Co-registeration and Inter-sensor Comparison of Modis and Landsat 7 Etm+ Data Aimed at Ndvi Calculation

To evaluate accuracy of low resolution vegetation mapping for hydrological purposes, a comparative study of NDVI images derived from MODIS and Landsat 7 ETM+ data has been done. Main goal is to understand how effective MODIS images can be for vegetation characterization on large areas, as compared to the Landsat 7 ETM+ ones. In this paper a methodology is proposed with the aim of measuring the difference between NDVI values derived from the two different data, considering synthetic parameters and investigating their dependency on the geometric resolution of the images. Great attention was paid to the problem of the geometric co-registration of the two types of data. This is a very sensitive parameter for the subsequent analysis. A mixed approach was adopted: images were firstly orthoprojected to eliminate sensor geometry and relief displacement effects; subsequently, a refining image-to-image co-registration procedure was carried out through a homographic transformation implemented in a self-developed routine. Two pairs of contemporary images (MODIS and Landsat 7) were used as benchmarks for our tests. Simplified procedures aimed at calibrating images and at removing atmospheric noise were performed. The resulting corrected images were used to calculate NDVI images. These ones (two pairs) were then compared through a statistical approach in order to investigate how a different geometric resolution can influence the NDVI values. The proposed approach is not a traditional image based (matrix comparison) but a new one. NDVI value correspondences were considered between the MODIS pixel and the group of Landsat pixels belonging to the polygon which represents the considered MODIS pixel in the Landsat image space. Statistics extracted on-the-fly from these Landsat pixels were used to investigate in depth the relationship between them and NDVI value of the corresponding MODIS pixel. NDVI differences were calculated between the single NDVI MODIS values and a synthetic parameter (mean value) of the homologous Landsat pixel group. A direct comparison between the NDVI values obtained from MODIS and Landsat 7 images has shown a systematic error that can be read as bias (MODIS NDVI over estimation). This led the authors to determine a suitable model in order to eliminate the bias, whose presence would have conditioned later comparisons. Original MODIS image was then corrected through the defined model. This has been designed to be suitable for any MODIS image acquired over the same area (parameterization was used). New NDVI differences were calculated using the corrected MODIS images and the previous Landsat 7 ones. In order to investigate the nature of the residual differences and to try to recognize the critical MODIS pixels, some considerations were made concerning the statistics of each corresponding group of Landsat pixels. A classification of the MODIS pixel was generated according to the behaviour of their differences with respect to the adopted statistics.