# Harmonizing Multi-Source Remote Sensing Images for Summer Corn Growth Monitoring
## 3 Results and Analysis  
### Accuracy Assessment Results of Image Harmonizing  
The spatiotemporal fusion of images using the improved Kalman filter preserves the high temporal resolution of low spatial resolution images, but the product results also inherits high spatial resolution characteristics. The spatial resolution of the harmonized images is 30 m, and the temporal resolution is determined by the time interval of the MODIS source images. For example, if all MOD09 GA images are available, then the temporal resolution of the harmonized images is daily. Similar spatial details can be used to evaluate qualitatively the accuracy of image harmonizing using the improved Kalman filter, and temporal variation trends can be obtained. To reveal the spatial details of harmonized image clearly, we chose a farmland with an area of about 5000 m \(\times\) 5000 m, as shown in Figure 4a-e, which is consist of farmland (about 75%), residential areas (about 20%) and other areas (about 5%). The Figure 5 represents spatial values for the NDVI calculated using GF-1 image, MODIS image and harmonized image, respectively. As it can be seen from Figure 5, there are only four image pairs of GF-1 image and MODIS image on DOY 156, DOY 178, DOY 227 and DOY 252. Excepting for these four image acquisition dates, there are seven MODIS images without simultaneous collected GF-1 images. In order to verify the accuracy of spatiotemporal fusion model, the image acquired on DOY 178 and DOY 227 are not used for image harmonizing, and only for assessing the image harmonizing accuracy. Figure 5 shows that the harmonized images on DOY 156, DOY 178, DOY 227 and DOY 252 have a similar spatial variation pattern with the simultaneous MODIS images, visually. Moreover, the harmonized images on DOY 178 and DOY 227 have very similar spatial details with the simultaneous GF-1 images. These two similarities reveal the harmonized images carry on the time trajectory of MODIS time-series images and detailed spatial specifics of GF-1 images. From the time-series harmonized images and MODIS images, the corn phenological growing changes in the study area can be inferred clearly. NDVI shows a tendency to increase slowly at early growing stage,  
Figure 4: The subset area was chosen to demonstrate results for better visualization: (**a**) is the study area; (**b–e**) are the subset area of GF-1 image (R\(\backslash\)G\(\backslash\)B = Band 3\(\backslash\)2\(\backslash\)1) in DOY 156, 178, 227 and 252.  
and change to increase quickly at the middle growing stage, and then decrease slowly at the end of growing stage. From the zoomed time-series images, we can see that the phenological changes are basically consistent for the harmonized images and MODIS images. Normally, the NDVI values with the maximum probability density can show the temporal and spatial variations and differences for studied summer corn more clearly, because they represent the overwhelming majority of summer corn plants growing condition. Figure 6 shows the time series of NDVI value with the maximum probability density resulting from time-series GF-1 images, harmonized images and MODIS images. Specifically, the NDVI value with the maximum probability density generally ranges from 0 to 0.5 before DOY 163, and increases to around 0.5 on the DOY 178, and increases to a range from 0.6 to 0.9 on DOY 194. Moreover, there are two peaks for the probability density distribution of NDVI values of GF-1 and harmonized images on DOY 227, with one lower peak around 0.4 and another higher peak around 0.8. Unfortunately, the time-series MODIS images show a single peak only, which cannot capture this little spectral variation. This comparation reveals that the harmonized images can carry on the fine spatial resolution of the GF-1 images. Another comparation is about the difference of three maximum probability density curves of GF-1, MODIS and harmonized images on DOY 178 shown in Figure 6. The mean NDVI value of three maximum probability density functions are all around 0.5.