Footnotes ...

<1>	When there's a risk of an area code running out of phone numbers, an 'Overlay Area Code' is created that has the same geographic boundaries as the existing area code. In this case, the 757 code has been Overlayed with the 948 area code. New phone numbers in the Philadelphia area will be assigned with one of these codes: 757 or 948. As a result, placing a call in the Philadelphia area will require 10-digit dialing (where you enter both the area code and then the phone number).
<2>	The elevation is an estimate for the area around Philadelphia - it will be higher in some places and lower in others.
<3>	Longitude & latitude values may not be the geographic center of Philadelphia, rather it may be a prominent intersection or the center of population, etc. It will vary depending on the source.
<4>	Name of the USGS 7.5 Minute topographic map which includes Philadelphia.
<5>	Every year, the US Census Bureau creates a population estimate for the preceeding year. Keep in mind that this value is an estimate and not an actual count. The most recent Census estimate is for the year 2019.