Silph road tracker??

The tracker is pretty useless in my opinion. It doesn't actually tell you where a pokemon is, it simply keeps track of the possible area a pokemon can be in. You have to enter whether or not you can see a pokemon you are looking for on your nearby list as you move from location to location, and the tracker shows you the area left where the pokemon could be. However it's far too time consuming and mobile unfriendly to be useful on the go.

The Silph Road tracker is only useful in areas without Pokéstops. Once they enter your sightings list, the tracker becomes useless. Also, if more than 9 Pokémon are in your vicinity, it can affect the reliability of the tracker's results.

If neither of those conditions exists in your current location, load up the tracker when the Pokémon you're hunting is on your sightings list. Tap "Begin Pursuit": a big doughnut-shaped circle will appear. Walk for a bit to one of the shaded portions of the circle then tap either "Not on Sightings" or "Visible on Sightings" after checking your radar. The app will then remove a portion of the circle where your Pokémon cannot be. Keep walking to areas that are still shaded and updating the app. Eventually, you will either find the Pokémon or it will despawn.

The app basically uses a process of elimination via geolocation. I find it works pretty well in Pokéstop-starved locations.