Map Projections Explained: Types, Properties and Which One to Use

A map projection is a systematic way of transferring positions from the curved surface of the Earth onto a flat plane: a formula that takes a latitude and longitude and returns an x and y on paper or screen. Every flat map, from a road atlas page to the map app on your phone, is drawn in some projection.

The reason there are so many projections is that none of them can be perfect. A sphere has curvature in every direction, and Gauss proved in 1827 (his "remarkable theorem") that a curved surface like that cannot be flattened without stretching or tearing. Try pressing half an orange peel flat: it splits. A projection decides where the stretching goes.

There are four things a flat map can try to keep:

• Area: equal areas on the ground stay equal on the map.
• Shape (angles): small shapes and all angles stay correct.
• Distance: true distances along some lines or from some point.
• Direction: true bearings from a central point.

No projection keeps all four. In particular, no projection can be both conformal (true angles) and equal-area. Choosing a projection is choosing which errors you can live with for the job at hand.

A projection is not the same thing as a datum or a coordinate format. The datum (such as WGS84) defines the shape of the Earth and where latitude and longitude are measured from; the projection flattens those coordinates; the format is just how you write them. See how coordinate formats differ for the last part.

Classic projections are grouped by the simple surface they imagine wrapping around the globe, a surface that can be unrolled flat without stretching:

Two refinements change how each family behaves:

• Tangent or secant. A tangent surface touches the globe along one line (or at one point), where scale is exact. A secant surface cuts through it along two lines, spreading the error more evenly. The Albers map of the U.S. uses two standard parallels for that reason.
• Aspect. The surface can sit in its normal position, turned 90° (transverse) or at any angle (oblique). Transverse Mercator, the basis of the UTM grid, is a cylinder wrapped along a meridian instead of the Equator.

Many modern world projections, such as Winkel Tripel and Equal Earth, are defined directly by formulas and do not come from unrolling any surface, but they still look like one of these families.

The property a projection keeps matters more than its family:

• Conformal projections keep angles, so small shapes look right everywhere, but areas swell or shrink. Examples: Mercator, transverse Mercator, Lambert conformal conic, stereographic.
• Equal-area (equivalent) projections keep every area in proportion, so any two regions can be compared, but shapes are sheared, especially near the edges. Examples: Equal Earth, Mollweide, Albers equal-area conic, Lambert azimuthal equal-area, Gall-Peters.
• Equidistant projections keep true distance along certain lines: from the center of an azimuthal equidistant map, or along every meridian of the plate carrée. No projection keeps all distances.
• Azimuthal projections keep true directions from the center point. Some are also conformal or equal-area or equidistant.
• Compromise projections keep nothing exactly and spread small errors in area and shape over the whole map so that nothing looks badly wrong. Examples: Robinson, Winkel Tripel, Natural Earth.

The figure draws the same world in four projections, one per property, with identical circles on the ground (Tissot's indicatrix) placed every 30° of latitude. Where a circle grows, area is inflated; where it becomes an ellipse, shapes and angles are distorted.

Read the circles, not the coastlines. On Mercator they stay round but balloon toward the poles. On Equal Earth they all cover the same area but flatten near the edges. Natural Earth lets them grow and flatten a little everywhere. On the polar azimuthal equidistant map, every circle's distance from the North Pole is true, but the circles in the south stretch sideways into broad ovals.

These are the projections you are most likely to meet, with what each one keeps and where it is used:

Robinson and Winkel Tripel are both compromises with no exact property; Winkel Tripel averages the coordinates of the equirectangular and Aitoff projections and shows high latitudes with less area inflation than Robinson. Equal Earth was designed in 2018 as an equal-area world map with a look similar to Robinson.

Many national maps of the contiguous United States, including USGS maps of the whole country, use the Albers equal-area conic projection with standard parallels at 29.5° N and 45.5° N. Three reasons:

1. The U.S. is wide and mid-latitude. A cone touching the globe between those two parallels hugs the country much more closely than a cylinder around the Equator.
2. Areas are exact. Thematic maps (population, income, land cover, election results) shade states and counties by value, and equal-area projections stop Montana from looking larger than it should next to Texas.
3. Shape errors stay small. We computed the scale along the parallels on a sphere: exactly 1 on both standard parallels, 0.990 at 37.5° N between them, and 1.012 to 1.013 at 25° N and 49° N. North-south scale is the inverse, so area stays exact while shapes are distorted by only about 1% across the lower 48 states.

The same calculation shows why Alaska does not fit: by 60° N the scale along the parallels reaches 1.11 and by 70° N 1.37. Composite layouts such as d3's "Albers USA" therefore draw Alaska and Hawaii in their own Albers projections and place them as insets. Our U.S. blank maps use that composite.

For conformal U.S. maps, Lambert conformal conic plays the same role: with standard parallels at 33° and 45° its scale stays within about 2.5% from 25° to 49° N. It is the projection of U.S. aeronautical sectional charts and of most state plane zones that are wider east-west than north-south.

Online maps use Web Mercator for practical reasons: the world becomes a square that divides into identical tiles at every zoom, north is always up, and at street level the projection is conformal, so intersections and buildings keep their shape. Its weakness only shows at world scale, where Greenland, Canada and Russia are inflated. The Mercator projection guide covers the scale factor, the Greenland example and the 85.05° cutoff in detail.

A practical consequence for anyone measuring on a web map: never read distances or areas off the screen at small scale. The country size comparison tool and the other measuring tools here compute on the WGS84 ellipsoid, then draw the result on the Mercator base map, so the numbers stay right even when the drawing is stretched.

Start with what the map has to show, then with the size and shape of the area:

1. Comparing sizes or mapping data by area (population density, land cover, choropleths): use an equal-area projection. Equal Earth or Mollweide for the world, Albers for a mid-latitude country, Lambert azimuthal equal-area for a continent or polar region.
2. Navigation, angles or local shapes: use a conformal projection. Mercator for charts at sea, transverse Mercator or UTM for a narrow north-south area, Lambert conformal conic for a wide east-west one.
3. Distances or directions from one place (flight ranges, radio coverage, "how far from here"): azimuthal equidistant centered on that place.
4. A general world reference map that should look natural: a compromise such as Winkel Tripel, Robinson or Natural Earth.
5. Small areas (a city, a county): the choice hardly matters for appearance. For measurement, use the local UTM zone or state plane system, where distortion is a fraction of a percent. Our UTM to latitude/longitude converter moves coordinates between the two.

When in doubt, avoid Mercator for anything at world scale where sizes might be compared, and say on the map which projection you used.