pyproj

PyProj - Coordinate Projections and Geodetic Math

PyProj is essential for any spatial analysis that requires high precision. It allows you to transform coordinates between thousands of different reference systems and compute distances on the Earth's ellipsoidal surface using the most accurate formulas (Vincenty, Karney).

When to Use

• Converting GPS coordinates (WGS84) to local projected systems (e.g., UTM, Albers, State Plane).
• Calculating precise distances, bearings, and areas on the Earth's surface (Geodetic math).
• Defining custom Coordinate Reference Systems (CRS).
• Handling transformations between different vertical datums.
• Working with "Great Circle" paths and geodesics.
• Identifying the best UTM zone for a specific location.

Reference Documentation

Official docs: https://pyproj4.github.io/pyproj/
PROJ (Engine): https://proj.org/
EPSG Registry: https://epsg.io/ (Essential for finding CRS codes)
Search patterns: pyproj.Transformer, pyproj.CRS, pyproj.Geod

Core Principles

CRS (Coordinate Reference System)

A definition of how numbers (coordinates) map to the real world. Can be defined via EPSG codes (EPSG:4326), PROJ strings, or WKT (Well-Known Text).

Transformer

An object optimized for converting many points from one CRS to another. Always use Transformer for bulk operations rather than one-off calls.

Geod

A class for performing "ellipsoid math". Use this for calculating distances on Earth without projecting to a flat map.

Quick Reference

Installation

pip install pyproj

Standard Imports

import pyproj
from pyproj import CRS, Transformer, Geod

Basic Pattern - Transformation

from pyproj import Transformer

# 1. Define the transformation (WGS84 to Web Mercator)
# always_xy=True ensures (lon, lat) order instead of PROJ default (lat, lon)
transformer = Transformer.from_crs("EPSG:4326", "EPSG:3857", always_xy=True)

# 2. Transform coordinates
lon, lat = -74.006, 40.7128 # NYC
x, y = transformer.transform(lon, lat)

print(f"Projected Meters: X={x:.2f}, Y={y:.2f}")