georeferencing errors

dmspilot

dmspilot

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The screenshot is from iFlightPlanner, but Skyvector and the Avare app for Android do the same thing. Why aren't the plot and sectional chart coincident? It's always about a half-mile off for all the Navaids and airports in the area.
 

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I checked it out on Garmin Pilot, same thing.

My guess - it's a printing deal on the sectional. That VOR legend symbol takes of a whole lot more space than the real VOR itself. Since it's right next to the road, they printed it a little bit offset so it wouldn't overlap. The route symbol is probably being displayed in it's proper position.


edit: I just noticed you said, "all the navaids in the area", so its' not just this one.

I looked at a local navaid, local to me, and it's right on. So it's still probably just something that's a little offset on the printing of the chart.
 
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Switch the chart from section to IR Low/high and look again. It's right on.
 
It's an artifact of the way these services handle their georeferncing. Try a course involving FAK in a plan that stays entirely within the WASHINGTON sectional (or switch to the WAC or IFR charts).

It's fun to get this right in a mosaic (says the man who made his fortunate mosaicking georeferenced spy satellite images together).
 
Matthew said:
Switch the chart from section to IR Low/high and look again. It's right on.
Click to expand...

Interesting.

flyingron said:
It's an artifact of the way these services handle their georeferncing. Try a course involving FAK in a plan that stays entirely within the WASHINGTON sectional (or switch to the WAC or IFR charts).

It's fun to get this right in a mosaic (says the man who made his fortunate mosaicking georeferenced spy satellite images together).
Click to expand...

Whether the route stays on the same chart doesn't seem to make a difference.

Assumedly all of the services I've checked are using the same formula to determine where to draw the plot because the error is in the same amount and direction, at least when looking at the same chart.

The WAC is also inaccurate, but the error is in a different direction. The IFR charts seem to be much more precise. Do the IFR charts use a different projection? (Sectional is Lambert conformic conal or whatever it's called.)
 
dmspilot said:
The screenshot is from iFlightPlanner, but Skyvector and the Avare app for Android do the same thing. Why aren't the plot and sectional chart coincident? It's always about a half-mile off for all the Navaids and airports in the area.
Click to expand...

They are likely non congruent mapping formats. There is more than one georeferencing global model. Usually on systems there is a reference model option to select the one in use for that cartographic system that takes care of these issues. BTW, this is one of the reasons IFR gear needs to be certified, to make sure this doesn't happen.
 
So the VFR sectional is off a little bit. This this for visual flying...will it make a difference?
 
Brad Z said:
Either way, does it matter?
Click to expand...

It's not a big deal, but I have my reasons...and it gets on my nerves. Shouldn't an electronic tool be more accurate than a paper one? In this case, it's less.
 
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dmspilot said:
... Avare app for Android do the same thing. Why aren't the plot and sectional chart coincident? It's always about a half-mile off for all the Navaids and airports in the area.
Click to expand...

Hi DMS and everyone.
The reason the Chart in Avare is off it's because it gets scanned and there are some errors attributed to that process.
If and when you create a FPlan the data is extracted from the Digital database and is as accurate as FAA publishes it.TV
 
dmspilot said:
Planning a cross-country with VFR checkpoints means that the checkpoint could be shown on one side of the course when it's actually the opposite side.
Click to expand...

:rofl: Like you expect to be right on the centerline. :)

dtuuri
 
Any paper map is a "projection" of the earth's curved surface onto a flat surface. That's why the edges of sectionals don't match up exactly. https://en.wikipedia.org/wiki/Map_projection

I don't know, but I would be surprised if the plotting software takes the map projection distortions into consideration and reverses the calculations. An easy way to check would be to plot something that's in the center of a sectional and compare the error to a plot of something that's near the edge.
 
dtuuri said:
:rofl: Like you expect to be right on the centerline. :)
Click to expand...

In practice it may not be a big deal, but I think electronic tools ought to be more accurate than pencil and paper, not less.
 
