Difference between two polygons in Earth Engine

Question

I have a reservoir and its buffer (see below). I want to do the difference between the 2 so that my buffer is just a contour around the polygon. I tried the 2 following scripts but cannot get the "contour buffer" to appear. I also tried with symmetricDifference, but same errors.

First:

var buffercontour = reservoir.difference(reservoirbuffer, ee.ErrorMargin(1));
Map.addLayer(buffercontour, {}, 'buffercontour');

Error message:
reservoir.difference is not a function
    in <global>, line 118
    in <global>, line 176

Then:

var reservoirbufferPolygon = ee.Geometry.Polygon(reservoirbuffer);
var reservoirPolygon = ee.Geometry.Polygon(reservoir);

var buffercontour = reservoirPolygon.difference(reservoirbufferPolygon, ee.ErrorMargin(1));
Map.addLayer(buffercontour, {c}, 'buffercontour');

Error message:
buffercontour: Layer error: GeometryConstructors.Polygon, argument 'coordinates': Invalid type. Expected: List<Object>. Actual: FeatureCollection.

Any ideas?

Answer 1

Here is a simple script that demonstrates how to take the difference of two ee.Geometry.Polygon() objects, using ee.Polygon.Geometry.difference():

var poly_inner = ee.Geometry.Polygon(
        [[[-119.14, 38.01],
          [-119.07, 37.95],
          [-118.91, 38.00],
          [-119.01, 38.06]]]);

var poly_outer = ee.Geometry.Polygon(
         [[[-119.17, 38.00],
           [-119.09, 37.93],
           [-118.93, 37.96],
           [-118.86, 38.01],
           [-118.96, 38.06],
           [-119.05, 38.07],
           [-119.15, 38.02]]]);

var poly_diff = poly_outer.difference(poly_inner);

Map.centerObject(poly_outer);
Map.addLayer(poly_inner, {color:'red'}, 'poly_inner (red)', false);
Map.addLayer(poly_outer, {color:'blue'}, 'poly_outer (blue)', false);
Map.addLayer(poly_diff, {color:'green'}, 'poly_diff (green)');

Answer 2

I tried ErrorMargin in the same way as you did and it worked.

In your first example, I think you commited an error in line where you wrote reservoir.difference. Better, try to do this example in other code editor to avoid writing wrongs.