Monthly Global NDVI at 5 km based on AVHRR product - 1982 to 2019

Long-term monthly NDVI calculated through daily images of AVHRR (NOAA/CDR/AVHRR/NDVI/V5), which were cloud screened and aggregated by percentile 90th on Google Earth Engine-GEE. All the images were exported from GEE to R environment where the follow procedures were executed on a pixel-basis:

Outlier removal based on a temporal moving window, used to identify and remove NDVI outlier values (i.e. greater or less than 1.5 standard deviation calculated on 60-months time window) 

Smoothing and gap filling approach implemented by the greenbrown package (TSGFssa method)

The final product is composed by 456 NDVI global images without gaps and for each month was produced an ancillary image indicating:

Disregarded pixels: -25000 (removed) and -32000 (nodata)

Gapfilled pixels: -20000

Smoothed pixels: all values bigger than -20000 indicate the difference between the original and smothed AVHHR NDVI

To access and visualize the maps use: OpenLandMap.org

Publication in preparation.

All files were internally compressed using "COMPRESS=DEFLATE" creation option in GDAL.
File naming convention (veg_ndvi_avhrr_p90_5km_s0..0cm_20000101..20000131 _v1.0):

veg = Vegetation theme

ndvi = Normalized Difference Vegetation Index 

avhrr = NOAA CDR AVHRR V5 product

p90 = Percentile 90th to aggregate the daily images

5km = 5 km of spatial resolution

s0..0cm = land surface as vertical reference

20000101..20000131 = from 2000-01-01 to 2000-01-31 (time reference)

v1.0 = version number

本数据集为长期月度归一化植被指数（Normalized Difference Vegetation Index, NDVI），基于NOAA/CDR/AVHRR/NDVI/V5版本的甚高分辨率辐射计（Advanced Very High Resolution Radiometer, AVHRR）每日影像计算得到；所有影像已在Google Earth Engine（GEE）平台完成云掩膜处理，并通过90百分位数聚合得到月度数据。

所有影像均从GEE导出至R语言环境，并在此环境中逐像素执行以下处理流程：
1. 异常值剔除：基于时间滑动窗口识别并剔除NDVI异常值，即通过60个月时间窗口计算标准差，剔除绝对值超出1.5倍标准差的NDVI数值；
2. 平滑与空隙填充：采用greenbrown工具包中的TSGFssa方法完成影像平滑与空隙填充。

最终数据集包含456幅无空隙的全球NDVI影像；同时针对每个月份生成配套辅助影像，各取值含义如下：
- 被剔除像素：取值为-25000（已移除的异常值像素）与-32000（无数据像素）；
- 空隙填充像素：取值为-20000；
- 平滑像素：所有取值大于-20000的像素，代表原始AVHRR NDVI与平滑后NDVI的差值。

可通过OpenLandMap.org访问并可视化该数据集影像。
相关研究成果正在筹备发表中。

所有文件均采用GDAL的"COMPRESS=DEFLATE"创建选项进行内部压缩。

文件命名规则如下（以veg_ndvi_avhrr_p90_5km_s0..0cm_20000101..20000131_v1.0为例）：
- veg：植被主题；
- ndvi：归一化植被指数；
- avhrr：NOAA CDR AVHRR V5产品；
- p90：每日影像聚合所采用的90百分位数；
- 5km：空间分辨率为5千米；
- s0..0cm：以地表为垂直参考基准；
- 20000101..20000131：时间范围为2000年1月1日至2000年1月31日；
- v1.0：数据集版本号。