The tool that coordinates the physical and the digital.
It translates physical reality into digital data with precision, and vice versa.
Two models of the Earth, two datums.
Position: the ellipsoid, a smooth mathematical shape that approximates the Earth. The geodetic datum anchors it with a centre, axes and a date.
Height: the geoid, or an approximation of it: mean sea level extended under the continents. The vertical datum fixes zero there.
The network puts the datum on the ground.
GNSS stations and control points fix position; a tide gauge and levelling benchmarks, height.
Latitude, longitude and height.
Two angles and a height on the datum. They work for the whole planet, but not in metres.
3.70° Wdatum ETRS89
Projecting means moving to a Euclidean plane.
On the ellipsoid, neither a ruler nor Pythagoras works. The projection is an abstraction: a plane where you design with everyday geometry, in exchange for a small distortion.
Easting and Northing, in metres.
Once projected, the point is written in metres. Datum and projection together have an EPSG code.
4 474 610 NEPSG 25830 · in m
6 250 816 NEPSG 7856 · in m
8 667 092 NEPSG 32718 · in m
Height goes separately, with its own code, and is combined with it.
Rotation is measured from grid north.
The building is drawn square and the official map rotates it. The angle starts from grid north, not true north.
A grid metre is not a site metre.
The projection and the elevation change distances. You work on the grid or on the ground, and write down which.
Two heights for the same point.
GNSS measures above the ellipsoid; drawings, above the geoid. The difference, N, varies with location.
Each country, its framework.
Each country's mapping agency sets datum, projection and heights. Choose yours and use its EPSG code.
The model inherits the surveyor’s system.
The surveyor works from the network and delivers coordinates in a known system.
Far from the origin, everything jitters.
With large coordinates, every program loses precision. That is why you do not model directly in UTM.
N ≈ 4 400 000
Integers eat the decimals.
A floating-point number has a limited number of digits. With millions of metres, few are left for millimetres.
More digits, more bytes.
Double precision doubles the size of each coordinate: it shows in a point cloud, not in a model.
Plumb lines that dance.
A UTM point cloud linked as is trembles. It is not the scanner: it is numerical precision.
Subtract the large, keep the small.
The offset is stored once; each vertex carries only the small part, with all its decimals.
Model locally, locate separately.
The geometry lives close to the origin. A transformation places it in the real world.
Three origins, one connects.
Only the georeferenced point links the model to the real world.
ETRS89, UTM and Alicante.
It is the official framework. The BEP, Civil 3D and the IFC write it as an EPSG code.
In IFC, only one way is right.
Since IFC4: IfcMapConversion plus an EPSG system.
Decide it once and write it down.
Eight decisions that go into the BEP from day one.
Each program has its radius.
The usable zone around the origin depends on the program. Few vendors publish it.
Same points, other names.
Each program has its own equivalent of the Survey Point.
The Survey Point is not the origin.
It is just a marker. Clipped, it sits at the shared origin; unclipped, at a control point whose coordinates it shows.
Acquire or publish.
A single model sets the coordinates and the others acquire them.
Each building, its local system.
Each building, square and close to its origin. The site model places it, rotates it and publishes its position to it.
One link, several positions.
The typical dwelling is linked as many times as it repeats; each instance stores its named position.
Translation first, then rotation.
A cloud in UTM is brought close to the origin before it goes into Revit.
Moving in the viewer is a patch.
If a model arrives out of place, fix it in the source file.
Each platform reads it its own way.
They all federate models from many programs, but not all of them look for the georeference in the same place.
Two points, same order.
Civil 3D knows the reference system; Revit only stores a transformation. The bridge is two common points.
The floating CAD.
A plain DWG brings origin and rotation; a Civil 3D one, its EPSG as well. In both, the drawing's elevation is added to the level's.
The three most frequent.
Four questions per model.
A model can look right and be a hundred metres off. Before federating: where it is, which way it faces, at what elevation and in which system.
Five ways to check.
Some look at the native model and others at the IFC; some are configured and others are scripted.
The requirement, in one file.
A buildingSMART XML file states what the IFC must contain. It goes with the contract and every checker reads it the same way.
Checking with code.
An open-source Python library validates the IDS and also takes the check point to UTM to compare it with the surveyor’s.
Checking inside Revit.
Before export, a rule set checks the native model and takes you to each element that fails.
Rules on the federated model.
It checks everyone’s IFCs with rules and, since 25.12, reads georeferencing as project properties.
What your program already has.
Nothing to install: a check cube at each control point, read in your program and in a neutral viewer.
From model to setting out.
What is measured on the ground goes back to it. Four rules so that it lands in the same place.