Understanding MODIS pixel size, CRS projection SR-ORG:6974 in Google Earth Engine

Question

I am calculating the area of an ice cover by summing the number of pixels having a value above a certain threshold then multiplying by the pixel area. This is done for a collection of images over a number of years.

The resolution of the MODIS dataset I'm using is 500 m. When I use the function ee.Reducer.sum() I set the scale to this value. When my area is calculated however, the pixel size appears to be 463.3 m with areas reflecting this pixel size (0.216 km2 instead of 0.25 km2).

I am not a remote sensing or GIS scientist by trade and am struggling to understand the relationship between projection (when a pixel is inspected it shows, SR-ORG:6974) and scale. This MODIS data set uses a sinusoidal projection which I understand to be area-preserving. That being said when I look at pixels nearer to the equator, they also appear to have a size of 463.3.

My bottomline is that I need to present the most accurate measure of area and make sure I have not caused an error.

Are the areas I've calculated using this approach correct or should they be based on a 500 m pixel?

My code is quite long and may confuse the main question, but I've included the main parts below to show my general method. It's not run-able but I can provide the rest if need be.

//Create a number of masks and other functions to clean up images and highlight ice, there is no //use of scale up to this point

var maskedModis_ice_AllQAcloud= sample
  .map(maskLand)
  .map(addice)
  .map(getclouddata)
  .map(getcirrusdata)
  .map(getQAdata)
  .map(maskAllQAcloud);

//Function to create threshold and multiply by pixel area to determine ice area

var icethreshold= 3000;
var icefunction = function(image){
  var icejunk= image.select(['Ice_index']);
  var wateretc= icejunk.lt(icethreshold);
  var solidice = icejunk.gte(icethreshold);
  //image = image.updateMask(solidice);
  var area2= ee.Image.pixelArea();
  var iceArea1 = solidice.multiply(area2).rename('iceArea1');
  var iceArea= iceArea1.divide(1000*1000).rename('iceArea');
  return image.addBands(iceArea);

//highlight only pixels marked as ice through thresholding and combine with all filtering steps

var filteredAlliceQAcloud=maskedModis_ice_AllQAcloud.map(icefunction);
print (filteredAlliceQAcloud);

//Apply to specific regions and plot

var chart = ui.Chart.image.series(filteredAlliceQAcloud.select('iceArea'), regions, ee.Reducer.sum(),500)
    .setChartType('LineChart')
    .setOptions({
      //title: 'Beisfjord',
      hAxis: {title: 'Date'},
      vAxis: {title: 'Ice Area, QA and cloud filtered (m^2)'},
      fontSize: 20,
      legend: {position: 'right'}
    });

Answer 1

I have not checked your outputs, but your data are likely correct because of how the MODIS data are delivered.

MODIS data is produced in a geographic coordinate system (GCS), not a projected coordinate system. This means that the underlying data are referenced to a spheroid, not a plane or flat sheet. Instead of measuring distance in meters GCS measure in angle. Because the shape of the Earth/spheroid is not perfectly spherical, the distance of a given angle in meters changes. The 500m pixel quoted is the 'typical' or average value of a pixel within the spheroid.