## Materials and Methods
### Module Description
#### 2.3.3 Orthorectification  
Orthorectification compensates for sensor orientation and terrain displacement through a geometric transformation onto a projected coordinate system. The required input is the satellite image, its corresponding RPC files, and a DEM that covers the satellite scene.  
Satellite image products are typically delivered in the Standard2A or ORStandard2A processing level, which means that the pixel coordinates are not translated into geographical positions. Some deliveries took place in 3A format, so these were already orthorectified. Off-nadir angles range considerably, and are as high as 30 degrees in record images in side-looking configurations in areas of high cloud density; it is an important cause of inconsistent georeferencing between images. One is generally advised to work with off-nadir angles below 20 degrees [36], otherwise the images will present problems in plant canopy studies. Our orthorectification is applied to all images that are not yet orthorectified.  
Several solutions exist for performing this task, and, in a study [37] that compared orthorectification methods for optical satellite data, it was found that ENVI, Erdas Imagine, PCIGeomatics, and Xdibias provided very similar absolute geometric accuracies; in our workflow implementation, GDAL is used. The DEM is used to reduce localization errors induced by jagged surface topography. A visual investigation of the Advanced Land Observing Satellite (ALOS) DEM provided by JAXA and the SRTM DEM by NASA, both freely available and with resolution of 1 arc-second (30 m), revealed that these two datasets are similar in the STARS use case areas. Eventually, we selected the SRTM because the ALOS DEM is, in essence, a Digital Surface Model (meaning that it does not account for understory vegetation height), and it is filled-in from other DEMs in areas with cloud coverage. The needed SRTM tiles covering our use case areas were downloaded in GeoTIFF format from the EarthExplorer website, stitched together, subsetted to the areas of interest, and projected onto a map coordinate system (WGS 84/respective UTM zone).  
We evaluated the geometric accuracy of a delivered WV-3 ORStandard2A image acquired on 18 May 2015 at 10:46 over Sukumba, Mali, the respective orthorectified product, and the co-registered product on a master image, as described in Section 2.3.4. The comparison was made against 41 Ground Control Points (GCPs) recorded in the field with a differential GPS device (Table 3). We used the 30 cm panchromatic image for better clarity to distinguish the GCPs cross marks. The results present considerably lower RMSE\({}_{\text{X}}\) and RMSE\({}_{\text{Y}}\) for the co-registered image (1.547 m and 0.361 m respectively), in comparison to the raw image (6.616 m and 1.886 m respectively). However, when the latter was orthorectified, it produced a similar RMSE\({}_{\text{Y}}\) (1.521 m) but a higher RMSE\({}_{\text{X}}\) (11.527 m). Looking into the mean and standard deviation of the absolute residual values, it is clear that the orthorectified product has a higher mean but a considerably lower standard deviation of the absolute residual value, which translates into a larger average misregistration but, at the same time, a consistent shift throughout the image, contrary to the raw product, which has higher standard deviations and residuals across the images that are variable. Obviously, the co-registered (i.e., to a master) image produces an overall RMSE that is lower than the spatial resolution of the specific multispectral image, which is 1.2 m.