To be honest I've never really understood this fixation of using scanned paper sectional charts for electronic moving maps. The sectional is designed to have all (or as much as possible) information on a sheet of paper. It's this way because of the limitations inherent in a paper map. There is no good reason for an electronic moving map to follow this format.
 
dmspilot said:
Whether the route stays on the same chart doesn't seem to make a difference.
Click to expand...
It did for me. If I did KRIC-FAK-KMEM it dropped the FAK waypoint about 4 mm too low on the map. If I did KRIC-FAK-KROA
Assumedly all of the services I've checked are using the same formula to determine where to draw the plot because the error is in the same amount and direction, at least when looking at the same chart.
Click to expand...
Actually, you'd be surprised. The code to read GEOTIFFS (which is what these sectionals are published by the FAA in) is pretty much the same, all based on the TIFF reader written by Sam Leffler at SGI with the GEOTIFF add ons by Niles Ritter and Frank Warmerdam with the tables borrowed from EPSG). The actual transformation code is almost certainly the government furnished PROG4 which just about every product out there be it commerical or open source uses.
The WAC is also inaccurate, but the error is in a different direction. The IFR charts seem to be much more precise. Do the IFR charts use a different projection? (Sectional is Lambert conformic conal or whatever it's called.)
Click to expand...
The FAA IFR charts are not conformal because they don't expect you to put a plotter to the thing to compute angles but rather to just use the airways. The sectionals and WACs are indeed lambert conformal conics. The FAA IFR enroutes are in mercator projection (note I believe that Jepp uses lambert conformal even for the enroutes).
 
Silvaire said:
To be honest I've never really understood this fixation of using scanned paper sectional charts for electronic moving maps. The sectional is designed to have all (or as much as possible) information on a sheet of paper. It's this way because of the limitations inherent in a paper map. There is no good reason for an electronic moving map to follow this format.
Click to expand...

Because Raster Scan was the initial form of digitizing charts, Vector Scan didn't come out for years later. I forget what datum that Sectionals were drawn on, but it wasn't WGS-84 which is the native for most GPS based charting systems.
 
Sectionals are NAD1983. Again, this shouldn't be an issue if things are done right in the software.
 
Drive down a highway that is on the sectional and see where your GPS tells you you are WRT the road on the sectional. IME it's usually off the width of the roadway or more.
 
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Did you mean PROJ4? Or is that Oracle specific?

flyingron said:
It did for me. If I did KRIC-FAK-KMEM it dropped the FAK waypoint about 4 mm too low on the map. If I did KRIC-FAK-KROA



Actually, you'd be surprised. The code to read GEOTIFFS (which is what these sectionals are published by the FAA in) is pretty much the same, all based on the TIFF reader written by Sam Leffler at SGI with the GEOTIFF add ons by Niles Ritter and Frank Warmerdam with the tables borrowed from EPSG). The actual transformation code is almost certainly the government furnished PROG4 which just about every product out there be it commerical or open source uses.



The FAA IFR charts are not conformal because they don't expect you to put a plotter to the thing to compute angles but rather to just use the airways. The sectionals and WACs are indeed lambert conformal conics. The FAA IFR enroutes are in mercator projection (note I believe that Jepp uses lambert conformal even for the enroutes).
Click to expand...
 
Henning said:
Because Raster Scan was the initial form of digitizing charts, Vector Scan didn't come out for years later. I forget what datum that Sectionals were drawn on, but it wasn't WGS-84 which is the native for most GPS based charting systems.
Click to expand...

My point is that, for an electronic format the paper sectional carries a lot of unnecessary clutter. There is no need to have every notice and detail statically displayed on an electronic map as it must be on a paper map. Also, as noted previously the paper sectional is meant to accommodate a plotter and pencil, you're never going to do that on your iPad.
 
Silvaire said:
My point is that, for an electronic format the paper sectional carries a lot of unnecessary clutter. There is no need to have every notice and detail statically displayed on an electronic map as it must be on a paper map. Also, as noted previously the paper sectional is meant to accommodate a plotter and pencil, you're never going to do that on your iPad.
Click to expand...

You definitely have point with that, and with an SVT display it becomes even less necessary. Garmin Pilot has native vector format VFR charts with much less clutter, as well as the sectional format, and I find myself using the native format more. The sectionals are drawn to the detail they are for visual pilotage/feature recognition, when the GPS is placing your position on the chart, the need for that level of detail disappears.
 
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