Abstract. National airborne laser scanning (ALS) datasets
provide essential three-dimensional information for large-scale
vegetation metric calculation, but their ecological use is constrained
by multi-year acquisition cycles, seasonal variation, heterogeneous
point densities, and inconsistent point classification. These factors
reduce the reliability and comparability of vegetation metrics derived
directly from national ALS archives. We developed a harmonised,
reproducible point-cloud preprocessing framework for national-scale
ecological applications of ALS data. The framework corrects metadata and
file-format inconsistencies, removes overlap-related sampling artefacts,
applies digital terrain model (DTM)-based height normalisation, and
performs deterministic rule-based reclassification using height
information, seasonal normalised difference vegetation index, and
ancillary vector data. The workflow was applied to selected 2019–2024
acquisition campaigns of the Estonian national ALS archive, comprising
approximately 132 000 LAZ files from leaf-off and leaf-on
surveys. Harmonised preprocessing increased thematic consistency of
vegetation, ground, building, and water classes across acquisition
campaigns by correcting systematic classification and sampling
artefacts. Removal of overlap-flagged points reduced local point-density
inflation by approximately 25 %, thereby improving comparability of
later grid-based vegetation metrics. Rule-based reclassification
reassigned a median of 32% of previously unclassified returns to
vegetation, increasing completeness and seasonal consistency of
vegetation representation. DTM-based height normalisation produced
stable and interpretable height distributions across campaigns while
retaining expected seasonal structural differences.By providing a more
consistent point-level representation of vegetation structure, the
framework offers a robust basis for vegetation metric calculation and
national-scale ecological analysis using heterogeneous ALS archives.
Reproducibility review available at: https://doi.org/10.17605/OSF.IO/75pwn