Can someone explain what ".lt" is in the code below, and how it works?

var hydrosheds = ee.Image('WWF/HydroSHEDS/03VFDEM');
var terrain = ee.Algorithms.Terrain(hydrosheds);
var slope = terrain.select('slope');
var before1 = before.mask(slope.lt(5));
var after1 = after.mask(slope.lt(5);
Map.addLayer(after1, {palette:"red"},'flooded areas - red',1);
  • 3
    lt = less than. So in your example: Mask everything where the slope is more than 5. Also look at the Earth Engine documentation. You will find everything there.
    – JonasV
    Commented Nov 11, 2020 at 10:08

1 Answer 1


lt means LowerThan, so basically you've masked any slope which is higher than 5 (degrees I'm assuming). If you ever have a doubt on what a function does, click on the 'Docs' tab on GEE and type in the name of the function you're unsure about, a short description comes out.

  • Thank you so much for your clear explanation Commented Nov 25, 2020 at 6:25
  • You are welcome! Could you accept the answer so I may gain points? ^^
    – M. Nicolas
    Commented Feb 4, 2021 at 20:42

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