The goal is to create path animations, such as with a travel route overlayed on a map.
I'm not sure if anyone has mentioned the standard way of doing this effect. In Photoshop, or something similar, import the map and lay down the "path" on a separate layer. Save the map with the line in some form that can be imported by the video editing software. In Photoshop (or something similar), delete the map layer, but not the layer containing the path. Apply a gradient to the path so that it is black on one end and white on the other. Save just the path as a separate file, with alpha channel preserved (use TIF or PSD). In you video edit, import both map files, i.e. the one with and the one without the path, as well as the file containing the separate path. Place the two map photos adjacent and apply a gradient wipe transition to the photos. Stretch the transition so that it covers both completely. In the gradient wipe transition controls, specify the separate path file as the mask. Render.