Coordinate Systems & Height Datums for Drone Surveying

What is a coordinate reference system (CRS)?

A CRS defines how a position on the curved Earth becomes numbers you can work with. It has two parts: a horizontal CRS (easting/northing, or latitude/longitude) and a vertical datum (height). A geographic CRS uses degrees on the globe (like WGS 84); a projected CRS flattens an area onto a grid and uses metres (like a national grid). Survey work almost always uses a projected national grid so you can measure directly in metres, and reads height in a national vertical datum. Every well-known system has an EPSG code — a short number (e.g. 3006) that names it unambiguously, so "EPSG:3006" always means SWEREF99 TM and nothing else.

Horizontal coordinate systems across Europe

Most European countries use a national grid based on ETRS89 (the European realisation of WGS 84), usually a Transverse Mercator projection in one or more zones. Use the one your project or mapping authority requires — mixing them, or using plain UTM where a national grid is expected, shifts your data by hundreds of metres to hundreds of kilometres.

Horizontal coordinate systems in the US & North America

North America uses NAD83. Local survey work typically uses a State Plane zone — and each state's zone comes in both metres and US survey feet, a classic source of scale errors if you pick the wrong one. Wider areas use UTM. Note that NAD83 and WGS 84 differ by 1–2 m, so they are not interchangeable for survey work.

Height matters: vertical datums and the geoid

Raw GPS gives ellipsoidal height — height above a smooth mathematical model of the Earth. Surveyors need orthometric height — height above mean sea level, defined by the geoid (the Earth's gravity field). The difference, the geoid undulation, can be tens of metres (roughly +30 m in Sweden, ‑34 m in parts of the US), so you must apply the correct geoid/height model — a raw GPS altitude is not a usable ground height. Getting this wrong is one of the most common and expensive drone-survey errors.

Datums, epochs and transformations

A datum pins a coordinate system to the Earth. Two things trip people up. First, WGS 84 and ETRS89 are not the same: ETRS89 is fixed to the stable part of the European plate, while WGS 84 follows the global frame, so they drift apart by about 2.5 cm per year — decades of drift now add up to well over half a metre. Second, precise frames carry an epoch (a date), because the ground itself moves with tectonic plates. For centimetre work you must transform between datums with the right method and epoch, not just relabel the numbers. Orthosite uses the authoritative PROJ transformation pipelines so a reprojection is a real datum shift, not a cosmetic one.

Common coordinate mistakes that ruin a survey

• Wrong horizontal CRS — data lands hundreds of metres or kilometres away; the classic "why is my map in the sea?".
• Ellipsoidal vs orthometric height — heights off by tens of metres because a raw GPS height was used instead of a geoid-corrected one.
• Metres vs US survey feet — a State Plane zone in the wrong unit scales everything by ~3.28.
• Mixing datums — GCPs surveyed in one datum, project set to another (e.g. NAD83 vs WGS 84), leaving a 1–2 m offset.
• Ignoring the epoch — using ETRS89 numbers as if they were WGS 84 (or vice-versa) on precise work.
• Lat/lon axis order — swapping latitude and longitude in imported files.

How Orthosite handles coordinates

Set your project's CRS from 3,000+ EPSG codes and Orthosite reads out coordinates on screen, in accuracy reports and in every export in that exact system — with proper vertical datums and geoid models so heights are correct, not just ballpark. Reprojection uses real PROJ datum transformations, GCPs are interpreted in the datum you surveyed them in, and you can switch between metres and feet anytime; it updates every measurement and report consistently.

Choosing the right one

Use the official national grid and height datum your deliverable requires — for example SWEREF99 TM + RH2000 in Sweden, or a State Plane zone + NAVD88 in the US. Confirm three things before you fly: the horizontal CRS (and zone), the vertical datum, and the units. When in doubt, match exactly whatever your ground control points were surveyed in, and check with the client or local mapping authority — it's far cheaper to confirm than to re-deliver.