Waypoint coordinates

Waypoint coordinates

Postby dhplane » Wed Feb 05, 2020 1:34 pm

I saw a number of posts about GPS coordinates but none applies to my question, so here goes...

I was doing a practice flight that included the RNAV-B approach into KEMT and noticed that the waypoints on final on ForeFlight are not the same as in the GTN 750 database, which is updated to the latest available version (Sept/2019). When I looked up WALLY on the FAA Fixes/Waypoints database they list GPS coordinates that are different from ForeFlight and also different from the GTN.

Should I just ignore those differences? I don’t want to create user waypoints that are going to be wrong. My main concern is not having the required precision when shooting the approach on the network.

Thx as always.

Joey
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Re: Waypoint coordinates

Postby Keith Smith » Wed Feb 05, 2020 7:34 pm

Any chance some of the coordinates are decimal degrees and the others are in minutes and seconds? Can you post all the relevant data so we can review?
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Re: Waypoint coordinates

Postby mh81 » Wed Feb 05, 2020 7:37 pm

I have seen a couple of approaches in X-Plane that entirely didn't match the real approach, and have just avoided those approaches in the sim. I've also noticed that some of the headings can be a degree or two off, but I've never had a problem related to that. Also, there are published approaches that are not in X-Plane. I've learned to check the GPS in the sim before requesting an approach.
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Re: Waypoint coordinates

Postby dhplane » Thu Feb 06, 2020 5:42 am

WALLY intersection coordinates:

ForeFlight:
N34.11
W118.11

FAA:
N34-06-33.2300
W118-06-44.8700

RXP GTN 750:
N34 05.95
W118 05.45
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Re: Waypoint coordinates

Postby Keith Smith » Thu Feb 06, 2020 6:35 am

As suspected, you're comparing two different notations with regards to ForeFlight and the FAA data. The latter is in degrees, minutes and decimal seconds. If you convert the FAA to decimal degrees, it's 34.105583. I suspect ForeFlight is just rounding to the nearest hundredth, hence 34.11 for ForeFlight. Same deal for the west coordinate.

I'm not sure what the GTN is showing, though.
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Re: Waypoint coordinates

Postby dhplane » Thu Feb 06, 2020 6:46 am

Thx Keith, that makes sense…

I’m going to try to reconcile the Foreflight and GTN notations so that I can create user waypoints that are more accurate (for those fixes that are missing from the GTN database). In the practice approach I flew, the geo-referenced plate showed a deviation from the final approach course that was hardly egregious but certainly noticeable.

Thx again!

Joey
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