Geuneullo bets shade-routing wins Korea's heat: solo student app hits App Store #1

Geuneullo (그늘로: 뚜벅이의 여름길, 'Walk in the Shade') is a free iOS map app that computes the sun's position and building/tree shadows from public data, then routes pedestrians through shade, showing route shade percentage, bus-seat sun exposure and cooler subway car sides.

That walkers choose a longer shaded route when an app computes tree and building shadows in real time — and a solo student can win Korea's summer without a company.

Yoo Min-jun, a 23-year-old Seoul National University student double-majoring in mathematics education and venture management, built the app in about two weeks after his own summer commute from Gyeonggi province left him drenched in sweat. Using AI plus public data (OpenStreetMap and government building, street-tree and transit datasets), he calculated sun altitude and shadow positions by time of day and released Geuneullo on the iOS App Store in mid-July 2026 (Korea Herald: July 18).

As Korea's record heat wave pushed temperatures toward 40°C, the app spread by word of mouth on social media: 20,000 people signed up on Aug 4 alone. Navigation downloads jumped from rank 209 to 119 between Aug 1 and Aug 3 (UPUP via Chosun Ilbo), then it hit #1 in Apple's free navigation chart on Aug 5 and the overall free-app chart on Aug 6, surpassing Google Maps, Kakao Map and Naver Map. TV Chosun reported the service also became available inside the Toss super-app, with an Android version still under review.

It bet on a question incumbents don't answer — where the shade is at this hour — so it faced no direct competition from Google Maps, Kakao Map or Naver Map.

It rode a real, time-limited surge: a record heat wave made shade urgent and searchable, turning a student side project into the #1 free app within days.

Public data (OpenStreetMap plus government building, tree and transit datasets) let one person build what a maps company would need a fleet for.

Word of mouth, not marketing, did the work: 20,000 sign-ups in a single day show the product itself was the channel.

A solo developer can out-route map giants by owning one narrow, urgent question — where is the shade right now — when the data is public and a heat wave makes people search for it every day.

As of August 7, 2026, Geuneullo sat at #1 in Apple's free navigation chart and had topped the overall free-app chart the day before, with an Android version still under review and the service also accessible inside Toss. The developer remained a student with no company, budget or funding and hoped the app would improve access to urban shade; user reviews told everyday stories such as guiding a father who walks his dog without coming home soaked in sweat. Whether it becomes a company or the heat-driven spike fades with the season was still open.