4.5 Mercator, UTM, Lambert, and Friends

Key takeaways

• Web Mercator renders web tiles; UTM is the metric workhorse for regional metre-accurate analysis.
• Lambert Conformal Conic and Albers Equal Area each specialise for mid-latitude regions.
• National grids exist for exactly this reason — optimised projections for one country.

Introduction

The previous lesson was the theory; this one is the concrete set of projections you'll deal with every day. We'll cover Web Mercator, UTM, Lambert Conformal Conic, Albers Equal Area, Robinson, and a handful of national grids.

Web Mercator (EPSG:3857)

Standard for consumer web mapping (Google, OSM, Mapbox, Apple, etc.). Based on spherical Mercator — uses a sphere rather than the WGS84 ellipsoid for simpler math at the cost of small distortion. Properties:

• Conformal — local angles preserved.
• Cylindrical normal — meridians are vertical lines, parallels horizontal.
• Square tiles — perfect fit for slippy-map tile schemes.
• Terrible at high latitudes — Greenland appears larger than Africa.
• Y-axis clipped at ±85.05112878°.

Use it for: tile rendering, web map display. Don't use it for: area calculations, thematic choropleth for densities.

UTM — Universal Transverse Mercator

The metric workhorse for regional analysis. Properties:

• 60 zones, each 6° of longitude wide (zone 1 = 180°W–174°W, zone 60 = 174°E–180°E).
• Each zone is a transverse Mercator, conformal, with scale factor 0.9996 at the central meridian.
• North and South hemispheres have separate codes (e.g., EPSG:32610 for Zone 10 North, EPSG:32710 for Zone 10 South).
• Coordinates in metres with a false easting of 500 000 m (to keep x positive) and a false northing of 0 (northern hemisphere) or 10 000 000 m (southern).

Typical UTM coordinates for a point in central California: (553 340 E, 4 183 073 N) in Zone 10N.

Why UTM is great:

• Low distortion (~1:2500 at zone edges).
• Consistent metre units.
• Widely supported.

Why UTM is awkward:

• Zone boundaries — features crossing zones need to choose one.
• Scale factor means measured distances are ~0.04% off at the central meridian, increasing outward.
• Different zones can't directly share coordinates.

For analyses covering multiple UTM zones, consider using a regional Albers or Lambert Conformal Conic instead.

Transverse Mercator (national versions)

Many countries use a transverse Mercator tuned to their area:

• British National Grid (EPSG:27700) — 2° W central meridian, OSGB36 datum.
• Gauss–Krüger — used across continental Europe; essentially transverse Mercator.
• South Africa National Grid (Hartebeesthoek94).

These act like "one-zone UTM" tuned to fit the country with minimal distortion.

Lambert Conformal Conic (LCC)

A conic projection with two standard parallels (secant cone). Used extensively for:

• France — Lambert-93 (EPSG:2154). Standard parallels 44° and 49° N, origin near Paris.
• United States (state-level) — many State Plane zones use LCC for east-west elongated states.
• Aeronautical charts — ICAO mandates LCC for most Jeppesen and FAA charts; great circles on LCC are very nearly straight lines.

Why it's good:

• Conformal — true angles locally.
• Minimal distortion across mid-latitude bands.
• Intuitive for east-west features.

Why it's awkward:

• Not equal-area — don't use for density maps spanning the whole projection extent.
• Latitude distortion at extremes.

Albers Equal Area Conic

Also conic with two standard parallels, but designed for area preservation. Used for:

• Contiguous United States — USGS's standard for thematic maps (EPSG:5070).
• Australia — regional thematic.
• China, Mexico — official thematic maps.

Good for:

• Choropleths of density, proportions.
• Regional thematic cartography.

Bad for:

• Precise angle-preserving navigation.
• Local metric analysis at the metre scale (use UTM for that).

State Plane (US only)

The USA's State Plane Coordinate System breaks each state into one or more zones, each using a projection optimised for that zone:

• East-west-elongated states → Lambert Conformal Conic.
• North-south-elongated states → Transverse Mercator.
• Alaska's panhandle → Oblique Mercator.

