GPS accuracy on a Foreflight Sectional

[Sac Arrow]

Kind of curious to know how accurately a GPS location is referenced on a sectional map within Foreflight, or any other similar aviation sectional map application. Sectionals aren't vector data (at least I'm pretty sure they aren't) but rather digital UTM coordinate raster maps somehow rubber banded to conform with the coordinates within the display.

Anybody have any insight on that?

 

[Yellowbird]

I've occasionally noticed a slight difference between positions of the little dot that indicates a route waypoint (say, a VOR) and the position shown on the sectional overlay. It's not much, but unless you are using Foreflight for geocaching, it's not going to be enough to throw you off course.

 

[Sac Arrow]

Navigation isn't the issue. Shaving airspace could be.

 

[Let'sgoflying!]

I would keep my eyes down at my ipad, and weave in and out of towers using only FF as a reference its that good.
Well no I wouldn't be crazy, but they do appear to be exactly where FF puts them.

 

[Sac Arrow]

I've occasionally noticed a slight difference between positions of the little dot that indicates a route waypoint (say, a VOR) and the position shown on the sectional overlay. It's not much, but unless you are using Foreflight for geocaching, it's not going to be enough to throw you off course.

Actually that was a good little exercise. I picked out a route of four VOR's, including one in Class B. Three were a little off, and varied slightly depending on the map zoom. The one in Class B was pretty much on.

But, on the low altitude IFR enroute chart, they were all spot on. Presumably those are stored as vector data, judging by the comparative data file sizes.

 

[waldo]

Actually that was a good little exercise. I picked out a route of four VOR's, including one in Class B. Three were a little off, and varied slightly depending on the map zoom. The one in Class B was pretty much on.

But, on the low altitude IFR enroute chart, they were all spot on. Presumably those are stored as vector data, judging by the comparative data file sizes.

Just curious . . . how much is a little off to you? Obviously I wouldn't be concerned about 1/8 mile over a VOR. I haven't found any significant errors on my geo ref charts . . . maybe 20-30 feet or so, but that's about it.

 

[flyingron]

Kind of curious to know how accurately a GPS location is referenced on a sectional map within Foreflight, or any other similar aviation sectional map application. Sectionals aren't vector data (at least I'm pretty sure they aren't) but rather digital UTM coordinate raster maps somehow rubber banded to conform with the coordinates within the display.

Anybody have any insight on that?

Not quite. They are not UTM. They're in lambert conformal conic projection. The GEOTIFFs that NACO put out have the parameters of the projection to accurately determine the precise geo position on any point on the map.

I've got no idea how Foreflight deals with things internally, but I would expect rather than trying to reproject the maps into some idealized global space that they just plot the stuff that's not on the map (including the current projection) into the coordinate system of the base map (at least that is the way it is done in the RemoteView which I wrote), but I could be wrong.

Sac: What do you mean off? Do you mean that the waypoints which were the VORs didn't correspond to the actual printed location on the chart?

 

[Sac Arrow]

Just curious . . . how much is a little off to you? Obviously I wouldn't be concerned about 1/8 mile over a VOR. I haven't found any significant errors on my geo ref charts . . . maybe 20-30 feet or so, but that's about it.

Not quite. They are not UTM. They're in lambert conformal conic projection. The GEOTIFFs that NACO put out have the parameters of the projection to accurately determine the precise geo position on any point on the map.

I've got no idea how Foreflight deals with things internally, but I would expect rather than trying to reproject the maps into some idealized global space that they just plot the stuff that's not on the map (including the current projection) into the coordinate system of the base map (at least that is the way it is done in the RemoteView which I wrote), but I could be wrong.

Sac: What do you mean off? Do you mean that the waypoints which were the VORs didn't correspond to the actual printed location on the chart?

Correct - not important for the VOR's themselves, or airports or ground features, but it can be important if you are operating very close to airspace. I do a lot of flying under and right up next to Bravo and Charlie airspace. "Off" to me is indicating I'm outside of the depicted airspace when I'm in fact in it. 20 or 30 feet is no big deal but that is roughly the accuracy of the device anyway. The deviations I've noted from the printed map are more than that.

 

[Pilawt]

Airport symbols on sectional charts are not always accurately placed. My home airport is an example. The airport symbol is about 1-1/2 to 2 miles too far north.

In the Foreflight screenshot below, the blue airplane symbol accurately shows where I was on the ground, rolling out at the west end of VUO's runway. When you're on downwind north of the airport, the chart makes it look like you're south of the airport.

(There is a more-accurate 1:250,000 VFR depiction of the Portland area, but it's just an inset in the margin of the paper Seattle sectional, not a separate Terminal Area Chart -- thus, it's not available in Foreflight.)

Interestingly, airport symbols on IFR enroute charts seem to be very accurately placed.

 

[waldo]

Very interesting indeed! Maybe that's why I haven't noticed, because I always use Low IFR Charts.

 

[Captain]

The accuracy is displayed right there on Foreflight...10 meters in the pic above.

If 30 feet is the difference between hitting the airspace and not then I'd say maybe you're cutting it too close.

 

[Clark1961]

The accuracy is displayed right there on Foreflight...10 meters in the pic above.

If 30 feet is the difference between hitting the airspace and not then I'd say maybe you're cutting it too close.

Uhh, Cameron, the discussion is about accuracy on the chart. It is not about the accuracy of the GPS. Two different things.

 

[Sac Arrow]

The accuracy is displayed right there on Foreflight...10 meters in the pic above.

If 30 feet is the difference between hitting the airspace and not then I'd say maybe you're cutting it too close.

On your first point, see below. Regarding your second, I agree 30 feet is cutting airspace too close. But what about one or two thousand feet? I know on a panel mount GPS such as a 430 object locations and airspace boundaries are pretty accurate. The real question is just how much berth airspace depictions should be given on a Foreflight sectional.

IFR low charts appear to be pretty accurate but they aren't very informative for Class C and D, and Bravo inner rings.

Uhh, Cameron, the discussion is about accuracy on the chart. It is not about the accuracy of the GPS. Two different things.