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Old 05-09-2011, 11:31   #601
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Re: Charts II: NGA - 2700 Charts

I have two charts entered, 22205 and 22250, which have a similar problem.

The opecpn grid lines are offset about a minute N and W of the lines on the chart. I've rechecked everything I can think of, but the offset persists.

I'd appreciate it if someone else could take a look at it and find what I'm missing.

-- rec --
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Old 05-09-2011, 12:55   #602
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Re: Charts II: NGA - 2700 Charts

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Originally Posted by cagney View Post
What the charts are called, coastal etc, is a bit of "fluff" for the bsb file that some chart plotters use, not including OpenCPN or SeaClear.
If I can remember, it is used to give the order of the charts in the lower bar ? or maybe OpenCPN uses directly the scale of the chart ?
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Old 05-09-2011, 13:24   #603
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Re: Charts II: NGA - 2700 Charts

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Originally Posted by recri View Post
I have two charts entered, 22205 and 22250, which have a similar problem.

The opecpn grid lines are offset about a minute N and W of the lines on the chart. I've rechecked everything I can think of, but the offset persists.

I'd appreciate it if someone else could take a look at it and find what I'm missing.

-- rec --
I doublechecked everything. The entries are OK. I saw on OpenCPN that 2205 offsets are even worse : 2' E and 1.3' S wen comparing with the CM93. Both charts have a big difference in the x and y positions of the corners. maybe to big for an automatic correction ? But I have heard that some charts in south of Chile have errors of that order. Have you tried other charts of Chile ?
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Old 05-09-2011, 13:48   #604
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Re: Charts II: NGA - 2700 Charts

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I doublechecked everything. The entries are OK. I saw on OpenCPN that 2205 offsets are even worse : 2' E and 1.3' S wen comparing with the CM93. Both charts have a big difference in the x and y positions of the corners. maybe to big for an automatic correction ? But I have heard that some charts in south of Chile have errors of that order. Have you tried other charts of Chile ?
Everything else at General scale down the west coast of South America that I've done is working fine. (Though I mislabeled most of them as Coastal.)

The west half of the Straits of Magellan, chart 22032, was entered by someone else and had a bad problem which I missed on first proofing: a one degree error in the NE corner latitude. I just fixed that, it should help it out a lot.
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Old 05-09-2011, 14:17   #605
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Re: Charts II: NGA - 2700 Charts

Well, I am afraid, i am the culprit ... I made a batch of charts and am verifying them at the moment. thanks
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Old 05-09-2011, 15:03   #606
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Re: Charts II: NGA - 2700 Charts

Verifying.......

I will try to put a "Calibrators manual" together.
Call for inputs - especially "Best practices for verifying"

Thomas, I think you have a lot of experience how to proceed and what to check.

The debates on corrections (datums), how to place the PLYs, type of charts are already very valuable.

Hubert
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Old 05-09-2011, 15:14   #607
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Re: Charts II: NGA - 2700 Charts

Quote:
Originally Posted by recri View Post
I have two charts entered, 22205 and 22250, which have a similar problem.

The opecpn grid lines are offset about a minute N and W of the lines on the chart. I've rechecked everything I can think of, but the offset persists.

I'd appreciate it if someone else could take a look at it and find what I'm missing.

-- rec --
Recri & Nohal

Similar problem to #586. Using the same header as the produced kap with pixels from the web, I get a tight fit in single mode, doing the kap manually, with a full size jpg. In quilted mode, however, it deteriorates slightly due to the skew => triangular wedges top and bottom.

Thomas
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Old 05-09-2011, 15:37   #608
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Re: Charts II: NGA - 2700 Charts

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Originally Posted by recri View Post
Everything else at General scale down the west coast of South America that I've done is working fine. (Though I mislabeled most of them as Coastal.)

The west half of the Straits of Magellan, chart 22032, was entered by someone else and had a bad problem which I missed on first proofing: a one degree error in the NE corner latitude. I just fixed that, it should help it out a lot.
...don't worry,the Siff's link page I quoted also says this...

