Biomass Estimation Using Digital Photogrammetric Cameras

Estimating and modelling biomass can be helpful for biomass management. It is also interesting to calculate the amount of biomass in a certain area for carbon implications, as indicated by the Kyoto protocol. In the past remote sensing techniques have been able to estimate biomass by using reflectance in the both the red and the near infrared wavelengths. Nowadays the sensors in the digital photogrammetric cameras gather information not only in the visible wavelengths but also in the near infrared wavelengths, which provides the possibility of using digital photogrammetric data for environmental studies. The National Plan of Aerial Orthophotography (PNOA) updates the photogrammetric information (e.g. orthophotographs) in Spain every two years, but so far none of the data derived from the photogrammetric flights is used to extract thematic or biophysical information. Therefore it would be interesting to explore the possibility of establishing a relationship between the biomass and the radiometric information captured by the digital photogrammetric cameras, as an added value to the PNOA deliverables. The aim of this research is determining the suitability of the Ultracam X and Ultracam Xp for biomass estimation in grasslands. There are two different study areas in this research. Field A is located in a grassland area in Barakaldo (Bizcaia, Spain). Field B is located in a grassland area in Cogollos (Burgos, Spain). The aerial photograph of Field A was captured by a digital photogrammetric camera Ultracam X with a Ground Sampling Distance (GSD) of 7 cm, while Field B was flown as part of the PNOA with a digital photogrammetric camera Ultracam Xp and a GSD of 25 cm. The images were calibrated to at-surface reflectance using ten portable reflectance targets with nominal reflectance values of 0%, 25%, 50%, 75% and 100%. An empirical line calibration was performed using the reflectance values for the targets and the corresponding Digital Numbers (DNs) in the images. 20 sample plots were placed in each study area to estimate their biomass and to be characterized radiometrically. Each of the 1 m x 1 m sample plots were radiometrically characterized by using the average radiance/reflectance values obtained for the sub-plots (0.50 x 0.50 m) using an ASD Fieldspec®3 spectrometer. The dried biomass weight was calculated to characterize each one of the plots to be used as a surrogate for the aboveground dry biomass. The data analyses were conducted in two ways: (i) the relationships between the radiometric values registered by the spectrometer for each sub-plot and the biomass estimation were studied and (ii) the relationships between the radiometric data gathered/derived for each sub-plot/plot by the aerial camera and the biomass estimation were investigated. The data was then analyzed by study area and the results of the study areas were compared, in order to study the influence of GSD. * Corresponding author.