regionalThreshold_Norway

//// can fill in threshold that you want, and the date
// will create a binary image 

var Innlandet = 
    /* color: #d63000 */
    /* shown: false */
    ee.Geometry.Polygon(
        [[[11.460955859374987, 60.611548923543275],
          [12.087176562499987, 60.41145339155949],
          [12.625506640624987, 60.54678843711464],
          [12.416766406249987, 60.93874631934527],
          [12.740275020404779, 61.029216241573465],
          [12.735369921874987, 61.17269004771807],
          [12.076190234374987, 61.57795147913667],
          [10.933612109374987, 61.225618493844735]]]);


var thresholdVal = 9.73 // [FILL IN HERE]
var date = '2018-03-29' //[FILL IN HERE]

var terraNDSI = ee.ImageCollection("MODIS/006/MOD10A1")
  .select(['NDSI_Snow_Cover'])
  .filter(ee.Filter.date(date , '2020-06-30')) //2018-01-01 for final
  .map(function(image){return image.clip(Innlandet)})

var Vis = {
  min: 0.0,
  max: 100.0,
  palette: ['black', '0dffff', '0524ff', 'ffffff'],
};

Map.addLayer(terraNDSI.first(),Vis,'terra NDSI')

var snowBinary = terraNDSI
            .map(function(img){
            var snow0 = img.select('NDSI_Snow_Cover').gte(0)
            .and(img.select('NDSI_Snow_Cover').lt(thresholdVal))
            .remap([1],[0])
            var snow1 = img.gte(thresholdVal)
            .remap([1],[1])
            return img.addBands(ee.ImageCollection([snow0.byte(),snow1.byte()]).mosaic())
            })

var Innlandetmap = snowBinary.map(function(image){return image.clip(Innlandet)})

Map.setCenter(11.9924, 61.2108);
Map.addLayer(Innlandetmap.first(), {min:0,max:1}, 'binary')

//// zonal statistics

var histogram = Innlandetmap.first().reduceRegion({
        reducer: ee.Reducer.frequencyHistogram().unweighted(),
        geometry: Innlandet,
        scale: 30,
        maxPixels: 20000000,
});

print('histogram',histogram)
/* to find the % snow pixels take the number of pixels for snow (1) and divide it by the 
total number of pixels (number of pixels for class 0 and class 1), excluding the null values. We exclude the null values because
we don't know whether they are snow or snow. 
*/