Superimpose one area over another in mapbox

In order to demonstrate the difference between two city area’s I took the outline of one city and superimposed it over the other city by shifting all the points in the 1st city’s polygon over the center of the 2nd city.

   $line = //a geojson array of points in a polygon;

   $newline = array();

   foreach($line as $l){

    //shift all the points over based on the center of the new location's distance from the center of the previous location's center

    $newline[] = array($l[0]+.193,$l[1]-.018);