Where do GPS units set the center of an airport?

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Doug F

Pre-takeoff checklist
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DougG
If I'm sitting at the end of the runway and I have the airport as my 'direct to' airport, my Garmin G300 shows 0.4 miles. (KGVL, 05-23)

The runway is 0.96 miles from end stripe to end stripe so 0.4 is just a tad short of the middle of the runway. I haven't checked the distance on the crossing runway (11-29).

I also haven't checked any other airports I fly in and out of.

The slight discrepancy got me wondering where the GPS units set the center of any given airport and does it vary as a function of the GPS manufacturer?
 
Doug F said:
If I'm sitting at the end of the runway and I have the airport as my 'direct to' airport, my Garmin G300 shows 0.4 miles. (KGVL, 05-23)

The runway is 0.96 miles from end stripe to end stripe so 0.4 is just a tad short of the middle of the runway. I haven't checked the distance on the crossing runway (11-29).

I also haven't checked any other airports I fly in and out of.

The slight discrepancy got me wondering where the GPS units set the center of any given airport and does it vary as a function of the GPS manufacturer?
Click to expand...

I would expect them to be programmed with the Lat and Long that is listed for the airport in the Chart Supplement.
GEOGRAPHIC POSITION OF AIRPORT—AIRPORT REFERENCE POINT (ARP)
Positions are shown as hemisphere, degrees, minutes and hundredths of a minute and represent the approximate geometric center of all usable runway surfaces.
 
Wheels said:
I would expect them to be programmed with the Lat and Long that is listed for the airport in the Chart Supplement.
GEOGRAPHIC POSITION OF AIRPORT—AIRPORT REFERENCE POINT (ARP)
Positions are shown as hemisphere, degrees, minutes and hundredths of a minute and represent the approximate geometric center of all usable runway surfaces.
Click to expand...

Oh, that's good too. I should be able to use Google Earth to find the spot on any given airport.

Thanks
 
It is at the "balance point" of the runways. If there is only one runway, it is at the center of that runway. If there is more than one runway, imagine the runways are thin rulers on a weightless plate (the rulers have weight) and is is the point where they "balance". It is sort of like finding the CG, but in two dimensions. More properly known as the geometric center (of the runways).
 
At my home drome, it is on the state line between Tennessee and Mississippi which runs through midpoint of runway.
 
I was flying yesterday and popped open ForeFlight to do track logs. I zoomed in on GVL and it turns out the airport center is marked on the map! It was pretty much where you could guestimate as the center. Ain't technology wunnerful?
 
After my PPL, when I started spending a bit more time with my GPS, I noticed this as well. I did some experiments similar to what the OP described, and found that most of the medium-sized airports I flew in and out of averaged 0.4-0.6 miles from the end of the runway to the point that the GPS was reporting the airport to be at. So I just started subtracting half a mile from any distance to the airport the GPS gave me. Not that it matters, really, since if IFR I'd be shooting the approaching instead of eyeballing and guessing. I was however, a bit irritated that that particular common discrepancy wasn't mentioned during my training.
 
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