Building segmentation for Croatian DOF5 orthophotos (U-Net / ResNet-34)
Binary building-footprint segmentation model for Croatian official
digital orthophotos (DOF5) published by the State Geodetic
Administration (Državna geodetska uprava, DGU). Given a 3-band RGB
orthophoto chip at ~0.5 m/pixel, it predicts a per-pixel probability of
"building". Developed for a longitudinal construction-monitoring study of
the City of Kaštela (Split-Dalmatia County), 2011–2023.
- Author: Dino Novak
- Architecture: U-Net with ResNet-34 encoder (ImageNet-initialized),
built with segmentation-models-pytorch
- Input: 3×512×512 RGB, per-chip per-channel z-score normalization
- Output: 1×512×512 logits (sigmoid → building probability)
- Weights:
unet_best.pt (PyTorch state_dict, ~98 MB)
Training data (all public/open sources)
Imagery — DGU DOF5 digital orthophoto, 2023 survey. The official
state orthophoto map of the Republic of Croatia at 1:5000 scale, RGB,
native 20–25 cm GSD, served openly via the DGU Geoportal WMS
(
https://geoportal.dgu.hr/services/inspire/orthophoto_2023/wms,
"Digitalni ortofoto 2023. – WMS za anonimne korisnike"). Downloaded and
resampled to 0.5 m/px over the study area (City of Kaštela and
surroundings, ~215 km², HTRS96/TM EPSG:3765). Terms: Croatian Open Data
Licence (
https://data.gov.hr/otvorena-dozvola), attribution:
Državna
geodetska uprava, Republika Hrvatska.
Labels — DGU cadastral building polygons (INSPIRE Buildings theme).
"Zgrade iz Digitalnog katastarskog plana – INSPIRE" served via public WFS
(https://api.uredjenazemlja.hr/services/inspire/bu/wfs, layer
BU.Building). ~38,000 building polygons within the study area were
rasterized over the 2023 mosaic as binary masks. Attribution: Državna
geodetska uprava / Uređena zemlja.
Dataset: ~1,900 chips of 512×512 px (0.5 m GSD), all chips containing
buildings plus 25% of building-free chips (hard negatives: orchards,
vineyards, bare karst, roads, greenhouses). 85/15 train/validation split.
Label noise is inherent and accepted: unregistered buildings appear as
unlabeled roofs; a small number of registered buildings post-date the
2023 imagery. The model demonstrably still learns to detect unregistered
buildings, since they are visually identical to registered ones.
Training procedure
BCE + Dice loss, AdamW (lr 3e-4, cosine annealing), batch 16, mixed
precision, early stopping on validation IoU. Augmentation: flips, 90°
rotations, and photometric augmentation designed for epoch transfer
(per-channel gamma 0.6–1.6, Gaussian blur σ 0.5–1.8, sensor noise) so the
model generalizes from the 2023 survey to older, hazier DOF5 epochs
(2011, 2014/16, 2017, 2019, 2021) despite different cameras and
atmospheric conditions. Per-chip z-score input normalization additionally
removes linear radiometric differences between epochs.
Evaluation
Pixel validation IoU on held-out 2023 chips: 0.526 (40 epochs, early
stopping; best at epoch 30). Note that this is measured against noisy
cadastre labels, so instance-level performance is materially better than
the pixel score suggests. An ablation without photometric augmentation
scores nominally higher on same-epoch validation (0.541) but transfers
far worse to older DOF5 epochs (e.g. 25.5% vs 61.5% cadastre coverage on
the 2017 epoch) — the augmented model is the published one.
Cross-epoch recall proxy — share of cadastre buildings (≥25 m²) covered
30% by predicted footprints, City of Kaštela (note: the cadastre is
cumulative-to-2026, so early epochs are expected to cover less of it —
buildings did not exist yet):
| DOF5 epoch | coverage >30% | median coverage |
|---|
| 2011 | 48.7% | 0.27 |
| 2017 | 61.5% | 0.60 |
| 2019 | 57.6% | 0.52 |
| 2021 | 69.6% | 0.72 |
| 2023 | 80.5% | 0.87 |
Intended use
- Building-footprint extraction from DGU DOF5 RGB orthophotos
(2011 and newer epochs, 20–50 cm native GSD, used at 0.5 m/px).
- Multi-epoch change detection / construction monitoring: comparing
footprints between DOF5 surveys to locate new construction, including
buildings absent from the cadastre.
- Geography: trained on Dalmatian coastal settlement patterns (terracotta
and flat grey/white roofs, karst, Mediterranean vegetation). Expected to
transfer to similar Croatian coastal areas; validate before relying on
it for continental Croatia or other countries.
Out of scope / limitations
- Not trained on satellite imagery, other national orthophoto programs,
or GSD coarser than ~0.6 m — performance there is unknown.
- Touching buildings in dense historic cores can merge into one polygon
(raise the threshold to ~0.6 to mitigate).
- Occasional confusion with large containers, paved terraces.
- Not a legal instrument. Outputs are probabilistic image analysis and
must not be the sole basis for enforcement, legalization, or property
decisions. Croatian regulations treat the DOF5/2011 orthophoto as a
statutory reference for legalization; any official determination
requires human verification against authoritative sources.
How to use
1import numpy as np, torch, segmentation_models_pytorch as smp
2
3model = smp.Unet("resnet34", encoder_weights=None, in_channels=3, classes=1)
4model.load_state_dict(torch.load("unet_best.pt", map_location="cpu"))
5model.eval()
6
7def zscore(img): # img: float32 (3,H,W)
8 m = img.mean(axis=(1, 2), keepdims=True)
9 s = img.std(axis=(1, 2), keepdims=True) + 1e-6
10 return (img - m) / s
11
12chip = ... # (3,512,512) uint8 RGB from a DOF5 orthophoto at 0.5 m/px
13x = torch.from_numpy(zscore(chip.astype(np.float32))[None])
14with torch.no_grad():
15 prob = torch.sigmoid(model(x))[0, 0].numpy() # (512,512) in [0,1]
16mask = prob > 0.5
inference_example.py in this repository is a complete sliding-window
script: GeoTIFF/VRT in → building-footprint GeoPackage out.
Attribution & license
Model weights and card: CC-BY-4.0. The model derives from open data of
the Republic of Croatia; when using it or its outputs, attribute:
Imagery and cadastral data: Državna geodetska uprava (DGU), Republika
Hrvatska — DOF5 digital orthophoto (geoportal.dgu.hr) and INSPIRE
Buildings / Digitalni katastarski plan (uredjenazemlja.hr), Croatian
Open Data Licence (data.gov.hr/otvorena-dozvola).
