How to change filtering by date for bimonthly GIMMS imagery to filtering by property 'year'?

Question

In an earlier post at How to select features in a feature collection simultaneously row by row in Google Earth Engine?, I got help for creating a code to compare conflicts and bimonthly GIMMS imagery.

Now I received a different dataset, GIMMS annual maximum NDVI data, to use tor the time 1981-2015. The filtering from previously date ('system:time_start') which was added before, doesn't work now.

Also the NDVI (now called 'b1') band is now between -99999 to +9999.

//Dates for filtering the GIMMS Image Collection // With annual data no filtering nexessary, 1981-2015 used
var startDate = 1982 //adjust with time for the 35
var endDate = 2015
print(startDate);

//Loading and filtering (by date) the UCDP conflict data (CSV) 1989-2015
var conflicts = ee.FeatureCollection('users/welp1992/Conflicts_Deaths_Greater25')
  .filterMetadata('year', 'greater_than', 1989)//min 6 years, but as long as possible
  .filterMetadata('year', 'less_than', 2010) //including 2010, to 2015 = 6 years
print(conflicts.limit(1000),'conflicts');


//filter image GIMMS by date and band and add time 't' as a band  
var timeSeries = ee.ImageCollection('users/welp1992/NDVI_Annual_Max_81_2015')
    .select(['b1'])
    .map(function (image) {
      return image.addBands(
        ee.Image(image.getNumber('system:time_start')) //'year'?
          .subtract(ee.Date(startDate).millis())
          .divide(1000*3600) // days (*365 = year)
          .int() //casts the input value to a signed 32-bit integer
          .rename('t')
      )
    })

print(timeSeries,'time');

This is the Error message I get:

ImageCollection (Error) Error in map(ID=Max_05): Image.constant: Parameter 'value' is required.

Unfortunately I cannot share the annual max NDVI data (which I have uploaded to GEE). The script is basically the same as the earlier version in the link I shared above.

Do you have any advice?

Answer 1

The error is caused by the line image.getNumber('system:time_start') because there is no property called 'system:time_start'. The only properties available for each image within timeSeries are system:asset_size, system:footprint, system:index. I found this out using print(timesSeries.first()).

According to your comment, you would like to add a property called "year" to each image within timeSeries so that you don't encounter this error. To do so, use the fact that the images within timeSeries are named sequentially such as "Max_01" etc. until "Max_35" corresponding to the years 1981 to 2015. To add "year" as a property, use:

function addYear(image){ // function to add "year" property
  var year = ee.Number.parse( //convert string to number
            ee.String( // cast to number
                image.get('system:index')) // get name of image
                .replace('Max_','')) //remove leading "Max_" string
              .add(1980); // add 1980 as 1 corresponds to 1981? 
  var image = image.set('year',year)
  
  return image
}

With the above function, now you can add a band to each image like so:

var timeSeries = timeSeries.map(addYear)

var timeSeries = timeSeries
    .select(['b1'])
    .map(function (image) {
      return image.addBands(
        ee.Image(image.getNumber('year')) //'year'?
          // .subtract(ee.Date(startDate).millis()) // not needed anymore since "year" is already in years
          // .divide(1000*3600) // days (*365 = year)  // not needed anymore since "year" is already in years
          .int() //casts the input value to a signed 32-bit integer
          .rename('t')
      )
    })

Link to complete working code