Data quality can be checked by applying strong stretches to the data and examining spectra of known material, as well as visually examining the image for missing data lines or rows. Surprisingly the data acquired in 1972 was the best quality, while data acquired in 1980 was missing a large area of data as well as scattered lines throughout the scene. It was important to determine these data quality complications early so that they could be accounted and corrected for throughout the image processing procedures. This step allows for specific pixels to be correlated to specific geographic areas on earth. Later this information can be integrated with geocoded statistical data or georegistered political lines and boundaries.
Radiometric alignment is vital to any change analysis.
These differences are even more acute when data is acquired by different sensors. In order to compare values of reflectance from one scene to another, it is necessary to align the DN values of each scene to a reference scene. These are areas that should remain spectrally constant over time and therefore should be represented by equal values in all scenes. Such areas generally include deep reservoirs, and pure sand piles or soil with little human or natural disturbance or vegetation. It is important to use a wide range of intensities for improved accuracy.