Route on Polyconic

[ktcanuck]

I wondered whether the new Beta would fix this but it seems not. I have a route that runs through the St Lawrence section of NOAA polyconic charts.

If I show the chart and overlay the route from an import it may show correctly initially but as soon as the scale is changed via the scroll wheel the route does not follow correctly.

Also the polyconic chart does not display north up.

[HappySeagull]

Originally Posted by ktcanuck

I wondered whether the new Beta would fix this but it seems not. I have a route that runs through the St Lawrence section of NOAA polyconic charts.

If I show the chart and overlay the route from an import it may show correctly initially but as soon as the scale is changed via the scroll wheel the route does not follow correctly.

Also the polyconic chart does not display north up.

ktcanuck,Hi...Are these OLD charts? I have some polyconics,too so I made a route through em,-not exactly importing as you describe but maybe you can try this too and see if the problem isn't in the "import" which is a different kettle of fish.

They actually work pretty well as I zoom up and down through various charts and only some of these are polyconic...But...some charts I have, have INSETS of "plans".If you happen upon one of these (They show up in the bottom bar EVEN if location is wrong..)and draw a route through it,it'll do as you describe, when consequently zoomed out into a larger area chart...
If this is what's happening,you'll need be careful you are in the location you want be by zooming in first from a larger area chart.Does that help?

...the UTM and Transverse mercator charts also can be skewed to look at but doesn't mean they are wrong.Place some "marks" on them and see if they agree with other charts....And,(I shoulda said this first!) in OPenCPN settings there's a box to check.="show skewed charts as North UP"...Have you tried this?

There are other solutions too but they are more complicated!

[ktcanuck]

Hi Seagull. The easy ones first. The charts are not new but they are the current ones from the NOAA website. I do run OpenCPN with skewed charts north up.

I think I follow your drift and I will check your suggestion.

I wish I could remember what I used to create the route originally. I think it was OpenCPN. It does show correctly on a Mercator chart. As it happens I have a chip for my chart plotter for this area so it isn't a navigation issue for me. I would just like to understand why I am getting these effects.

Keith

[ktcanuck]

I used 14773 and 14774 as my test. First, they did not display north up. I placed a route across the boundary between the two. I then left clicked down to move the map and the route was displaced. I zoomed in and out and the route moved around the map.

Is there something obvious I am missing?

[bdbcat]

ktcanuck....

I think I can understand this. Let me restate:

Condition:
1. Single chart mode (not quilted)
2. Toolbox->Settings "Show skewed raster as north-up" is SET.
3. Skewed Polyconic BSB charts from NOAA, around Grindstone Island, for instance.

Result:
Route drawing is incorrect. Route geolocation and rendering changes with pan and zoom.

However, if the Toolbox->Settings "Show skewed raster as north-up" is CLEAR, then the routes are rendered correctly on the skewed charts.

Do I have this right?

If so, sure looks like a legitimate bug to me.

Thanks
Dave

[ktcanuck]

Not quite. It doesn't matter whether the show skewed north is on or off. It seems that when a polyconic chart is in view, the route does not stay put.

[ktcanuck]

Let me add something. My route also passes through chart 14802 which is also polyconic. I can zoom in and out on this chart without the route moving!

[bdbcat]

ktcanuck...

Sorry, cannot reproduce the problem with North-up checkbox CLEARed.
It seems to work fine in this scenario.

Also, for reference, this scenario works fine for skewed Mercator charts, as are found in US ICW, say South Carolina. North-up and natural mode work fine. Further, non-skewed polyconics (e.g. 14802) work OK.

So clearly the problem relates only to skewed polyconics.

Can you upload some screenshots of the polyconic problem?

Thanks
Dave

[HappySeagull]

ktcanuck...
Yes,per"My route also passes through chart 14802 which is also polyconic. I can zoom in and out on this chart without the route moving! ".Ditto with mine.But my polys aren't skewed and are mostly large scale harbour plans.so near enough to mercator it don't matter...
As to GreatLakes,I only have 14800 and it's a Mercator.

My guess as to "skew" and I hope someone'll correct me !-when you check the "View skewed chart as North Up" box,Open CPN will look into the actual chart data and IF a skew is actually noted in the chart's "header",OpenCPN can correct for it.But if it's not written in there,it won't make any difference,checked or not.
I think OpenCPN just treats Polyconics and other non-mercators as Mercators.I can imagine the difficulty-there would have to be some odd jig-jogs in a route that crossed small scale chart boundaries where one chart looked like an "orange-slice" or just plain tilted and the rest were "flat-earth" rectangles....(I have some Transverse Mercs tilted like this but I straightened 'em and rebuilt em as Mercators without noticeable ill effect...)

[HappySeagull]

AHA!I see what you mean,ktcanuck!!

[bdbcat]

Happy....

Well, yep, I see the problem on your attached shot. Clearly broken.

I wonder what is the edition date of the 14773 chart? It seems that the edition you are using has no skew specified in the KAP header. It is polyconic....

Hover over the selector bar to get the chart info window.

My version is Source edition 17, 2004-09-01. It shows as skewed polyconic.
The "conic" icon in your chart bar is overridden by the "skew" icon on my chart set.
From the .KAP file header:

KNP/SC=15000,GD=NAD83,PR=POLYCONIC,PP=-76.050,PI=1.000,SP=,SK=44.9370824
    TA=134.9370824,UN=FEET,SD=GREAT LAKES LOW WATER,DX=1.50,DY=1.50
KNQ/EC=RF,GD=NARC,VC=UNKNOWN,SC=GLLW,PC=PH,P1=-76.050,P2=UNKNOWN
    P3=NOT_APPLICABLE,P4=NOT_APPLICABLE,GC=NOT_APPLICABLE,RM=POLYNOMIAL
CED/SE=17,RE=01,ED=09/01/2004

I shall go onto NOAA right now and check the latest available chart.

A mystery, but we'll work it out.....
Dave

[bdbcat]

Happy....

OK, the current NOAA download KAP header looks like this:

KNP/SC=15000,GD=NAD83,PR=POLYCONIC,PP=-76.05,PI=1.000,SP=,SK=0.0000000
    TA=90.0000000,UN=FEET,SD=GREAT LAKES LOW WATER,DX=1.50,DY=1.50
KNQ/EC=RF,GD=NARC,VC=UNKNOWN,SC=GLLW,PC=PH,P1=-76.050,P2=UNKNOWN
    P3=NOT_APPLICABLE,P4=NOT_APPLICABLE,GC=NOT_APPLICABLE,RM=POLYNOMIAL
CED/SE=17,RE=01,ED=09/01/2004

Skew is 0.0 (!)

What are we to make of this? The chart is obviously skewed....

Stumped
Dave

[HappySeagull]

Hi!Bdbcat...
gee yer too quick.I was just going to say...
Yeah,SKEW=0! You better keep your older version.!Also I got 14774_1...same-no skew.
Oh,and for "fun" I did this-wrote a guess for skew.45.It's better with ShowNorthUp"set...sort of.



edit...I'll try the skew from your earlier post...

[cagney]

Dave & Happy

This is clearly a chart error.
A few ideas. Of course OpenCPN could calculate the correct SK from the frame around the charts (the PLY:s), but this is an overhead that we could do without. A script could crawl a folder an do the correction as well.
But ... as it is a chart error, and probably restricted to a few charts that may be corrected in the next edition, let us treat it as such.
We can create a chart correction page for this kind of errors on OpenCPN.org with instructions for the user how to fix the error.
With US charts we could even host an updated version.

Thomas

[ktcanuck]

Happy to see it wasn't me being stupid! Thanks for the work!