Charts II: NGA - 2700 Charts

[cagney]

Nohal
You were to quick for me with the DTM bug......
But there is still something fishy with 53266, and the only thing I can think of is the skew correction. Do you remember a couple of weeks ago when I found that "rotate" warped the picture? I think that we have a case of that here. The web chart looks like this

The second set of PLY lines comes from a manual conversion I just did on the full original jpg. My calibration point are very close to the web version, but the same corner looks like this with my kap.

Where the PLY from my kap agrees tightly with the chart, in spite of the slight skew, and the web PLY is west of the real position. All this indicates a warped chart, no longer a strict Mercator chart.

Thomas

[HappySeagull]

I see a very slight but very consistent shift in all my East Africa charts...(this is 61180)
the "simple" mark is never in the cross.These are just about all "easy" WGS charts.

[nohal]

Originally Posted by cagney

Nohal
You were to quick for me with the DTM bug......
But there is still something fishy with 53266, and the only thing I can think of is the skew correction. Do you remember a couple of weeks ago when I found that "rotate" warped the picture? I think that we have a case of that here. The web chart looks like this
Attachment 31146
The second set of PLY lines comes from a manual conversion I just did on the full original jpg. My calibration point are very close to the web version, but the same corner looks like this with my kap.
Attachment 31147
Where the PLY from my kap agrees tightly with the chart, in spite of the slight skew, and the web PLY is west of the real position. All this indicates a warped chart, no longer a strict Mercator chart.

Thomas

Thomas,
can you please mail me your manual calibration for analysis? Still can't figure out how the rotation could warp the chart, but really it looks like there's something wrong...

Thanks

Pavel

[bcn]

A clarification needed:
if the chart the Datum Note states:
"To position WGS72 coordinates on this chart
add 5.09 seconds to Latitude
subtract 4.66 seconds from Longitude"

this translates to
0.09' Latitude
0.08' Longitude

Now in the application:
"Corrects Chart Datum to:
Adjustment north-/southward: 0.09' Northward
Adjustment east-/westward: 0.08' Westward"

I understand that the Datum Note indicates how to transpose a GPS date correctly on the chart.
Does the application go the other way around, moving the chart?
So in case of adding in the paper chart we have to move pixels southward?
Or in case of subtracting longitude on paper we move the chart pixel westward?

Hubert

[cagney]

Originally Posted by bcn

A clarification needed:
if the chart the Datum Note states:
"To position WGS72 coordinates on this chart
add 5.09 seconds to Latitude
subtract 4.66 seconds from Longitude"

this translates to
0.09' Latitude
0.08' Longitude

Now in the application:
"Corrects Chart Datum to:
Adjustment north-/southward: 0.09' Northward
Adjustment east-/westward: 0.08' Westward"

I understand that the Datum Note indicates how to transpose a GPS date correctly on the chart.
Does the application go the other way around, moving the chart?
So in case of adding in the paper chart we have to move pixels southward?
Or in case of subtracting longitude on paper we move the chart pixel westward?

Hubert

Good observations.
I have never seen this correction given in seconds on the chart, this s a first.

The normal case is like this. The correction is given in minutes on the chart.
This is what you enter in the webform.
This is then, by a script, converted to seconds and the signs reversed before it goes into the header of the kap file. The signs are reversed, as you observed, because we are doing the reverse to the case stated on the charts.

Look at your #579 where we had a correction on the chart of

0.03' Southward
0.03' Eastward

Now open 53266.kap in a text editor and

DTM/1.8,-1.8

This is according to the BSB2 protocol.

In this case: just put a note in the comment box, as we will have to correct the DTM values manually, not to loose any accuracy.

Thomas

[recri]

I've entered a bunch of region 2 charts, working south from Ensenada along the coast.

There have been a few charts that want to specify the seconds for the scale latitude.

Many of the "reasons" for failure can be true at once: there are multiple rectangles, there are non-rectangles, there is chart junk inside the chart boundary, the corner clips don't show the chart corners. So maybe some of that should be broken out as a check box menu rather than making a longer and longer radio box menu of status types?

A rectangle with a wart on one edge is a common pattern that could be partly done in phase one, the wart would be cut off in the quilt, but the core of the chart could still be completed.

-- rec --

[ChuckSK]

Many NOAA BSB charts have blocks of text and LORAN LINEAR INTERPOLATOR graphs withing the navigation field. Blocking out chart titles and interpolation tables with special plys may be an unnecessary complication.

Chuck

[siff]

Firstly, many thanks for your great work. I'll try to help doing the calibration from Uruguay and southwards, in which I am interested ...

May I suggest a small simplification to the labelling of the charts :
It could be possible to automate the labelling of the chart type using the scale of the chart (cf this publication of NGA). It would speed a little the entries and avoid some interpretation of the different terms. I am still wondering what a "small craft route" chart looks like .

