Projected Coordinate System
Definition
A projected coordinate system (PCS) is a flat, two-dimensional representation of Earth coordinates derived from a map projection, with linear units like meters or feet for direct measurement. PCSs specify the projection method, parameters (central meridian, standard parallels), the geodetic datum, and units. They enable accurate distance, area, and buffering within their design region but introduce distortion outside it. Examples include UTM zones, State Plane, British National Grid, and custom local systems for engineering. Choosing the correct PCS prevents measurement errors and misalignment between datasets. Modern GIS performs on-the-fly reprojection, but analysis should still be run in an appropriate PCS to avoid distortion-driven bias.
Application
Engineers design infrastructure in local PCSs; land records and utilities maintain data in jurisdictional grids; environmental analysts choose equal-area PCSs for fair comparisons; web maps often display in Web Mercator but reproject for analysis.
FAQ
Why not analyze in geographic (lat/long)?
Degrees are angular, not linear; distances and areas vary with latitude. Use an appropriate PCS for metric calculations.
How do datums affect PCSs?
A PCS relies on a datum; mixing datums can shift data by meters to tens of meters. Always record and transform datums explicitly.
Can a single PCS cover a large country?
Yes via national grids optimized for extent, but local accuracy may still benefit from regional zones.
How to choose among equal-area vs conformal?
Equal-area preserves area totals; conformal preserves shape/angles. Pick based on the analysis need—demography vs navigation, for instance.