Coordinate System Converter — GPS WGS84 GCJ02 BD09

WGS84 — The Global GPS Standard

WGS84 (World Geodetic System 1984, EPSG:4326) is the coordinate system used by GPS satellites worldwide. Google Maps outside mainland China, Apple Maps globally, OpenStreetMap, and virtually all international mapping services use WGS84 as their native coordinate system. If you record a GPS waypoint with your phone anywhere in the world outside China, you are recording WGS84 coordinates. WGS84 is geographically accurate — a latitude of 39.9042, longitude of 116.4074 in WGS84 points to the correct location on Earth's surface with no intentional offset. This makes WGS84 the universal interchange format for geospatial data.

GCJ02 — China's Encrypted Offset (Mars Coordinates)

GCJ02, sometimes called 'Mars coordinates' by developers, is mandated by Chinese national standard GB/T 17278 for all maps published in mainland China. It applies a non-linear cryptographic offset to WGS84 coordinates, typically shifting positions by 100 to 500 metres in a direction that varies based on location. The offset algorithm is deliberately obfuscated to prevent reverse-engineering. Mapping services that use GCJ02 include AutoNavi (Gaode Maps), Tencent Maps, and Apple Maps when viewed in China. If you are developing an app for the Chinese market or consuming data from these platforms, you must convert GCJ02 coordinates to WGS84 before storing them in a standard database — and convert back before displaying on Chinese maps.

BD09 — Baidu's Proprietary Second Layer

BD09 is Baidu Maps' proprietary coordinate system that adds an additional offset on top of GCJ02. This means converting WGS84 → BD09 requires two steps: WGS84 → GCJ02, then GCJ02 → BD09. The double offset pushes coordinates up to 700 metres from their true WGS84 position. Baidu Maps is one of China's most widely used mapping platforms, and the BD09-BD09LL specification is documented in the Baidu Maps LBSYUN developer SDK. The reverse conversion (BD09 → WGS84) is an approximation that is accurate to within 1-2 metres for coordinates inside mainland China. This converter uses open-source algorithms validated against the Baidu Maps API.

Plan astrophotography sessions with converted coordinates → Telescope FOV Calculator

When You Need This Converter

You need coordinate conversion whenever you mix data from different mapping ecosystems. Common scenarios include: (1) You are a developer integrating a Chinese mapping API (AutoNavi/Tencent/Baidu) and need to convert API responses to WGS84 for database storage. (2) You are a GIS analyst preparing map data for regulatory submission in China, which requires GCJ02 coordinates per GB/T 17278. (3) You are debugging a map display issue where pins appear offset — this is almost always a WGS84/GCJ02/BD09 mismatch. (4) You are using a GPS device in China that outputs WGS84 but needs to display the location on a Chinese map application. The converter handles all three systems bidirectionally: WGS84 to GCJ02, GCJ02 to BD09, and WGS84 to BD09. Outside mainland China, GCJ02 equals WGS84 with no offset applied, and the converter handles this edge case automatically.

Compute Local Sidereal Time from your converted GPS location → Sidereal Time Calculator