Ground coordinates
See each point's combined scale factor, export a ground-scaled point file, CSV, DXF and LandXML, and key ground coordinates off a plat to stake.
On this page
A state plane grid distance isn't what a tape measures. The projection stretches or shrinks distances by up to a few tens of parts per million, and height above the ellipsoid adds about 5 ppm for every 100 ft. The combined factor relates the two: grid ÷ combined factor = ground. OpenSurvey3D works it out for every point and can export your project scaled to ground.
Before you start
- A project with a coordinate system set. See Choose a coordinate system. Without one, Tools → Ground coordinates… is greyed out.
- Collected points. The project factor is the mean of their own factors.
Each point's factors
For every collected point, the app works out:
| Factor | What it is |
|---|---|
| Grid scale factor | Grid distance per ellipsoid distance at the point, from the projection |
| Elevation factor | Ellipsoid distance per ground distance at the point's height |
| Combined factor | The two multiplied. Grid = ground × combined factor |
The elevation factor wants the height above the ellipsoid. The app uses the point's ellipsoidal height when it has one, otherwise its elevation plus the geoid separation recorded with the fix, otherwise its elevation alone. The ground CSV says which it used. A point with no height at all gets an elevation factor of 1.
The factors are always computed from the point's current position and height in the project's current zone, never stored. Re-observe or edit a point and its factors follow.
Where you see them:
- On the point. Open a point in the asset list. With a zone set, the Position section shows Combined factor, for example 0.99991234 (−87.7 ppm).
- In Export CSV. Every row gets
grid_scale_factorandcombined_factornext to its northing and easting.
Export ground coordinates
- Go to Tools → Ground coordinates….
- Read This project: the Mean combined factor of the collected points, its Range and the number of Points. The footer says how much error one factor for the whole job puts into a short distance at the extremes, in ppm and feet a mile.
- Under Factor, keep the project mean, or turn on Use a factor I enter and type the office's factor, as grid ÷ ground. It must be between 0.99 and 1.01.
- Under Origin, choose what to Scale about:
- Grid origin (0, 0): ground coordinates differ from grid by hundreds of feet, so nobody can mistake one for the other.
- A project point: ground coordinates stay close to grid near that point.
- Check Example, which shows one point's grid and ground northing and easting.
- Tap Export ground coordinates (ZIP) and choose where to send it.
Every point is scaled by the same factor about the same origin: ground = origin + (grid − origin) ÷ factor. One factor keeps the file consistent: the distance between two scaled points is the scaled distance between them. Elevations aren't scaled.
The factor and origin are remembered for the project when you export, so later exports, and ground coordinates you key in to stake, use the same pair.
What's in the ZIP
The ZIP is named after the project, ending in -ground, and holds:
| File | Contents |
|---|---|
…-ground-pnezd.txt |
A PNEZD point file: point, ground northing, ground easting, elevation, description |
…-ground.csv |
Ground and grid northing and easting side by side, with each point's grid scale factor, elevation factor, combined factor and ppm, convergence, height basis, and the factor and origin used |
…-ground.dxf |
The points and lines on ground, for CAD |
…-ground.xml |
LandXML on ground. It carries no EPSG code, because ground isn't state plane |
ground-scaling.txt |
How the scaling was done: the zone, the factor and where it came from, the origin, the formula both ways, and your surveyor statement |
Every file states that it's ground coordinates, not grid, with the factor and origin, and how to return to grid.
Key a ground coordinate to stake
When a plat gives ground coordinates, you can key them in directly:
- Go to Tools → Stake a typed coordinate.
- Set Coordinates to Ground. The switch is remembered.
- Check the footer. It shows the conversion in full, for example Grid = origin + (ground − origin) × 0.99991234, about grid origin (0, 0).
- Enter the northing and easting off the plat, then save and stake out as usual.
The app converts the ground values back to grid with the factor and origin chosen under Tools → Ground coordinates…. If nobody has exported with a chosen factor yet, it uses the project mean about grid (0, 0), and the footer says so in red. If the plat used another factor or origin, set them under Tools → Ground coordinates… and export first.
The point is stored on grid. Its notes keep what you keyed and how it was scaled, for example keyed as ground N 5000.000, E 3000.000 ftUS, to grid × 0.99991234 about grid origin (0, 0), so it can be checked against the paper later.