Force an image to be computed in a given projection and resolution.
Image.reproject(crs, crsTransform, scale)Image
this: imageImageThe image to reproject.
crsProjectionThe CRS to project the image to.
crsTransformList, default: nullThe list of CRS transform values. This is a row-major ordering of the 3x2 transform matrix. This option is mutually exclusive with the scale option, and replaces any transform already on the projection.
scaleFloat, default: nullIf scale is specified, then the projection is scaled by dividing the specified scale value by the nominal size of a meter in the specified projection. If scale is not specified, then the scale of the given projection will be used.


Code Editor (JavaScript)

// Use of ee.Image.reproject is rarely needed and should generally be avoided.
// Defining the projection and scale of analysis should be handled by "scale",
// "crs", and "crsTransform" parameters whenever they are offered by a function.
// It is occasionally useful for forcing computation or visualization at a
// desired scale and projection when alternative methods are not available. In
// this example it is used to compute and visualize terrain slope from a DEM
// composite.

// Calculate mean elevation from two DEM datasets. The resulting composite
// image has a default CRS of WGS84 with 1 degree pixels.
var dem1 = ee.Image('NASA/NASADEM_HGT/001').select('elevation');
var dem2 = ee.Image('CGIAR/SRTM90_V4').select('elevation');
var demMean = ee.ImageCollection([dem1, dem2]).mean();

// Display the DEMs on the map, note that they all render as expected.
var demVisParams = {min: 500, max: 2500};
Map.setCenter(-123.457, 47.815, 11);
Map.addLayer(dem1, demVisParams, 'DEM 1');
Map.addLayer(dem2, demVisParams, 'DEM 2');
Map.addLayer(demMean, demVisParams, 'DEM composite');

// Calculate terrain slope from the composite DEM (WGS84, 1 degree pixel scale).
var demCompSlope = ee.Terrain.slope(demMean);

// Because the composite has 1 degree pixel scale, the slope calculation
// is essenstially meaningless and difficult to even display (you may need to
// zoom out to see the individual 1 degree pixels).
Map.addLayer(demCompSlope, {min: 0, max: 0.3}, 'Slope');

// We can use ee.Image.reproject to force the slope calculation and display
// the result with a reasonable scale of 30 m on WGS84 CRS, for example.
var slopeScale = ee.Terrain.slope(
    crs: 'EPSG:4326',
    scale: 30
Map.addLayer(slopeScale, {min: 0, max: 45}, 'Slope w/ CRS and scale');

// To more precisely control the reprojection, you can use the "crsTransform"
// parameter instead of the "scale" parameter or set the projection according to
// a reference image. For example, here the input composite image for the slope
// function is set to match the grid spacing and alignment of the NASADEM image.
var nasademProj = dem1.projection();
var demMeanReproj = demMean.reproject(nasademProj);
var slopeRefProj = ee.Terrain.slope(demMeanReproj);
Map.addLayer(slopeRefProj, {min: 0, max: 45}, 'Slope w/ reference proj');
print('Reference projection', nasademProj);
print('DEM composite projection', demMeanReproj.projection());

// An alternative method for changing the projection of image composites
// (not accepting the default WGS84 CRS with 1 degree pixel scale) is to
// explicitly set the default projection using ee.Image.setDefaultProjection,
// which will not force resampling, like ee.Image.reproject will.
var demMeanProj = ee.ImageCollection([dem1, dem2]).mean()
var slopeProj = ee.Terrain.slope(demMeanProj);
Map.addLayer(slopeProj, {min: 0, max: 45}, 'slope w/ default projection set');