should use OSTN15 rather than a rough seven-parameter shift if you need metre-level accuracy. For heights, use OSGM15. Check that your GIS is applying the transformation, not just displaying one layer over another. If you export to GeoJSON, remember that the format assumes WGS 84 (EPSG:4326), so a projected layer may be silently reprojected on export.

5. Doing analysis in the wrong coordinate system

Projection mistakes are not only cosmetic. They change measurements. If you calculate area, length or buffers on a layer in degrees or Web Mercator, the numbers can be wrong enough to change a decision.

• Area: A polygon’s area in EPSG:3857 is not in square metres on the ground. In the UK, the inflation varies with latitude, so comparing a Highland council area with a London borough on Web Mercator is doubly misleading.
• Buffers: A 1 km buffer in EPSG:4326 is not 1 km. One degree of longitude is about 70 km at 51°N and about 55 km at 58°N. A “kilometre” in degrees is a different distance in Cornwall, Manchester and Shetland.
• Length: Distance calculations in geographic coordinates can be acceptable with geodesic tools, but plain planar length in degrees is meaningless.

Fix: Project to a metric coordinate system before measuring. For Great Britain, EPSG:27700 is usually suitable. For Ireland and Northern Ireland, use EPSG:2157 or EPSG:29903. For UK-wide area statistics, choose an equal-area projection. If you only need point-to-point distance, use a geodesic calculation rather than flat geometry in degrees.

6. Failing to document the projection

A map can be technically correct and still cause problems if the projection is hidden. Six months later, a colleague opens the file, assumes it is in Web Mercator, overlays a different layer and wonders why the coastlines do not line up.

Documentation matters for data as well as maps. A shapefile may have a .prj file, a GeoPackage stores the CRS, and a GeoJSON file is supposed to be WGS 84 by default. But files get copied, converted and renamed. Metadata gets lost.

Fix: Record the EPSG code, datum and transformation for every layer. Put the projection in the map credits or legend. Add a short note about known limitations: “Area comparisons are approximate; map shown in Albers Equal Area centred on the UK.” If you transform data, save the new version with a clear name and metadata rather than overwriting the original.

If the projection is not in the metadata, it is not in the map.

A quick pre-publication checklist

1. Purpose: Does the map need to preserve area, shape, distance or direction?
2. Extent: Is this a city, county, UK, Ireland or global map?
3. CRS: Is every layer in the same coordinate system, or correctly transformed?
4. Analysis: Were areas, lengths and buffers calculated in a suitable projected CRS?
5. Metadata: Are the EPSG code, datum and transformation recorded?
6. Communication: Does the legend or caption explain the projection and any limitations?

None of this requires advanced geodesy. It requires deciding what the map is for, matching the projection to that purpose, and being honest about what the flat version leaves out. Do that, and the small lie your map tells will be one you chose deliberately.

Photo: Tama66 / Pixabay