I live in Johannesburg SA. We have a neitz minyan every morning. We take neitz from a time calender we use in Jhb which seems to be a time taken at sea level. The problem is we are on a plateau about 1600 meters above sea level. Now if we were on the edge of the plateau next to the sea we would see the sun before the people would see it at sea level. We however are in the middle of the plateau and therefor see the sun later than at sea level. I have checked and the sun actually pops up 5 minutes later than our calender time. The question is, do we daven neitz at the calender (sea level) time or at the time we actually see the sun come up , which is 5 minutes later. I have spoken to Rabbi Flegg about this and if you are not clear of my question you can consult with him when he returns to Israel. The time would also affect skiyah, Motzie Shabbos Fast times etc
Poskim have long debated the correct time for haneitz hachamah and other times of the day in circumstances where the sun is hidden from sight by topographical factors.
In your case, if the entire city, or a large and separate part of the city, is situated on the plateau, the time would be measured according to sighting the sun in the area of the plateau itself. However, the question would be where on the plateau to measure the time, for the time of sunrise at the edge would be much earlier than the time in the middle. The generally accepted answer to this is that the time is measured according to the earliest sunrise in the town, which would be the part of the town closest to the edge of the plateau. In Jerusalem, for instance, which is on a hill, the time is measured according to the highest point, which yields the earliest time.
However, because there several topographical factors that must be considered, and there are a number of variant opinions concerning which factors must be taken into account, and how an entire “city” is evaluated, it would be best to consult with a Rav who is familiar with the topography and with the city, and with the current custom in Johannesburg.