"In the classification system used by NIMA, the sailingcharts are incorporated in the general charts classification(smaller than about 1:150,000); those coast charts especially
useful for approaching more confined waters (bays, harbors)are classified as approach charts. There is considerable over-
lap in these designations, and the classification of a chart isbest determined by its use and by its relationship to othercharts of the area. The use of insets complicates the place-ment of charts into rigid classifications.
"

anyways,I made the same mistake...I figured anything larger scale than The BlackSea or say,1:250,000 is ok as "Coastal"
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Old 05-09-2011, 15:50   #609
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Re: Charts II: NGA - 2700 Charts

Thomas?... if I change NAD27 to WGS84,no shift happens..(it's quite a bit different than WGS84)..But I recall,SeaClear,did shift my charts in an annoying though legitimate,way.
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Old 05-09-2011, 16:00   #610
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Re: Charts II: NGA - 2700 Charts

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Well, I am afraid, i am the culprit ... I made a batch of charts and am verifying them at the moment. thanks
You're quite welcome. It's easy to do, and easy to fix once you notice that the overlaid grid coordinates are way off.
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Old 05-09-2011, 16:06   #611
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Re: Charts II: NGA - 2700 Charts

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Thomas?... if I change NAD27 to WGS84,no shift happens..(it's quite a bit different than WGS84)..But I recall,SeaClear,did shift my charts in an annoying though legitimate,way.
I had a few Central American charts which gave instructions for correcting NAD-1927 to the chart datum where the chart datum was otherwise identified as WGS. The charts appeared to come out fine specified as datum WGS.

I wonder if the note is more an aid to navigators who are transferring NAD-1927 referenced coordinates than a correction to the chart datum?
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Old 05-09-2011, 16:25   #612
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Re: Charts II: NGA - 2700 Charts

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Verifying.......

I will try to put a "Calibrators manual" together.
Call for inputs - especially "Best practices for verifying"

Thomas, I think you have a lot of experience how to proceed and what to check.

The debates on corrections (datums), how to place the PLYs, type of charts are already very valuable.

Hubert
I've been using opencpn in unquilted mode, restricted to a test chart directory. I turn on the grid overlay and check that the labeled chart tick marks match the overlaid grid and the labels.

I usually scroll all the way around the frame using the cursor motion keys because you'll get mismatches on one edge that don't appear on the opposite edge. The chart is zoomed to a comfortable size, but not too large -- if you zoom too large, nothing matches very well, and if you zoom too small, everything matches.

Things are usually matching pretty well on the first try, if there aren't typos in the corner coordinates. If there's a consistent error in meridian or parallel matching toward a corner or an edge, I go back and tweak the corner pixel(s) in or out one pixel, which requires ten clicks on the appropriate fine adjustment button.

One thing that's puzzled me is that opencpn's latitude lines appear to move around relative to the chart ticks as you scroll north and south, as if it's being computed with a round-off error of some kind. The longitude lines don't have the same problem, they're stable as you scroll east and west. I've been accepting the latitude match if the lines coincide somewhere while they're on screen.

This doesn't feel like "best practices", but it's what I've worked out over the past few days and seems to work in most cases.
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Old 05-09-2011, 20:05   #613
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Re: Charts II: NGA - 2700 Charts

Recri..well,if it says it's WGS,it is...and the note is from the paper chart,I guess.But when you get one that really wants a correction,you'll notice it.
Hereabouts, on a 1:40000 scale wgs84 chart,NAD27 looks like:


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Old 05-09-2011, 22:33   #614
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Re: Charts II: NGA - 2700 Charts

Happy --

So if you have a chart that really isn't on a WGS datum, there are two possibilities:

1) You find a datum note, enter the datum or the specific correction, and after the correction the grid lines on the chart won't come out aligned to opencpn's grid lines, they'll all be displaced, along with everything else, by the correction.

2) You don't find a datum note, no correction is applied, and the grid lines on the chart will be aligned with the opencpn grid lines, but all the fixed features on the chart will be displaced by the inverse of the missing correction. So the check is to overlay onto a known WGS chart and compare the two.

So my current checking doesn't even catch case 2) and can't catch it until I have layers of charts over given features and an easy way to compare the alignment of different layers.
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Old 05-09-2011, 22:54   #615
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Re: Charts II: NGA - 2700 Charts

Recri..that's right.The latlong gridlines are meaningless anyways unless you are using a pencil,dividers,and a chart table.People that take bearings and so on couldn't care less about datum.They just want everything in their places... Sextants ?well it's in their zone of error so they don't care either.But for a GPS it must agree...and you can't know if it does unless you have another reliable chart or List of Lights (with positions) for the area ...or unless you are there,Garmin in hand,atop the reef...Gee,I even used to change the datum in my GPS to agree with the map on occasion,but nobody wants to do that all the time.
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