What projection is being used in this dataset?

Question

I'm not from US and so don't know much about the CRS/EPSG there. Anyway, when reviewing the FAA 5010 Airport dataset on https://www.faa.gov/airports/airport_safety/airportdata_5010/#5010 (direct download link = https://www.faa.gov/airports/airport_safety/airportdata_5010/menu/nfdcfacilitiesexport.cfm?Region=&District=&State=&County=&City=&Use=&Certification=) I found myself confused about what projection was being used. Can anyone help?

There is a "data dictionary" file that can be downloaded but CRS/EPSG is not included:

• ARPLatitude = Airport reference point latitude (formatted).
• ARPLatitudeS = Airport reference point latitude (seconds).
• ARPLongitude = Airport reference point longitude (formatted).
• ARPLongitudeS = Airport reference point longitude (seconds).

Here is a sample of the lat/long fields being used:

ARPLatitude |   ARPLatitudeS    | ARPLongitude |    ARPLongitudeS

51-53-00.8954N | 186780.8954N | 176-38-32.9277W | 635912.9277W

56-56-19.2870N | 204979.2870N | 154-10-57.2000W | 555057.2000W

60-54-49.7150N | 219289.7150N | 161-29-35.9850W | 581375.9850W

60-54-28.3130N | 219268.3130N | 161-26-06.2780W | 581166.2780W

Answer 1

Latitude and Longitude being used strongly suggests that these are not projected coordinates.

The documentation of the fields says the ones without trailing S are "formatted" while the ones with an trailing S are "seconds".

Those angular measures are often described in Degrees Minutes Seconds (DMS) with varying amounts of "decimalisation" (I made that term up).

I would say your "formatted" coordinates are D-M-S.FractionOfS and your "seconds" coordinates are the total number of seconds (see https://en.wikipedia.org/wiki/Geographic_coordinate_conversion#Coordinate_format_conversion)

>>> D = 51
>>> M = 53
>>> S = 00.8954
>>> dd = D+M/60+S/3600
>>> dd
51.88358205555556
>>> seconds_total = dd*3600
>>> seconds_total
186780.8954
>>> 

Looks like it!

https://www.openstreetmap.org/#map=17/51.88358205555556/-176.64247991666667 is an airport so there you go.

Answer 2

To me this just looks like geographic latitude/longitude, so these are not projected coordinates.

According to the linked dataset the first row of your example stands for the Adak Airport in Alaska. This matches also the result from e.g. Google Maps if you "transfer" the first and third column to the "normal" formatting like so: 51°53'00.8954"N 176°38'32.9277"W