How coordinates are mapped
Erde stores each Site, Location, and Sample coordinate in whatever coordinate system you recorded it in — longitude/latitude, a UTM zone, a State Plane zone, and so on. To draw any of these on a map, Erde converts the stored coordinate to WGS84 (World Geodetic System 1984; longitude/latitude, EPSG:4326) on the fly. Your stored values never change; the map just shows everything in one common frame. This page explains what that conversion does, what transforms exactly, and the one legacy datum worth understanding before you trust a position.
Two operations: projection and datum shift
Converting a coordinate to the map can involve two distinct operations, and they are not the same thing.
- Projection flattens a grid system — like UTM or State Plane — back to longitude/latitude. A projected coordinate is a position on a flat map measured in meters or feet (easting and northing); the projection is the exact math that recovers the underlying longitude and latitude. This step is reversible mathematics with no accuracy loss.
- Datum shift accounts for the reference frame the coordinate is anchored to. A datum defines where "zero" sits and the shape of the Earth model the coordinate was measured against. NAD27, NAD83, and WGS84 are anchored differently, so the same longitude and latitude on two different datums describe two slightly different spots on the ground. Reconciling that difference is the datum shift, and it is the only place small position differences can come from.
Most coordinates need only the projection step, or no conversion at all. A datum shift matters only when the stored datum differs meaningfully from WGS84 — which, in practice, means NAD27.
What transforms with no meaningful error
For the coordinate systems Erde ships and that US environmental work uses day to day, the map position is exact for all practical purposes:
| Coordinate system | EPSG | What happens | Position on the map |
|---|---|---|---|
| WGS84 (longitude/latitude) | 4326 | Used as-is | Exact — no conversion |
| NAD83 (longitude/latitude) | 4269 | Treated as coincident with WGS84 | Exact for display (sub-meter) |
| NAD83 UTM zones (10N–19N) | 26910–26919 | Projection only | Exact for display (sub-meter) |
| Web Mercator | 3857 | Projection only | Exact — defined on WGS84 |
NAD83 and WGS84 were designed to align and remain effectively coincident across the US — the difference between them is well under a meter, far below what matters for a map pin, a reporting table, or a plume outline. So NAD83 data, including NAD83-based UTM zones, displays exactly where you expect with no datum shift applied. Web Mercator is defined on WGS84 to begin with, so converting from it is pure projection.
When you collect new positions, prefer WGS84 or NAD83 (or a NAD83 UTM zone). These map exactly and carry no datum-shift caveat. NAD27 support exists for legacy and historical Site records, not as a target for new data.
NAD27 — the legacy datum worth understanding
NAD27 (the North American Datum of 1927; EPSG:4267) is the older reference frame much US environmental data predates modern GPS in. It is built on the Clarke 1866 ellipsoid and anchored at a single survey monument — Meades Ranch, Kansas — rather than at the Earth's center. Because of that anchoring, NAD27 is genuinely offset from modern WGS84 by tens of meters, and the size and direction of the offset vary across the country. Drawing a NAD27 coordinate as if it were WGS84, with no datum shift, would place it tens of meters from its true location.
Erde does not do that. It applies a real datum transformation for NAD27 using the standard CONUS (the conterminous lower-48 US) parameters, so a NAD27 position lands close to where it actually is on a modern map.
Accuracy you can expect
In the lower 48 states, the NAD27 transformation is accurate to roughly 10–30 meters. That is calibrated for display, reporting, and general spatial analysis — placing a map pin, generating a reporting figure, drawing a plume outline, or assembling a regulator submission. It is not survey-grade and is not suitable for cadastral or boundary surveys. Survey-grade NAD27 conversion requires NADCON grid-shift files; for cadastral or boundary work, use a dedicated survey tool rather than the map position Erde shows.
The transformation parameters are calibrated for the conterminous lower-48 US. NAD27 data in Alaska, Canada, Mexico, or the Caribbean still transforms in the correct direction and far better than applying no shift at all, but it is less accurate there than within CONUS. If you work primarily outside CONUS and rely on legacy NAD27 data, treat those positions as more approximate.
How it fits together
The same conversion applies wherever Erde puts a coordinate on a map: the Location primary spatial coordinate behind a marker, a Site or Site Area boundary polygon, and the points you map in Data Analysis. You choose the coordinate system when you enter the data — on a coordinate measurement, on a boundary, or in an import column — and Erde reprojects from there. A coordinate it cannot transform is reported in a notice rather than shown in the wrong place; see Maps.
Figure: every coordinate is reprojected to WGS84 for the map; only NAD27 adds a datum shift.
Related
- Manage location coordinates — record a Location's positions and choose their coordinate system.
- Edit site and area boundaries — enter a boundary in any coordinate system; Erde reprojects it to WGS84.
- View results on a map — where reprojected coordinates are drawn, and what happens to ones that cannot be transformed.
- Enumerations and codes — the coordinate system types Erde recognizes.