Global Digital Terrain Models: Evaluation of vertical accuracy against different morphological contexts

Digital Terrain Models (DTMs) are essential representations of Earth’s surface, widely used in topographic mapping, hydrological modeling, engineering, and hazard assessment. Advances in satellite imagery, LiDAR, and interpolation techniques have significantly improved DTM generation. This study evaluates the vertical accuracy of four global, freely available DEMs, SRTM 30, ALOS World 3D, Copernicus 30, as DSMs and FABDEM as a DTM, against high-resolution LiDAR-derived reference data across three diverse case study areas in Italy: urban, mountainous, and flat terrains. The assessment framework combined pixel-wise error statistics with zonal analysis using the 2023 ESRI Land Cover dataset. Results highlight that terrain morphology and land cover significantly affect DTM accuracy. Copernicus 30 and FABDEM outperformed the others overall, showing low mean elevation errors and consistent performance across landscape types. Copernicus 30 achieved high accuracy in urban areas (mean difference: 0.48 m), while FABDEM performed well in vegetated and mixed terrains (1.48 m and −0.87 m in Trentino-Alto Adige and Valle d’Aosta, respectively), benefiting from vegetation and building artifact removal. ALOS World 3D showed the poorest performance, with high errors in forested and urban areas, failing to meet its nominal vertical accuracy threshold (<5 m). SRTM 30, while less accurate than Copernicus 30 or FABDEM, remained within its expected accuracy (<16 m) and performed reliably in simpler terrains. Error distribution analysis confirmed these trends: Copernicus 30 showed tightly clustered, low-bias errors; FABDEM had broader but centered distributions; ALOS World 3D exhibited wide skewed errors. Zonal statistics further showed that dense vegetation and urban features caused the largest discrepancies, while bare ground yielded the highest accuracy. Overall, Copernicus 30 is recommended for urban applications, FABDEM for vegetated and mixed-use landscapes, and SRTM 30 for general terrain. ALOS World 3D is currently unsuitable for applications that require high vertical accuracy.