Each zone has very low distortion (better than UTM for its area) but is limited to that zone. EPSG codes 2000+ for NAD83 feet, or 2000+ in metres.

Robinson, Winkel Tripel, and Equal Earth (world compromise projections)

These are compromise projections that don't perfectly preserve any property but look pleasing at world scale. Used by publishers and UN agencies:

• Robinson (1963) — National Geographic's default 1988–1998.
• Winkel Tripel (1921) — NatGeo 1998–2021.
• Equal Earth (2018) — equal-area, Robinson-shaped; now used by NatGeo, NASA, and many others.

For statistical world maps, prefer Equal Earth or Mollweide over Robinson / Winkel (which distort area).

Polar Stereographic

The go-to projection for polar regions (>60° latitude). Conformal azimuthal projection:

• EPSG:3031 — WGS84 / Antarctic Polar Stereographic.
• EPSG:3995 — WGS84 / Arctic Polar Stereographic (NSIDC).

Used for sea-ice extent maps, Antarctic research, Arctic shipping route analysis.

Goode Homolosine

An interrupted projection — the globe is sliced and displayed in pieces to preserve area while keeping continents un-distorted. Used for oceanographic maps and Goode's World Atlas.

National grid projections (worldwide)

Most countries maintain an official grid. A non-exhaustive list:

Country	Projection	EPSG
UK	Transverse Mercator (OSGB36)	27700
France	Lambert Conformal Conic (RGF93)	2154
Germany	Transverse Mercator (ETRS89 UTM Zones)	25832, 25833
Switzerland	Oblique Mercator (CH1903+)	2056
Netherlands	Oblique Stereographic (RD New)	28992
South Africa	Gauss Conformal (Hartebeesthoek94)	2046–2055
Australia	Lambert Conformal Conic / MGA UTM	3577, 28354–28356
Japan	Plane Rectangular (JGD2011)	6669–6687
Brazil	UTM + SIRGAS 2000	31965–31985
Canada	Lambert Conformal Conic (NAD83 CSRS)	3347

When working with a country's authoritative data, it's usually cheapest to use the national grid — transformations are built-in and accuracy is excellent.

Self-check exercises

1. Your colleague wants to publish an interactive web map of US county population. Which projection for the underlying map tiles? Which for the population area calculations?

Web Mercator (EPSG:3857) for the tiles — it's what every mapping library expects. Albers Equal Area (EPSG:5070) for the area calculations — population per km² requires preserving county area, which Web Mercator distorts. Report both: tiles rendered in Web Mercator can display values computed in Albers.

2. You're analysing a 200 km east-west transect in Germany. UTM would span two zones (32 and 33). What's a better projection choice?

Use ETRS89 / Lambert Azimuthal Equal Area (EPSG:3035) or a regional Gauss–Krüger in a wider zone. A single national projection avoids the zone boundary, giving consistent coordinates across the transect. Alternatively, keep the analysis in one UTM zone (32N) and accept the slightly higher distortion at the eastern edge.

3. Why is Web Mercator inadequate for mapping Arctic sea ice?

Web Mercator clips at ±85° latitude and dramatically distorts area above ~60°. A polar projection (EPSG:3995 Arctic Polar Stereographic) places the pole at the centre, preserves shapes locally, and gives sensible areal comparisons. Most cryosphere agencies (NSIDC, NASA) use Polar Stereographic by default.

Summary

• Web Mercator for rendering; UTM for regional metric analysis.
• Lambert Conformal Conic for east-west conformal; Albers Equal Area Conic for east-west equal-area.
• Polar Stereographic for polar regions; Equal Earth / Mollweide for world thematic.
• When in doubt about a country's data, use its national grid.

Further reading

• USGS Map Projections: A Working Manual.
• Snyder, J. P. — An Album of Map Projections.
• NSIDC documentation on Polar Stereographic.
• https://projectionwizard.org — interactive tool to choose a projection for your extent and purpose.