@stefan-- added sourceRoot support PR gruntjs#136

tasks/lib/sourcemap.js

@@ -136,7 +136,8 @@ exports.init = function(grunt) {
   // Add the lines of a given file to the sourcemap. If in the file, store a
   // prior sourcemap and return src with sourceMappingURL removed.
   SourceMapConcatHelper.prototype.addlines = function(src, filename) {
-    var relativeFilename = path.relative(path.dirname(this.dest), filename);
+    var destDirectory = path.dirname(this.dest),
+        relativeFilename = path.relative(destDirectory, filename);
     // sourceMap path references are URLs, so ensure forward slashes are used for paths passed to sourcemap library
     relativeFilename = relativeFilename.replace(/\\/g, '/');
     if (
@@ -162,7 +163,7 @@ exports.init = function(grunt) {
       var sourceMap = JSON.parse(sourceContent);
       var sourceMapConsumer = new SourceMapConsumer(sourceMap);
       // Consider the relative path from source files to new sourcemap.
-      var sourcePathToSourceMapPath = path.relative(path.dirname(this.dest), sourceMapDir);
+      var sourcePathToSourceMapPath = path.relative(destDirectory, sourceMapConsumer.sourceRoot || path.dirname(sourceMapPath));
       // Transfer the existing mappings into this mapping
       var initLine = this.line;
       var initCol = this.column;