1.) Center each map over the cities or regions you want to compare. You can use the search box on each map to search for a place
2.) Zoom with your mouse wheel when your cursor is over a map, or with the zoom icons, or with the +/- keys on your keyboard
3.) When dragging the maps, they will follow each other. You can stop this by toggling the "Lock Movement" checkbox
4.) Once you have your maps centered, click anywhere on the map to start drawing a polygon. Each click will add a point to the polygon. The polygon will be shown on both maps
5.) Start over by clicking "Erase Polygon"
Lock Movement (Press L key to toggle):
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