Export NDVI raster from Google Earth Engine

Question

I try to export a raster with NDVI values from Google Earth Engine. Although the export works fine, the original NDVI values (-1 to +1) are translated into 8bit values between 0 and 255 in the resulting image. How can I export the NDVI results conserving the original values?

Below the code, I´m using. If you import the "Sentinel-2 MSI: MultiSpectral Instrument, Level-2A" image collection and set the variable name to sent2_collection, you can run the code.

function add_ndvi_band(image){
  var ndvi = image.normalizedDifference(["B8","B4"]);
  return image.addBands(ndvi);
}

var time_filter = sent2_collection.filterDate("2019-01-01", "2019-12-31");
var time_filter_ndvi = time_filter.map(add_ndvi_band);

Map.addLayer(time_filter.median(), {bands:["B4","B3","B2"], min:0, max:3000}, "Cuautla - true color");
Map.addLayer(time_filter_ndvi.median(), {bands:"nd", min: -1, max: 1}, "Cuautla-NDVI");

var exportar_mapa = time_filter_ndvi.median().visualize({bands: ['nd', 'nd', 'nd'],max: 1.0});

Export.image.toDrive({image: exportar_mapa, description: 'ndvi',  scale: 10});

Answer 1

The purpose of the Image.visualize algorithm is to convert images to RGB in the range 0-255 for display. Remove it from your code if you want original values. Replace

var exportar_mapa = time_filter_ndvi.median().visualize({bands: ['nd', 'nd', 'nd'],max: 1.0});

with

var exportar_mapa = time_filter_ndvi.median().select(['nd']);

to selects the one band you want, but not rescale it to 8-bit values.