Specifying min and max values for image while exporting from Google Earth Engine

Question

I have this small code for getting nightlight image for a particular city using VIIRS Nighttime dataset.

var district = ee.FeatureCollection('ft:1PA2zwArj8EsplrX9eMxJ2H_TICyyx855KPnbJhC1','geometry')
.filter(ee.Filter.eq('name','Gurgaon'));
var nighttime = ee.Image(ee.ImageCollection('NOAA/VIIRS/DNB/MONTHLY_V1/VCMCFG')
              .select('avg_rad')
              .filterBounds(district)
              .filterDate('2013-05-01', '2017-05-31')
              .median())
var nighttimeVis = {min: 0.0, max: 60.0};
Map.addLayer(nighttime.clip(district), nighttimeVis, 'Nighttime');

Now I wish to export this image to my Google Drive. Usually I use the following script for exporting:

Export.image.toDrive({
  image: nighttime.clip(district),
  description: 'Nighttime_Image',
  maxPixels: 499295920080,
  scale: 30,
  region: district
});

But for this particular image, I need to specify the min and max values like I have done while using Map.addlayer (for viewing the image) by using nighttimeVis vector. How can I specify the same max and min before or in my Export command for this image?

Answer 1

Probably, you'd like something like this:

// Scale the image between 0 and 255 
// based on: min = 0, max = 60
var scaled = nighttime.unitScale(0, 60).multiply(255).toByte();
Map.addLayer(scaled.clip(district), {min: 0, max: 255}, 'Nighttime');

By adding toByte(), you image is stored as a 8-bit integer which would save you some storage space, but you could remove that to keep floating values. Note that now values above 60 are all saved as 255 values. But I think that is what you want.