Processing steps to generate a Digital Surface Model based on SPOT-7 tri-stereo images published in the study "An assessment of the effects of DEM quality and spatial resolution on a model for mapping lahar inundation areas at volcan Copahue (Argentina & Chile)" in the Journal of South American Earth Sciences https://doi.org/10.1016/j.jsames.2022.104138

The Digital Surface Model (DSM) was created from SPOT-7 tri-stereo images for the Copahue volcano between the border of Argentina and Chile. Two versions of the DSM are provided: an unfiltered product and a final, filtered product. The final product has a spatial resolution of 5-m and was used for lahar inundation modeling for the Copahue volcano (Viotto, Toyos, and Bookhagen 2022, https://doi.org/10.1016/j.jsames.2022.104138 : An assessment of the effects of DEM quality and spatial resolution on a model for mapping lahar hazard inundation at Volcán Copahue (Argentina & Chile). Journal of South American Earth Sciences ). The dataset provided should be cited together with the article. 

DSM processing 

The source images were given by a SPOT-7 snow- and cloud-free triplet (Nadir, Backward and Forward) of 1.5 m spatial resolution from 19 April 2018 (SPOT Image, Airbus Defence and Space GmbH, distributed by CONAE; Dataset ID:  SEN_SPOT7_20180419_142955500_000, delivered by CONAE as DS_SPOT7_20180419).

The data were processed with the suite of digital photogrammetry tools AMES Stereo Pipeline ASP (Beyer et al., 2018). The procedure for the generation of the DSM is summarized by following steps: 

The orbital parameters (RCP models) were adjusted using the bundle adjustment tool with no ground control points, since they were unavailable.

The scenes were map-projected onto the NASADEM (spatial resolution of 30 m)  elevation dataset, assisted by the results of the orbital adjustment in Step 1.

The stereo correlation of the map-projected scenes including the results of the adjusted orbital parameters, was performed three times, using as first scene (i.e., primary image) the nadir (N), backward (B), and forward (F) images . In each run, the order of images to perform the stereo correlation was: N-F-B, F-N-B, and B-N-F. Thus, three point clouds were generated. Specific ASP correlator settings (other than defaults parameters; for details see the provided stereo-default file) were set in the following way:  Correlation Kernel: 15 x 15 pixels; Sub-pixel Refinement Kernel: 21 x 21 pixels; Subpixel Refinement Mode: 2 (Weighted Affine Adaptive Window Correlator EM)

The three point clouds were merged into one point cloud with a regular grid of 5 m (unfiltered product, known as DSM_Copahue_UTM19S_WGS84_5m_raw.tif).

The quality of the final point cloud was assessed by comparing the unfiltered DSM with a spatial resolution of 12-m against the WorldDEMTM  elevation dataset (Collins et al., 2015). The WorldDEM was provided by Airbus Defence and Space GmbH under license for the scope of the Viotto et al., 2022 study. The comparison of the pixel-to-pixel heights above the ellipsoid (WGS84) between the two datasets resulted in a mean difference of 0.67 m and a standard deviation of +/- 4.82 m. 

Comprehensive details on the methodologies evaluated  to create the dataset with ASP, can be found in the corresponding master's thesis  “Topografía digital y modelado de lahares en el Volcán Copahue, Argentina-Chile” from S. Viotto (link: https://rdu.unc.edu.ar/handle/11086/15384). Recommended literature about processing DEMs from SPOT imagery is given by Mueting et al., 2021 (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021JF006330). 

Creation of the Final, Filtered DSM product

The corrections and improvements applied to the unfiltered product to create the final, filtered DSM (named DSM_Copahue_UTM19S_WGS84_5m_VoidFilled.tif) are summarized by following steps. 

Water Bodies Delineation

The delineation of the water bodies was based on a mask created from the free access water bodies datasets provided by the Instituto Geográfico Nacional of Argentina (https://www.ign.gob.ar/ NuestrasActividades/InformacionGeoespacia l/CapasSIG) and by the Ministerio de Bienes Nacionales in Chile ( https://www.ide.cl/index.php /aguas-continentales/item/1508-catastro-de-lagos). A total of 45 lakes within the area of interest were considered. Lakes with areas below or equal to 25 m2 were smoothed with a median filter in the last step. Lakes with areas  above this threshold were filled in with a constant value and their borders  were smoothed with a median filter to provide smooth shorelines.

2 . Void Filling

Voids (other than water bodies) were filled with the tool “Close Gaps” from Saga GIS software.  

3. Smoothing

Finally, the elevation dataset was smoothed with a median filter using a 3 x 3 pixel  window, excluding water bodies filled in the step 1.  

Final Remarks and Suggestion

The quality assessment of the final version by visual inspection of the hillshades suggested an improvement of the signal to noise ratio. However, the void filling process may be improved.

Dataset Description

Digital Surface Models

No Data Value = -9999

Format = float 32 bit

File Format = GeoTiff

Vertical Datum: WGS84

Projection information: EPSG 32719 (UTM19S)

Spatial Resolution: 5m (subfix: _5m) 

Versions: 

Unfiltered product: without corrections DSM_Copahue_UTM19S_WGS84_5m_raw.tif

Final, filtered product: smoothed and void filled DSM_Copahue_UTM19S_WGS84_5m_VoidFilled.tif

Water Bodies Mask

No Lake Value = 0

Lakes Values = 1 to 45

File Format= GeoTiff

Spatial Resolution: 5m (subfix: _5m)

Projection information : EPSG 32719 (UTM19S)

WB_mask_5m_UTM19S.tif

Repository structure

|__ 01_Scripts

    |+ run21_CopahueDSM_AMES_sviotto.sh

    |+ stereo.default

|__ 02_DSMs

    |+ DSM_Copahue_UTM19S_WGS84_5m_raw.tif

    |+ DSM_Copahue_UTM19S_WGS84_5m_VoidFilled.tif

    |+  WB_mask_5m_UTM19S.tif

References

Beyer, R. A., Alexandrov, O., & McMichael, S. (2018). The Ames Stereo Pipeline: NASA's open source software for deriving and processing terrain data. Earth and Space Science, 5, 537– 548. https://doi.org/10.1029/2018EA000409

Collins, J., Riegler, G., Schrader, H., Tinz, M., 2015. Applying terrain and hydrological editing to TanDEM-X data to create a consumer-ready worlddem product. Int. Arch. Photogram. Rem. Sens. Spatial Inf. Sci. 40 (7), 1149. https://doi.org/10.5194/isprsarchives-XL-7-W3-1149-2015.

Mueting, A., Bookhagen, B., & Strecker, M. R. (2021). Identification of debris-flow channels using high-resolution topographic data: A case study in the Quebrada del Toro, NW Argentina. Journal of Geophysical Research: Earth Surface, 126, e2021JF006330. https://doi.org/10.1029/2021JF006330

Viotto, S., Toyos, G., & Bookhagen, B. (2022). An assessment of the effects of DEM quality and spatial resolution on a model for mapping lahar hazard inundation at volcán copahue (Argentina & Chile). Journal of South American Earth Sciences, 104138.  https://doi.org/10.1016/j.jsames.2022.104138