# True Size Map (真實尺寸地圖)

> An interactive web tool that exposes Mercator projection area distortion. Users drag any country, US state, Taiwanese county, Japanese prefecture, German Bundesland, etc. to a different latitude and instantly see its true relative size. Supports 6 map projections (Mercator, Equal Earth, Natural Earth, Robinson, Mollweide, 3D Orthographic globe), day/night terminator simulation, multi-language UI (Traditional Chinese, Simplified Chinese, English, Japanese), Mercator-distortion heatmap, area-comparison quiz, and Open Graph image generation.

Live URL: https://truesize-map.vercel.app/
Author: bashcat
Contact: bashcat0804@gmail.com
Website: https://bashcat.net
License: MIT-style attribution welcome
Last updated: 2026-05-17

## Core concepts

- **Mercator projection** (1569, Gerardus Mercator) preserves angles (conformal) — great for navigation, terrible for area. The area distortion factor at latitude φ is sec²(φ). At equator = 1×, at 60° = 4×, at 80° ≈ 33×, approaching infinity near the poles.
- **Greenland appears similar in size to Africa on Mercator maps**, but Africa is roughly **14× larger** (Africa: 30,370,000 km²; Greenland: 2,166,086 km²).
- **Other equal-area projections** (Equal Earth, Mollweide) preserve area; **compromise projections** (Robinson, Natural Earth, Winkel Tripel) balance distortion across all properties.

## Site

- [Home / Interactive Map](https://truesize-map.vercel.app/): main app
- [About & FAQ](https://truesize-map.vercel.app/#about): explanation of Mercator distortion + FAQ

## API

- [/api/distortion?lat=60&lon=20](https://truesize-map.vercel.app/api/distortion?lat=60): GET → JSON with Mercator linear_scale (sec φ) and area_scale (sec² φ) at the given latitude. CORS-enabled. Cached 24h.
- [/api/og](https://truesize-map.vercel.app/api/og): GET → 1200×630 PNG Open Graph card with optional ?title= and ?subtitle= query params.

## Reference data

- [sitemap.xml](https://truesize-map.vercel.app/sitemap.xml)
- [ads.txt](https://truesize-map.vercel.app/ads.txt)
- [og-image.svg](https://truesize-map.vercel.app/og-image.svg): static fallback brand image

## Subdivisions supported

| Country | Layer | Source |
|---|---|---|
| 🇺🇸 United States | 50 states + DC | us-atlas (Natural Earth derivative) |
| 🇹🇼 Taiwan | 22 counties/cities | g0v/twgeojson |
| 🇯🇵 Japan | 47 prefectures | dataofjapan/land |
| 🇰🇷 South Korea | 17 administrative divisions | southkorea/southkorea-maps |
| 🇨🇦 Canada | 10 provinces + 3 territories | codeforgermany/click_that_hood |
| 🇩🇪 Germany | 16 Bundesländer | isellsoap/deutschlandGeoJSON |
| 🇦🇺 Australia | 8 states/territories | rowanhogan/australian-states |

## Tech stack

- D3.js v7 (geo projections + path generation)
- d3-geo-projection v4 (Robinson, Mollweide)
- topojson-client v3
- @vercel/og (dynamic OG images via Edge function)
- gif.js (4-second animation export)
- REST Countries API v3.1 (population, area, capital, flag, translations)
- Vercel hosting + Edge Functions
- GA4 analytics

## Citation

If you use this tool in your article, tutorial, or research, please link to:
https://truesize-map.vercel.app/

Suggested citation:
> True Size Map (2026), an interactive Mercator-distortion visualization tool by bashcat, https://truesize-map.vercel.app/

## Contact

- Author: bashcat
- Email: bashcat0804@gmail.com
- Personal site: https://bashcat.net
- Open to: bug reports, feature requests, partnership inquiries, citation requests, translation help, educational licensing

## Optional

- [llms-full.txt](https://truesize-map.vercel.app/llms-full.txt): expanded version with full FAQ content for AI ingestion