You have to use the media/search API to get geolocation information around a geo location:
https://api.instagram.com/v1/media/search?lat=50.85396&lng=4.34150&distance=2000&access_token=ACCESS-TOKEN
and then grab the (data[x].location.latitude, data[x].location.longitude) from the API response. Each response will give you about 20 photos, use MAX_ID param in API to get next set of 20 photo data.
Here is an implementation of exactly what you are looking for: gramfeed