# An Improved CASA Model for Estimating Winter Wheat Yield from Remote Sensing Images
## 2 Study Area and Data
### Data and Processing  
#### 2.2.1 Remote Sensing Data  
The remote sensing data used in this study came from two sources: HJ-1A/B satellite images with a spatial resolution of 30 m and MOD17A2H data with a spatial resolution of 500 m from 2008 to 2009. (1) HJ-1A/B Data Product  
China's independently developed environment and disaster monitoring and forecasting small satellite constellations A and B, called HJ-1A/B, are equipped with CCD sensors with a spatial resolution of 30 m, radiometric resolution of 8 bit, spectral resolution (visible and near-infrared bands:  
Figure 1: Study area: (**a**) Beijing in China; (**b**) Tongzhou and Shunyi districts in Beijing. Green dots are sampling points. 430-520 nm, 520-600 nm, 630-690 nm, and 760-900 nm). When satellites A and B are used in parallel, the return visit period can attain two days [35; 36]. The level-2 products of the HJ-1A/B satellite used in this work are taken from the China Centre For Resources Satellite Data and Application website ([[http://www.cresda.com](http://www.cresda.com)]([http://www.cresda.com](http://www.cresda.com))) [37]. From the beginning of October 2008 to the end of June 2009, high-quality images were downloaded following the principle of no cloud or less cloud in the study area. High-quality remote sensing image data for 39 scenes were screened and reserved for atmospheric radiation correction, geometric correction, image splicing, clipping, and other image preprocessing operations. According to the growth characteristics of winter wheat, the winter wheat growing season was divided into 54 stages at 5-day cycle intervals, 39 of which could be applied to the 39 scenes of the preprocessed HJ-1A/B images and used as input data for the CASA model to extract the NDVI of internal stages. In addition, Savitzky-Golay filter was applied to eliminate image noise for the preprocessed images [38; 39; 40]. Then, a supervised classification method of maximum likelihood combined with decision tree classification was used to complete the extraction of the winter wheat planting area based on processed HJ-1A/B images. The wheat extraction results are shown in Figure 2.  
(2) MOD17A2H Data Product  
MOD17A2 is the primary data product of the Moderate Resolution Imaging Spectrometer (MODIS) with a spectrum range of 400-14400 nm, 36 spectral bands, and a radiometric resolution of 12-bit. It offers the first periodic, near-real-time, repetitive monitoring of global vegetation, and provides Gross Primary Production (GPP) synthesized data products with a resolution of 1 km and 8-day [41; 42]. MOD17A2H is an upgrade of MOD17A2, which offers level-4 products with a spatial resolution of 500 m and temporal resolution of 8-day. For the 15 stages of the study not covered by the HJ-1A/B data with medium spatial resolution, MODIS land standard MOD17A2H data products were used for interpolation in 2008 and 2009,  
Figure 2: Winter wheat planting area in Tongzhou and Shunyi districts of Beijing in 2009.  
which were downloaded from the NASA website ([[https://ladsweb.modaps.eosdis.nasa.gov/](https://ladsweb.modaps.eosdis.nasa.gov/)]([https://ladsweb.modaps.eosdis.nasa.gov/](https://ladsweb.modaps.eosdis.nasa.gov/))) [43]. Its data preprocessing mainly included projection conversion, data mosaic, clipping, and so on. The MOD17A2H data products of spatial resolution were changed from 500 m to 30 m by resampling in order to extract the winter wheat images of single vegetation. In addition, the MOD17A2H value represented the daily cumulative value of GPP in 8-day, which was multiplied by \(\frac{5}{8}\) can be considered as the cumulative value of GPP for five days. Gross primary production (GPP) is the total amount of organic matter produced by photosynthesis of green plants, which is the sum of NPP and autotrophic respiration [44]. Since the NPP model was used to estimate the winter wheat yield in this work, the GPP generated by winter wheat photosynthesis was transformed into the NPP.