Fraction of green vegetation, f_g is needed as one of regular parameters for vegetation cover analysis. The paper explores the potentials of deriving this variable from thermal mapper (TM) normalized difference vegetation index (NDVI) data considering the leaf area index (LAI) of agricultural field. Geometric, radiometric and atmospheric correction of the images were performed before further analysis. According to the sub-pixel structure characteristic, we choose mosaic-pixel model for calculating percent vegetation cover. A new method was put forward to achieve LAI values of non-dense vegetation where soil line equation was considered. Two schemes are produced to obtain different LAI values and type-specific accuracy is evaluated using parameter defined in this paper.

Satellite data have been widely used in the detection of vegetation area changes, however, the lack of historical training samples seriously limits detection accuracy. In this research, an iterative intersection analysis algorithm (IIAA) is proposed to solve this problem, and employed to improve the change detection accuracy of Phragmites area in the Detroit River International Wildlife Refuge between 2001 and 2010. Training samples for 2001, 2005, and 2010 were constructed based on NAIP, DOQQ high-resolution imagery and ground-truth data; for 2002–2004 and 2006–2009, because of the shortage of training samples, the IIAA was employed to supply additional training samples. This method included three steps: first, the NDVI image for each year (2002–2004, 2006–2009) was calculated with Landsat TM images; secondly, rough patches of the land-cover were acquired by density slicing using suitable thresholds; thirdly, a GIS overlay analysis method was used to acquire the Phragmites information in common throughout the ten years and to obtain training patches. In the combination with training samples of other land cover types, supervised classifications were employed to detect the changes of Phragmites area. In the experiment, we analyzed the variation of Phragmites area from 2001 to 2010, and the result showed that its distribution areas increased from 5156 acres to 6817 acres during this period, which illustrated that the invasion of Phragmites remains a serious problem for the protection of biodiversity.