GIMMS NDVI3g V1

目前GIMMS NDVI3g更新到v1版本，空间覆盖全球，时间范围1981-2015年，空间分辨率1/12°（约8km）,时间分辨率为15天。是目前时间范围最长的NDVI数据产品。

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比较V0版本的变化是：

1. 数据做成了netcdf4文件格式。半年一个nc文件，每月2期数据。
2. flag简化。
3. 增加percentle变量。

Comments Version1:

version1 includes two major fixes (a and b), and three minor (c-g):

(a) Reprocessed Level 2 entire SeaWIFS mission for the land products to reduce artifacts in the data, particularly changes in calibration after 2006 that generates drops in ndvi lower values. OB.DAAC / Ocean Biology Processing group NASA/GSFC 616 (april 2016)

(b) Recovered ndvi negative values of snow-covered regions in winter Northern latitudes. In Version0, we masked them with zero values, creating artifacts in phenology parameters.

(c) Arranged data in ncd format, compiled it in two nc4 files a year. Each nc4 file includes 6 months of ndvi data (jan-jun and jul-dec), with a total of 12 (15-day) composites each semester.

(d) Rescaled ndvi values and splitted the flag values from them.

(e) Added a new variable, percentile, to represent the distribution of ndvi values in the time series. Range 10*[0, 100]

(f) Flag values are (simpler): flag 0: ndvi without apparent issues (good value) flag 1: ndvi retrieved from spline interpolation flag 2: ndvi retrieved from seasonal profile (possible snow/cloud)

(g) Flag values are embeded on the percentile variable: 2000*flag + percentile. Thus, the actual percentile three ranges [0 1000], [2000 3000] and [4000 5000] could provide direct information of how interpolation is affecting the time series.
Temporalrange: 1981-07-01 -> 2015-12-31
Year: 1981
RangeSemester: Jul 1 - Dec 31 (7:0.5:12.5)
SpatialResolution: 1/12 x 1/12 degrees
TemporalResolution: 1/24 a year
_fill_val: -32768
NorthernmostLatitude: 90
SouthernmostLatitude: -90
WesternmostLongitude: -180
EasternmostLongitude: 180