FarEarth automatically creates accurate and calibrated imagery. What you may not know is it also generates a standalone report with every product using the disparities calculated during processing. This is very useful for you to independently review the product accuracy compared to trusted references, such as Sentinel 2.
You can look at an example report here.
Below is an example of a satellite image which FarEarth has calibrated, band-aligned and precision orthorectified. The image was processed from a raw and uncalibrated small satellite dataset.
Geodetic accuracy assessment
Every report includes a diagram showing how well the image was geodetically corrected. Three measurements are shown.
- The geolocation of the raw image is outlined in blue. This would have been the location of the image, if only the satellite navigation and attitude data were used.
- With systematic processing the geolocation is improved as shown in the red outline. This is a good representation of the satellite’s calibrated pointing accuracy.
- To very accurately geolocate the image, FarEarth performs a precision alignment against reference data. This final refined outline is shown in green.
For our example, the report shows that with only systematic alignment, the image would have been geolocated 73 meters away from the true location.
A verification image below shows how well the resulting product aligns with the reference imagery after precision alignment. The same features are found and correlated on both images. These are called tie-points. The distance between each set of the points is calculated and highlighted in different colours. This disparity is expressed in the pixel size of the product image. If the product image has a pixel size of 3 meters, a ½ pixel disparity indicated in green means the alignment is within 1.5 meters.
The same tie-points can be visualized as a scatter plot. These points are shown in red with a disparity being the distance from the center. The red outline is the circular error at 95% boundary of these disparities. For easy reference, the pixel size of the product is shown in green (in this case 3 meters). The pixel size of the Sentinel 2 reference is shown in blue (in this case 10 meters).
For our example, this means that 95% of the tie-points are within approximately 2 meters of the reference (in other words, aligned within 0.2 pixels).
Band alignment accuracy
The previous measurements show how well our image is aligned to Earth. Another important aspect of a quality image is to align the different bands precisely with each other. For every band in our image, FarEarth reports the accuracy of the alignment with a scatter plot like this one. It shows how well the band aligns with another band in the same image.
In our example, the green circle indicates the resolution of the RED band (about 3 meters). The blue circle indicates the resolution of the NIR2 band (about 6 meters). The red tie points show the disparity between these two bands, along with a CE95 outline (about 2 meters). We can see that bands are well aligned within 0.3 pixels.
Product meta-data details
Another nice feature of the report is the product meta-data properties. It includes traceability information about the product generation such as the input products and ancillary references. It also shows details of the scene, such as solar angle, altitude and incidence angles.
The L1C quality report is available for download along with every product in the FarEarth Catalogue.