[paulr]

The versions of 82377 and 93340 in the database are defective.

I have redownloaded them from source and put new copies in the incoming folder on the ftp server.

Paul

[HappySeagull]

Originally Posted by bcn

A clarification needed:
if the chart the Datum Note states:
"To position WGS72 coordinates on this chart
add 5.09 seconds to Latitude
subtract 4.66 seconds from Longitude"

this translates to
0.09' Latitude
0.08' Longitude

Now in the application:
"Corrects Chart Datum to:
Adjustment north-/southward: 0.09' Northward
Adjustment east-/westward: 0.08' Westward"

I understand that the Datum Note indicates how to transpose a GPS date correctly on the chart.
Does the application go the other way around, moving the chart?
So in case of adding in the paper chart we have to move pixels southward?
Or in case of subtracting longitude on paper we move the chart pixel westward?

Hubert

It'd be 'way easier to apply the offered corrections at the corners ...I have a lot of old Geographic Datum-paper-charts-converted-to-WGS84 that were corrected this way by the "pros" .I don't think I have one with DTM corrections....

(so,Cagney,I am now confused by :
"The normal case is like this. The correction is given in minutes on the chart.
This is what you enter in the webform.
This is then, by a script, converted to seconds and the signs reversed before it goes into the header of the kap file. The signs are reversed, as you observed, because we are doing the reverse to the case stated on the charts."...
You are saying whatever I put in as a correction will be reversed by the script?while here I was ,attempting to shift the chart with the correction offered myself.No wonder if they don't come out right!

[bcn]

Happy,

"faking" the corner values might do it, but this a little bit a "quick & dirty" solution (or "chapuza" as we say here) - you are loosing the transparency of the process!
Who will remember this - or even know - touching the next time this chart.
Perhaps we have to consider seriously to write a small "manual for calibrators".
The one Pavel suggested.

Hubert

[HappySeagull]

bcn:not so "fake" or "quick and dirty" if the CHS and Maptech did it.as to remembering,it's maybe why the CHS called these charts "NAD 27" or NAD 83 and so on in the kap header,but we could put a note in.
These are all old charts.Actually showing lines and scales of Lat and Long on the chart is "Old school" and irrelevant on a digital chart and pretty much tells you how old these are and that they have been fudged whatever....

in fact,we are calibrating historical documents,if you like.

[HappySeagull]

Siff,that's a good point.Deciding if it's harbour or coastal can be based on the scale...(not that I've been sure why filling out this field is anyway relevant on a computer)
"Sailing charts are the smallest scale charts used for
planning, fixing position at sea, and for plotting the dead
reckoning while proceeding on a long voyage. The scale is
generally smaller than 1:600,000. The shoreline and topog-
raphy are generalized and only offshore soundings, the
principal navigational lights, outer buoys, and landmarks
visible at considerable distances are shown.
"General charts are intended for coastwise navigation
outside of outlying reefs and shoals. The scales range from
about 1:150,000 to 1:600,000.
Coastal charts are intended for inshore coastwise nav-
igation, for entering or leaving bays and harbors of
considerable width, and for navigating large inland water-
ways. The scales range from about 1:50,000 to 1:150,000.
Harbor charts are intended for navigation and an-
chorage in harbors and small waterways. The scale is
generally larger than 1:50,000."

[cagney]

Originally Posted by HappySeagull

bcn:not so "fake" or "quick and dirty" if the CHS and Maptech did it.as to remembering,it's maybe why the CHS called these charts "NAD 27" or NAD 83 and so on in the kap header,but we could put a note in.
These are all old charts.Actually showing lines and scales of Lat and Long on the chart is "Old school" and irrelevant on a digital chart and pretty much tells you how old these are and that they have been fudged whatever....

in fact,we are calibrating historical documents,if you like.

OpenCPN recognizes 101 chart datums internally, not just WGS 72 & 84.
To see all the details, have a look at georef.c in the source code.
IIRC NAD 83 is more or less the same as WGS 84 for the area concerned.(Norh America).


Regarding DTM etc, a lot of the nitty gritty in the kap header was ironed out
in OpenCPN when the NZ pictures were done originally. In preparation for the NGA calibration I summarized a lot of this in a commented header available here ->http://http://opencpn.org/drupal/imgkap

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. The suggestion of using the scale for this, is not bad, and can always be implemented later on, from info in the chart data base.

The exact latitude where the scale is valid is not mega important. If it is given down to the second, just use the nearest minute.

Thomas

[recri]

Chart 22225, which is rotated 90d in our image set, is now served North up in at the NOAA website.

Chart 22275, which is two charts further south down the Chile coast, is still sideways at NOAA.

-- rec --