Web Mercator (Pseudo Mercator, EPSG:3857)

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Introduction

Pseudo Mercator projection is a sphere-based conformal cylindrical projection. It converts longitude and latitude coordinates into plane rectangular coordinates by approximating the earth as a sphere and using a tangent cylindrical projection. It is widely used in global Web map services (such as Google Maps and OpenStreetMap) and supports seamless splicing and layered tile loading.

Projection Basics

Projection method: Pseudo-Mercator projection is based on the conformal cylindrical projection principle of Mercator projection, but it is simplified.
Earth model: The earth is regarded as a perfect sphere (not an ellipsoid), and the radius is the major semi-axis of the WGS84 ellipsoid, 6,378,137 meters.
Coordinate system: The projected plane coordinate system is in meters, with the X axis corresponding to longitude and the Y axis corresponding to latitude.

Pros

Computational efficiency: Simplify the earth into a sphere, simplify coordinate conversion and projection calculation, and adapt to the real-time rendering requirements of Web maps.
Directional consistency: Maintain the equiangular characteristics to ensure that the azimuth of any point on the map is consistent with the actual geographical orientation, suitable for navigation applications.
Global seamless stitching: Supports seamless stitching and smooth scaling of global maps, and adapts to layered tile map technology.
Standardization support: Adopts EPSG:3857 standard, compatible with mainstream Web map services and GIS platforms.

Cons

High-latitude deformation: The higher the latitude, the more serious the area deformation, and the polar regions cannot be effectively displayed.
Distance and area distortion: As the latitude increases, the actual distance and area are exaggerated, which is not suitable for accurate measurement.
Ellipsoid approximation error: Ignoring the ellipsoid shape of the earth, there is a certain loss of accuracy in coordinate conversion.
**Polar region restrictions: **Areas with latitudes exceeding ±85.05° cannot be projected and need to be cropped or specially processed.