TanDEM-X for Large-Area Modeling of Urban Vegetation Height: Evidence from Berlin, Germany

Abstract

Large-area urban ecology studies often miss information on vertical parameters of vegetation, even though they represent important constituting properties of complex urban ecosystems. The new globally available digital elevation model (DEM) of the spaceborne TanDEM-X mission has an unprecedented spatial resolution (12 × 12 m) that allows us to derive such relevant information. So far, suitable approaches using a TanDEM-X DEM for the derivation of a normalized canopy model (nCM) are largely absent. Therefore, this paper aims to obtain digital terrain models (DTMs) for the subsequent computation of two nCMs for urban-like vegetation (e.g., street trees) and forest-like vegetation (e.g., parks), respectively, in Berlin, Germany, using a TanDEM-X DEM and a vegetation mask derived from UltraCam-X data. Initial comparisons between morphological DTM-filter confirm the superior performance of a novel disaggregated progressive morphological filter (DPMF). For improved assessment of a DTM for urban-like vegetation, a modified DPMF and image enhancement methods were applied. For forest-like vegetation, an interpolation and a weighted DPMF approach were compared. Finally, all DTMs were used for nCM calculation. The nCM for urban-like vegetation revealed a mean height of 4.17 m compared to 9.61 m of a validation nCM. For forest-like vegetation, the mean height for the nCM of the weighted filtering approach (9.16 m) produced the best results (validation nCM: 13.55 m). It is concluded that an nCM from TanDEM-X can capture vegetation heights in their appropriate dimension, which can be beneficial for automated height-related vegetation analysis such as comparisons of vegetation carbon storage between several cities. Index Terms-Digital elevation model (DEM), digital terrain model (DTM), morphological filters (MFs), normalized digital surface model (nDSM), Tandem-X, trees, UltraCamX, urban vegetation. I. INTRODUCTION U RBAN vegetation is known to positively regulate urban ecological processes through air purification, noise reduction, urban cooling, and runoff mitigation, as well as to provide aesthetic and environmental benefits that are commonly characterized as urban ecosystem services [1]. In recent Manuscript

Deriving and Evaluating City-Wide Vegetation Heights from a TanDEM-X DEM

“…The comparison reveals an underestimation of mean heights for the total vegetation, and for trees as well (TanDEM-X CHM: 10.46 m; validation CHM: 12.02 m; TanDEM-X CHM for trees: 13.17 m; validation CHM for trees: 15.40 m). In comparison to an earlier study on urban vegetation height assessment [35], the results could be significantly improved (for the earlier study: mean height of 6.66 m for total vegetation in a subset of~70 km 2 ). Contrarily, the mean height for bushes/shrubs from the TanDEM-X CHM is larger than the validation height with 2.90 compared to 2.29 m. The distribution of the quantiles for CHMs for the total vegetation and trees indicates a constant offset to the validation heights, with similar deviations of the 50% and 75% quantiles (~2.25 m).…”

“…A pre-processed TanDEM-X IDEM was available for this study from a former approach [35] where the following steps had been applied: (a) conversion from ellipsoidal to orthometric heights to enable data integration, and (b) removal of random errors by water masking including the biotope map and a provided height error mask followed by local outlier detection using a method of Breunig [46]) was available for this study. The IDEM is a product derived from acquisitions from the first coverage with one baseline configuration only and thus does not profit from dual-/multiple-baseline techniques and incidence angles.…”

“…A pre-processed TanDEM-X IDEM was available for this study from a former approach [35] where the following steps had been applied: (a) conversion from ellipsoidal to orthometric heights to enable data integration, and (b) removal of random errors by water masking including the biotope map and a provided height error mask followed by local outlier detection using a method of Breunig and Kriegel [48]. For this study, the available LiDAR DTM (5 × 5 m) was resampled to the spatial resolution of the TanDEM-X data (12 × 12 m) by aggregating contiguous cells by bilinear interpolation.…”

“…For vegetation height assessments, the spatial resolution of 12 × 12 m (144 m 2 ) is unique, because a single pixel comprises mean heights at the individual level as well as mean heights at the stand level, a measure often used in forestry [32]. First studies derived promising results for the applicability of an intermediate DEM (IDEM) of the TanDEM-X mission for height assessments within a normalized digital surface model (nDSM) of buildings [33,34] and for deriving canopy height models (CHMs) for urban vegetation [35]. The latter study has shown great assets of a TanDEM-X DEM to derive a digital terrain model (DTM) and subsequently CHMs.…”

“…A recent study has also shown for TanDEM-X data that forest areas could be classified when the respective interferometric data is available [39]. In cases were such information is not available, data fusion with multi-spectral information has returned good results for identifying vegetation-covered areas in TanDEM-X DEM [35].…”