Creating opacity mask in Google Earth Engine?

Question

I have an NDSI snow layer loaded in Google Earth Engine. Since snow is present for values <0.4, I want to make values >0.4 opaque. I looked at the manual's examples, but this code is not giving me what I want. Instead I get black (<0.40) and white (>0.40).

What am I missing?

var winter = NDSI.filter(ee.Filter.calendarRange(1999,2018,'year')).filter(ee.Filter.calendarRange(6,8,'month'));
var medianWinter = winter.median();
var medianWinterMask = medianWinter.gt(0.4);
Map.addLayer(medianWinterMask);

Answer 1

To mask an image, use the method ee.Image.updateMask(). Masking is also covered in the Introduction to the Earth Engine JavaScript API tutorial.

There are many ways to present masked data, but here is one possibility using the variables from your example code:

Map.addLayer(medianWinter.updateMask(medianWinterMask), {min:0, max:100});

Answer 2

Very useful, Tyler, thanks! After looking at the documentation, I was able to apply a second mask to exclude water. So this shows snow pixels (NDSI < 0.4) over land as white – everything else is transparent, which is what I was going for. Looks great on top of the satellite imagery. I used the Hansen dataset to identify land pixels, based on the example in Google's tutorial.

//Select the datamask band from the Hansen data
var datamask = Hansen.select('datamask');
// Create a binary mask. For this band, 0=No Data, 1=Land, and 2=Water
var HansenMask = datamask.eq(1);

var winter = NDSI.filter(ee.Filter.calendarRange(1999,2018,'year')).filter(ee.Filter.calendarRange(6,8,'month'));
var medianWinter = winter.median();
var medianWinterMask = medianWinter.gt(0.4);

var doubleMask = medianWinterMask.updateMask(HansenMask);
Map.addLayer(doubleMask.updateMask(medianWinterMask));

This gets the job done, but there other more elegant ways of masking water, snow, and clouds – notably Fmask – here are two great tutorials with useful examples:

https://medium.com/upstream/find-water-and-remove-clouds-with-fmask-on-google-earth-engine-261c090cb62d

https://www.researchgate.net/publication/302589628_Generating_a_cloud-free_homogeneous_Landsat-8_mosaic_of_Switzerland_using_Google_Earth_Engine