Time series for MultiPoint geometries in Google Earth Engine

Question

I am trying to generate time series for a set of points in earth engine. I am attempting this for multiple datasets, but for now let's do NDVI with Landsat 8. My problem lies (i think) in step 6. below, as the plot is generated, but with no sign of mutiple sites...

1. Data import

var l8sr = ee.Image('LANDSAT/LC08/C01/T1_SR')

The method I am trying to use is as follows:

2. Create a multi point geometry, here called roi. N.B. The code presented in this question runs perfectly for a single point geometry, but not for multipoint

var roi = ee.Geometry.MultiPoint([[146.007413888888,-17.6536194444444],
           [146.061191666666,-17.4442805555555],
           [145.960902777777,-17.7890055555555],
           [151.937444444444,-24.3380305555555],
           [150.468130555555,-23.1586194444444],
           [148.506963888888,-21.088375],
           [148.591619444444,-21.3626416666666],
           [148.186125,-24.9189222222222]])

3. Set the time field as milliseconds since 1970-1-1 00:00

var timeField = 'system:time_start'

4. A couple of functions which I am using to clean and process data. These have been taken from sources that I dont have to hand, but will update question when I do.

4.a Function to mask cloud using the pixel_qa (quality assurance) band of L8SR data. This is from the code editor example on cloud masking

function maskL8sr(image) {
  var cloudShadowBitMask = 1 << 3
  var cloudsBitMask = 1 << 5
  var qa = image.select('pixel_qa')
  var mask = qa.bitwiseAnd(cloudShadowBitMask).eq(0)
  .and(qa.bitwiseAnd(cloudsBitMask).eq(0))
  return image.updateMask(mask).divide(10000)
  .select('B[0-9]*')
  .copyProperties(image,['system:time_start'])
}

4.b Create NDVI, time, and constant variables in data

var addVariables = function(image) {
  var date = ee.Date(image.get(timeField))
  var years = date.difference(ee.Date('1970-01-01'),'year')
  return image
  .addBands(image.normalizedDifference(['B5','B4']).rename('NDVI'))
  .addBands(ee.Image(years).rename('t'))
  .float()
  .addBands(ee.Image.constant(1))
}

5. Apply functions to data and filter by roi

var filteredLandsat = l8sr
.filterBounds(roi)
.map(maskL8sr)
.map(addVariables)

6. Plotting a time series of NDVI. The image that is output from this is one with red points only and no sign of multiple sites being present. I am trying my best to troubleshoot this, but have hit a bit of a wall in my understanding and thought to bring the question to a wider audience...

//Plot a time series of NDVI at a single location
var l8Chart = ui.Chart.image.seriesByRegion(filteredLandsat, 
                                            roi,
                                            ee.Reducer.mean(), 
                                            'NDVI')
                                            .setChartType('ScatterChart')
                                            .setOptions({
                                              title: 'Landsat 8 NDVI time series at ROI',
                                              vAxis: {title: 'NDVI'},
                                              lineWidth: 1,
                                              pointSize: 4,
                                              series: {
                                                        0: {color: '#FF0000'}, // Warrubellen
                                                        1: {color: '#FFFF00'}, // Ella Bay
                                                        2: {color: '#00FF00'}, // Nyleta Creek
                                                        3: {color: '#008000'}, // Deep Water
                                                        4: {color: '#00FFFF'}, // Mt Etna Quarry
                                                        5: {color: '#0000FF'}, // Eungella 1
                                                        6: {color: '#FF00FF'}, // Eungella 2
                                                        7: {color: '#800080'}  // Carnavon
                                                      }

                                            })
print(l8Chart)

Answer 1

Since posting this, (and with a helpful comment from @Sean Roulet) it seems the "correct" way to do this is to replace the ee.Geometry.MultiPoint() with a ee.FeatureCollection() containing several ee.Geometry.Point() as follows:

var roi_list = [ee.Feature(ee.Geometry.Point(146.007413888888,-17.6536194444444), {name: 'Warrubellen'}),
           ee.Feature(ee.Geometry.Point(146.061191666666,-17.4442805555555), {name: 'Ella Bay'}),
           ee.Feature(ee.Geometry.Point(145.960902777777,-17.7890055555555), {name: 'Nyleta Creek'}),
           ee.Feature(ee.Geometry.Point(151.937444444444,-24.3380305555555), {name: 'Deep Water'}),
           ee.Feature(ee.Geometry.Point(150.468130555555,-23.1586194444444), {name: 'Mt Etna Quarry'}),
           ee.Feature(ee.Geometry.Point(148.506963888888,-21.088375), {name: 'Eungella 1'}),
           ee.Feature(ee.Geometry.Point(148.591619444444,-21.3626416666666), {name: 'Eungella 2'}),
           ee.Feature(ee.Geometry.Point(148.186125,-24.9189222222222), {name: 'Carnavon'})]

var roi = ee.FeatureCollection(roi_list);

So now, the question presented here is more based on whether it is possible to do what I have done below, without using a FeatureCollection(), and instead just using ee.Geometry.MultiPoint(), or if not, why google earth engine is unable to do this...?

Answer 2

I agree with the OP that it's weird that it's not easy to convert an ee.Geometry.MultiPoint to an FeatureCollection. One workaround is to extract the coordinates as a list and then map through the list rebuilding points wrapped in features.

So if you have a Multipoint in mypoints you can get them as a FeatureCollection myfeats like this:

var myfeats = ee.FeatureCollection(mypoints.coordinates().map(function(x) {return ee.Feature(ee.Geometry.Point(x))}))