Heights, vertical references and geoid models
Ellipsoidal against orthometric, what a geoid model does, and how to install one.
5 min read
Two kinds of height
- Ellipsoidal height is what GNSS measures: height above a mathematical ellipsoid. It is not the height water flows down.
- Orthometric height is height above the geoid, near enough sea level, and the one engineering drawings use.
- The difference between them is the geoid separation, and it changes across a site.
How Backsight handles it
A vertical reference travels as a label on everything: points, surfaces, clouds, plots. If you say the work is on an assumed local datum, every product says so too.
No conversion without a model
Backsight will not turn an ellipsoidal height into an orthometric one unless a geoid model is installed, selected and verified. A converted height with no model behind it is a guess, and a guess in a levelling column is worse than no answer.
Installing a geoid model
- Get the model file from the authoritative source, for Canada, that is NRCan.
- Organization → Geoid models → Install, and upload it.
- Backsight records the file, its checksum and its licence, and reports the coverage it provides.
- Choose it in the project's coordinate settings. It is now available anywhere heights are converted.
Note
The checksum matters: it is how you prove months later that the heights you delivered came from the model you said they did.
Where the vertical reference shows up
- On the sheet, in the coordinate information panel.
- On every surface, so a volume states what its heights are on.
- On a point cloud, exactly as you stated it, clouds are never reprojected and their heights are never converted.
- On locate reports and cut sheets.
Read next
- Choosing a coordinate systemThe one decision everything else rests on, and how to be sure you have it right.
- Building a surfaceTriangulate points into ground you can contour, measure and stake from.
- Point clouds: reading, clipping and thinningUpload LAS, LAZ, E57, PLY or XYZ/PTS, see what is in it, and cut it down to the part you care about.
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