LoD vs GSD: what an LoD3 request asks for, and how to fly it
A client asks for an LoD3 model of a building, and your flight plan needs a number in cm/px. The two don't convert: LoD says which shapes the model holds, GSD says how fine the photos are, and accuracy is measured on its own.
By Sander Helgesen · Last checked
In short
- In a 3D city model (CityGML), LoD is geometric detail: LoD1 is a block, LoD2 a generalized shape, LoD3 keeps details such as balconies, dormers and sills. No level has a pixel size.
- In BIM, LOD usually means Level of Development: how far the team can rely on an element. Ask which one the client means.
- GSD is the size of one photo pixel on the surface. It sets the smallest detail you can see, not how accurate the model is.
- Accuracy is shown with checkpoints: surveyed points the processing does not use. A planned GSD is not a measured accuracy.
- Start from the smallest feature the model must show, pick the GSD from it, and fly the facades at the distance that gives it. See the mapping guide for GSD per height.
What LoD means in a 3D city model
The OGC's CityGML 3.0 standard has four levels of detail, LoD0 to LoD3, and sorts them by geometry. LoD0 is a footprint or a floor plan. LoD1 is a block, the footprint extruded straight up. LoD2 is a realistic but generalized shape, where smaller details such as bulges, sills, balconies and dormers are usually left out. LoD3 is the highest level and keeps all the shape details there are.
CityGML 2.0 had a fifth level, LoD4, for interiors. Version 3.0 dropped it: every level can now hold indoor elements, so a building can have its outer shell in LoD2 and its rooms in LoD1.
None of the levels is defined by a pixel size. The level says what the model contains; how precise it must be is for the project's own specification to state.
LOD in BIM is something else
In building information models, LOD usually stands for Level of Development, from the BIMForum specification (LOD 100, 200, 300, 350 and 400 in its 2015 edition). BIMForum keeps it apart from Level of Detail: detail is how much is drawn into an element, development is how far the team can rely on the element's geometry and data.
A scan-to-BIM job that asks for LOD 300 is describing the model the office will build, not your photos. For the accuracy of a measured building, the U.S. Institute of Building Documentation publishes a separate Level of Accuracy (LOA) specification. If the brief uses LOD and LOA together, ask for the LOA number: that is the accuracy question.
What GSD tells you, and how far from the wall to fly
GSD is the size of one photo pixel where it lands: on the ground for a photo straight down, on the wall for a photo of a facade. It follows from the camera and the distance: GSD = distance × sensor width × 100 / (focal length × image width). For a facade the distance is how far the drone is from the wall, not its height above the ground.
GSD sets the smallest thing you can see in the photos and the texture. It does not set where things are: without RTK or ground control, a model at 1 cm/px can still sit a metre or more off in absolute position, as the mapping guide explains.
Distance from the wall in metres for each GSD, camera square to the wall in the 12 MP mode, computed from the sensor width and focal length in DJI's specifications. Tilt the camera and the pixels get bigger.
| Drone | 0.5 cm/px | 1 cm/px | 2.5 cm/px |
|---|---|---|---|
| DJI Mini 4 Pro | 14 m | 28 m | 70 m |
| DJI Mini 5 Pro | 14 m | 28 m | 71 m |
| DJI Air 3 | 14 m | 28 m | 70 m |
| DJI Air 3S | 14 m | 28 m | 71 m |
| DJI Mavic 3 Pro | 19 m | 38 m | 94 m |
| DJI Mavic 4 Pro | 24 m | 48 m | 119 m |
Turn an LoD request into a flight plan
Do this before you plan the grid, ideally in the same message as the quote.
Ask which LoD the client means
CityGML's Level of Detail or BIM's Level of Development, and what they will deliver: a CityGML file, a textured mesh, a point cloud or a BIM model. The answer changes the job more than the number does.
List the smallest features that must be modelled
Windows, window reveals, cornices, gutters, roof details, signs. Measure or estimate the smallest one. That feature, not the LoD, decides the GSD.
Pick the GSD from the smallest feature
A feature only a pixel or two wide does not come out as a shape in the mesh, so pick a GSD several times finer than it. As starting points, Skyplanr uses 2.5 cm/px for LoD2, 1 cm/px for LoD3 and 0.5 cm/px for fine detail, and labels them as assumptions.
Fly the facades, not only the roof
A grid with the camera straight down sees roofs and ground. Walls need photos from the side: rows along each facade at the distance in the table above, about 80% overlap, and a second pass with the gimbal tilted down 20° to 30° so the photos cross.
