Ask which property the map preserves
USGS explains that different projections serve different tasks and introduce different distortions. Its projection FAQ notes strong area and distance distortion near the polar regions on a standard world Mercator map. An equal-area projection serves another purpose: preserving relative area does not mean every shape and direction remains unchanged.
A dramatic-looking country outline is therefore not a sufficient basis for comparing land area. Use the source’s numerical area measure or a suitable documented map.
Do not measure a route from the screen
A line on a television map can be a schematic itinerary rather than a surveyed route. The producer may move labels, simplify coastlines or curve a line for readability. Even on a formal projection, a straight line on the page need not represent the shortest surface path between two places.
USGS’s map-projection reference distinguishes a constant-direction line on Mercator from a great-circle route. A route that appears curved on one flat map can reflect Earth’s geometry rather than a needless detour.
Also check whether the map is oriented north-up and whether it shows the whole journey. A cut between maps can change scale without a conspicuous warning.
Make the geography verifiable
For a programme note, save the named places in order and identify the graphic as an illustration if no scale or projection is given. To compare distances, use an appropriate geographic source and state whether you mean straight-line surface distance, road distance or a transport itinerary.
Do not estimate travel time from a line’s length or assume a route shown in a past programme is currently available. A useful shared explanation says what the map is intended to show and which measurement it cannot support. Keeping the legend, scale and source credit with a screenshot helps another viewer evaluate the image rather than treating visual size or apparent straightness as self-explanatory facts.
An inset can show a remote island at a different scale from the main map. Read its border and scale separately; its placement beside the mainland does not establish that the two are geographically adjacent.
Related questions
Can one flat map preserve every property perfectly?
No. Projection choice involves different distortions.
Is the shortest route always straight on a world map?
No. It depends on the projection and route definition.
Sources and scope
Scope notes distinguish published documentation, supplied information and editorial methods. They do not report a hands-on service test.
- USGS: projection uses — Explains purpose-dependent distortion and Mercator limitations.
- USGS: map projections — Reference distinguishes projection properties, rhumb lines and great circles.
