I am trying to export yearly median MODIS-based NDVI products at 250m spatial resolution. I am using the 16 day MODIS/061/MOD13Q1 product. I have found this code which I am following (https://gis.stackexchange.com/questions/356857/exporting-cloud-masked-modis-image-collection-from-google-earth-engine).
After some modifications (i.e., I am interested in a single yearly median image) on the code, the image I produced looks like this (image attached).
Because I have very little knowledge in Google Earth Engine, I wanted to ask if the code to generate yearly median MODIS/061/MOD13Q1 product (free of cloud, cloud shadows and bad quality pixels) is correct.
I am asking this, because the areas highlighted in red are water bodies and, as far as I know, water bodies should have negative NDVI values. As you can see from the picture, the NDVI range of my image has no negative values (0.0066-0.7318).
Here is the code (https://code.earthengine.google.com/ecaeb390f70aeb105d0b076e653341cd).
MODIS Vegetation Index Products (NDVI)
MODIS vegetation indices, produced on 16-day intervals and at multiple spatial resolutions, provide consistent spatial and temporal comparisons of vegetation canopy greenness, a composite property of leaf area, chlorophyll and canopy structure. Two vegetation indices are derived from atmospherically-corrected reflectance in the red, near-infrared, and blue wavebands; the normalized difference vegetation index (NDVI), which provides continuity with NOAA's AVHRR NDVI time series record for historical and climate applications, and the enhanced vegetation index (EVI), which minimizes canopy-soil variations and improves sensitivity over dense vegetation conditions. The two products more effectively characterize the global range of vegetation states and processes.
The vegetation indices are retrieved from daily, atmosphere-corrected, bidirectional surface reflectance. The VI's use a MODIS-specific compositing method based on product quality assurance metrics to remove low quality pixels. From the remaining good quality VI values, a constrained view angle approach then selects a pixel to represent the compositing period (from the two highest NDVI values it selects the pixel that is closest-to-nadir). Because the MODIS sensors aboard Terra and Aqua satellites are identical, the VI algorithm generates each 16-day composite eight days apart (phased products) to permit a higher temporal resolution product by combining both data records. The MODIS VI product suite is now used successfully in all ecosystem, climate, and natural resources management studies and operational research as demonstrated by the ever increasing body of peer publications.
MODIS NDVI data can be downloaded from the following URL:
https://modis.gsfc.nasa.gov/data/dataprod/mod13.php
Lots of fun with the data.
Please check this link
https://www.geo.fu-berlin.de/en/v/geo-it/gee/2-monitoring-ndvi-nbr/2-2-calculating-indices/ndvi-modis/index.html
In it, there is code to calculate NDVI from MODIS with spatio-temporal resolutions 500m, 1 km and daily, respectively. If you need early, you can change the filter date (e.g., 01-01-2022 / 31-12-2022).
Looks like you're selecting only positive values in the maskEmptyPixels function, so you shouldn't expect any negative values on the map
D. Ermakov by removing the maskEmptyPixels function the water bodies had negative NDVI values. Thanks.
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