The halachic opinions used are based on Chabad custom taken from chabad.org and approved by their board of Rabbonim. Alos Hashachar at 16.9 degrees below the horizon, Misheyakir at 10.2 degrees, Hanetz at 0.833 degrees, Shkiah at 0.833 degrees, Tzeis at 6 degrees, end of Shabbat and Yom Tov at .5 degrees. Shaosh Zmanios are calculated from Hanetz Amiti untill Shkiah Amitis - both 1.583 degrees. See more details here.
The search is flexible; any location, address, or place can be searched. The result will be for the center of the location. In order to ensure proper accuracy, a location searched must fit within 0.09 square degrees in total. This will keep the results within an accuracy of a few seconds.
The zmanim can variate by about 2 minutes, and in extreme situations up to about 4 minutes, due to weather and atmospheric conditions.