Google Earth Engine – Get Image with lowest NDVI from a time period

google-earth-enginemaximumndvi

One of the answers to this question (Google Earth Engine – Get Image with highest max Scene NDVI from a time period) provides code which gives a Sentinel image/date with the highest NDVI for each region (https://code.earthengine.google.com/5165eca22f5dbfc8e114919d9315ab73).

This is half of what I want, but I also want the Sentinel image/date of the lowest NDVI for a region, but am not sure how. I have tried changing

return ee.Feature(meanFeatureCollection.sort('mean').first()

to descending order, but it didn't change the console output.

EDIT: I double checked and the link code doesn't actually do as described, it only prints the NDVI and ID of first Sentinel image in the collection. So I also need the correct script to identify highest AND lowest NDVI images.

Best Answer

In the link here, the code provided as an answer to the question you referred to.
For getting lowest value, you can change max to min

var highestMean = mosaicsWithMean.aggregate_max('mean')

var lowestMean = mosaicsWithMean.aggregate_min('mean')

Related Solutions

[GIS] Extracting position date of max and min value in NDVI time series in Google Earth Engine

You can get the minimum values similar to how you got the max values, but instead you'll need the first position to use arraySlice.

// for the max value sorted
var valuesMax = sorted.arraySlice(axes.image, length.subtract(1), length);
// for the min value sorted
var valuesMin = sorted.arraySlice(axes.image, 0, 1);

Then you can continue with the script similar as you did with the max values.

Here you can find a link to the full script.

[GIS] Max NDVI per month & year using Google Earth Engine to CSV – CSV gives -999 values only

You can obtain the answer you want with the small geometry you use using a different approach. Note that a Landsat pixel and the scale you are currently using is 30m. Your geometry is just about 10x10m.

Add the moment you are using an unweighted reducer (ee.Reducer.max), which will only take pixels into account if the centroid is within the geometry (see here). Neither the geometry nor the scale you are using apparently include the centroid of the pixels, thus you return no valid values (which you set to -999 afterwards).

You have two options:

1) Use a scale which will include the centroid and use the unweighted reducer. Set the scale for example at 10m or calculate approximately the scale of a square geometry using:

var scale = table.geometry().area(1).sqrt();

2) Use a weighted reducer which will include pixels which approximately include at least 0.5% of the pixel. An example would be using a mean reducer. In such a small region you probably aggregate one pixel, so that is fine for your max outcome.

var data = cloudlessNDVI.filterDate(ini,end).max().reduceRegion({
  reducer: ee.Reducer.mean(),
  geometry: feature.geometry(),
  scale: 30
});

Here an example returning both values (and you will see they are similar, confirming there is just one pixel in the region).

Best Answer

Related Solutions

[GIS] Extracting position date of max and min value in NDVI time series in Google Earth Engine

[GIS] Max NDVI per month & year using Google Earth Engine to CSV – CSV gives -999 values only

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