Level-03 Image-Product Geometry of UltraCam Condor 4.1
The delivered Vexcel UltraCam Condor 4.1 Gleisdorf dataset carries several co-existing Level-03 image products — panchromatic, RGB, colour-infrared, and a 270°-oriented colour variant — alongside surface models and an orthophoto mosaic. Every Level-03 product shares a single exterior orientation per exposure and differs only in ground sample distance, extent, and resampling orientation. Tested against 165 ground-control measurements, a frame-collinearity model fits to a 2.4 px median while treating the products as orthophotos through their world-file georeference leaves 50 px (≈18 m): the Level-03 products are perspective frames, not orthophotos. Panchromatic and colour co-register to ≈0.08 m. Because all products share one orientation, a single independent aerial-triangulation solution refines every product at once, and the colour inherits panchromatic accuracy without additional geometric loss.
1. Products
48 exposures over Gleisdorf, Austria; EPSG:25833. Each Level-03 image carries a TIFF world file only; a single block exterior-orientation file holds one record per exposure (X, Y, Z, ω, φ, κ). The separately delivered DTMOrtho mosaic — north-up, DTM-rectified, seam-lined, 552 tiles at 15 cm — is the dataset's only true orthophoto.
| Level-03 product | Pixels | GSD |
|---|---|---|
| 0° PAN, 8-bit (48 exposures) | 20488 × 14040 | 0.356 m |
| 0° RGB, 8-bit (48) | 48462 × 6150 | 0.150 m |
| 0° CIR, 8-bit (48) | 48462 × 6150 | 0.150 m |
| 270° RGB, 8-bit (48) | 6150 × 48462 | 0.150 m |
2. Shared exterior orientation
For a representative exposure every Level-03 product reports an identical scene centre and an identical grid rotation equal, to six figures, to the exposure's orientation κ. Each product is therefore the frame resampled into a grid aligned to the camera attitude and georeferenced from the shared orientation; they carry no independent orientations. The apparent difference in geometry between panchromatic and colour is a difference of ground sample distance, extent, and storage orientation only: the panchromatic product spans the full ≈7287 × 4994 m frame footprint at 0.356 m, the RGB/CIR products cover the full swath width but only the central ≈925 m along-track at 0.150 m (2.37× finer), and the 270° product is the same RGB data resampled to a grid rotated 90°.
3. Frame versus orthophoto
Whether the Level-03 products are perspective frames or orthophotos determines whether an aerial-triangulation solution can re-rectify them. Two models were evaluated against the 165 ground-control image measurements: a frame model (collinearity through the exterior orientation) and an orthophoto model (the inverse of the supplied affine world file, mapping ground position to pixel independently of height).
| Model | Median residual | Max |
|---|---|---|
| Frame (collinearity, exterior orientation) | 2.4px | 83 px |
| Orthophoto (world file) | 49.9 px (17.8 m) | 270.7 px (96 m) |
The frame model fits roughly 20× better. The world-file residual correlates with terrain relief and, read as relief displacement, implies a flying height consistent with the true ≈4.9 km above ground. The Level-03 products are therefore perspective frames; the world file is a nominal-elevation quicklook georeference that leaves tens of metres of uncorrected relief displacement. The true orthophoto is the separate DTMOrtho product.
4. PAN / colour co-registration
The 0.150 m colour strip and 0.356 m panchromatic frame for a common exposure were resampled to a shared 0.5 m grid through their world files and compared over their full overlap. Global normalised cross-correlation is 0.984 with a residual offset of 0.064 m (E) / 0.051 m (N) — ≈0.08 m total, 0.16 px at the test grid, below the resampling noise floor. Sharing the orientation and the same frame perspective, panchromatic and colour are geometrically locked: any orientation refinement shifts them identically.
5. Implications
Because all Level-03 products are perspective frames sharing one orientation per exposure, an independent bundle adjustment that refines that orientation applies to every product at once. The independent solution for this block — image σ₀ 0.93 px against the factory 1.87 px, fitted-control ground RMS 0.044 / 0.042 / 0.053 m — is below the factory image-space figure, so an AT-driven re-orthorectification supersedes the quicklook world-file georeference and the colour products inherit panchromatic accuracy without additional geometric loss. The full accuracy comparison is reported in the orthorectification case study.
6. Report
Product-geometry report (PDF, 3 pp)
Study data: Vexcel UltraCam Condor 4.1 — Gleisdorf, Austria (delivered dataset, public). Contact: info@ag2i.ai.