Check the distance against the clearance
At 1 cm/px a Mini 4 Pro has to be about 28 m from the wall, at 0.5 cm/px about 14 m. If trees, wires or a neighbour's house keep you further out, the GSD gets coarser: say so in the quote rather than promise the finer number.
Plan checkpoints if accuracy is part of the order
Survey points with an RTK receiver or a total station, spread over the site and at different heights, and keep some out of the processing as checkpoints. The ASPRS positional accuracy standard (Edition 2) asks for at least 30 checkpoints for a fully compliant accuracy assessment, reported as RMSE.
Planned GSD is not measured accuracy
Ground control points (GCPs) tie the model to coordinates and improve it; checkpoints are measured the same way but left out of the processing, so they show how far off the finished model is. Pix4D puts it shortly: checkpoints are not used in the optimization, only for accuracy reporting.
A plan at 1 cm/px tells you what the photos can show. Only checkpoints measured against the finished model tell you the accuracy, so quote accuracy as a number with the way it will be checked, for example RMSE at a stated number of checkpoints.
What to ask the client before you quote
- Which LoD: CityGML's Level of Detail (LoD0 to LoD3) or BIM's Level of Development (LOD 100 to 400)?
- Which features must be in the model, and the smallest of them in centimetres.
- The accuracy as a number, and how it will be checked: checkpoints, how many, RMSE or another measure.
- The coordinate system and height datum, or whether a model in local coordinates is enough.
- The file to deliver: CityGML, a textured mesh (OBJ, GLB), a point cloud, or a BIM model someone else builds from your data.
Where Skyplanr fits in
Write the LoD in the brief, and Skyplanr plans with a starting GSD labelled as an assumption and asks in the summary which features must be modelled and what accuracy and format the client needs, instead of stopping to ask. Facade rows are planned at the distance for that GSD, never closer than 8 m, and the plan is checked with 100 rule-based checks before you get the DJI Fly KMZ. Upload the photos afterwards and Skyplanr builds the point cloud and a textured 3D model, labelled as relative accuracy: it does not measure checkpoints yet.
See the technologyQuestions
Is LoD3 the same as 1 cm/px?
No. LoD3 in CityGML is geometric detail: the model keeps the shape details LoD2 leaves out. It has no pixel size. 1 cm/px is a common starting point for facades, and the one Skyplanr plans with, but the smallest feature the client needs decides the GSD.
What GSD do I need for an LoD2 model?
LoD2 is a generalized building shape, so roofs and walls must be clear but small details can be missing. Skyplanr starts at 2.5 cm/px, about 70 m from the wall with a Mini 4 Pro in the 12 MP mode.
What is the difference between LOD and LOA?
LOD (Level of Detail in CityGML, Level of Development in BIM) describes what the model contains and how far you can rely on it. LOA, the USIBD's Level of Accuracy, describes how accurate the measured data must be. A brief can name both; the LOA is the accuracy requirement.
Do I need ground control points for an LoD3 model?
For the shape alone, no: the details line up within the model without them. For a model in real coordinates with a known accuracy, yes: ground control points or an RTK drone, plus checkpoints left out of the processing to show the accuracy.
How far from the facade should I fly for 1 cm/px?
About 28 m with a Mini 4 Pro, Mini 5 Pro, Air 3 or Air 3S, 38 m with a Mavic 3 Pro and 48 m with a Mavic 4 Pro, in the 12 MP mode with the camera square to the wall. A tilted camera gives bigger pixels at the same distance.
Sources
Skyplanr is not affiliated with the third-party guides linked here.
- OGC: CityGML 3.0 Conceptual Model, §6.4.4 and §7.2.5 (Levels of Detail 0 to 3, LOD4 dropped) (docs.ogc.org)
- BIMForum: Level of Development Specification 2015, §2.2 Level of Development vs. Level of Detail (bimforum.org)
- USIBD: Level of Accuracy (LOA) Specification (usibd.org)
- LIDAR Magazine: The ASPRS Positional Accuracy Standards, Edition 2 (30 checkpoints, RMSE) (lidarmag.com)
- Pix4D community: check points versus GCPs (check points are only for accuracy reporting) (community.pix4d.com)
- Pix4D: computing the flight height for a given GSD (support.pix4d.com)
- DJI Developer: DJI WPML overview (the KMZ format DJI Fly reads) (developer.dji.com)
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