Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rating: Thread Rating: 3 votes, 5.00 average. Display Modes
Old 19-12-2011, 13:43   #1126
Registered User
 
HappySeagull's Avatar

Join Date: Dec 2010
Location: B.C.,Canada
Boat: 29'
Posts: 2,395
Re: Charts II: NGA - 2700 Charts

Quote:
Originally Posted by wdkester View Post
Happy,
I'm confused by "GE can't get the Greenwich meridian right (it's .249 min west)" Couldyou say a little more?
...oh,I was looking up the accuracy of GE with an eye as to quick-check our handiwork....
The above ".249" is just an illustrative gem in all that...if you go to Greenwich in GE,the observatory is about that westward of GE's 0 longitude.There's all kinds of reasons touted for that, but I think they'd all be news to the painstaking folks that drew our charts.
__________________

__________________
HappySeagull is offline   Reply With Quote
Old 19-12-2011, 15:42   #1127
Registered User

Join Date: Jan 2007
Location: Minneapolis
Boat: Irwin 37 CC
Posts: 660
Re: Charts II: NGA - 2700 Charts

From the fount of all knowledge "Wikipedia-
"The shift from several spheroids to one worldwide spheroid caused all geographical coordinates to shift by many metres, sometimes as much as several hundred metres. The Prime Meridian of these modern reference systems is 102.5 metres east of the Greenwich astronomical meridian represented by the stainless steel strip. Thus the strip is now 5.31 arcseconds West. The modern location of the Airy Transit is 5128′40.1247″N 00′5.3101″W / 51.477812417N 0.001475028W / 51.477812417; -0.001475028 (Airy Transit)"

This seems to be in the car park to the north of the observatoy.
__________________

__________________
David Kester
Pegasus IV
wdkester is offline   Reply With Quote
Old 19-12-2011, 17:04   #1128
Registered User
 
HappySeagull's Avatar

Join Date: Dec 2010
Location: B.C.,Canada
Boat: 29'
Posts: 2,395
Re: Charts II: NGA - 2700 Charts

I take away from that that they couldn't get Greenwich right,so there's a "modern" location....but GE will do to spot a 7 mile error for sure.
__________________
HappySeagull is offline   Reply With Quote
Old 19-12-2011, 21:42   #1129
Marine Service Provider

Join Date: Feb 2010
Posts: 279
Re: Charts II: NGA - 2700 Charts

On my version of GE the prime meridian is correctly marked 104 meters to the east of the old meridian strip If you look carefully it is now marked by a line of Sea Gull droppings
__________________
r.fairman is offline   Reply With Quote
Old 20-12-2011, 08:01   #1130
Registered User
 
HappySeagull's Avatar

Join Date: Dec 2010
Location: B.C.,Canada
Boat: 29'
Posts: 2,395
Re: Charts II: NGA - 2700 Charts


..anyways,I 'm toying with the idea to adjust the DTM in the header for a "GE-agree" version of some of these NGA charts (just for personal interest) but I'm wondering if it's a reasonable pursuit.
a DTM would be a compact and fast way of communicating chart shifts around...and it'd be easy to revert the chart too.
__________________
HappySeagull is offline   Reply With Quote
Old 20-12-2011, 12:55   #1131
Registered User

Join Date: Jan 2007
Location: Minneapolis
Boat: Irwin 37 CC
Posts: 660
Re: Charts II: NGA - 2700 Charts

Happy,
I think you are talking about editing the header of a downloaded NGA/KAP with a hex editor to create a personal copy fo the chart. Great. Tell the rest of us how to do it.

I realize that I may be a single voice; but I'm against altering the header data using the "adjustment" commands at "opencpn.info/en/nga-charts-upload-files" as this alters the archive copy. I would like to see any suggested datum changes so that I might apply them at my option. I feel that any two people are likely to disagree on the adjustments depending on how they arrive at those adjustments: e.g. using GE, a single WP, multiple WP a single track, another chat etc.

I think we need a repositoryfor information like this.
__________________
David Kester
Pegasus IV
wdkester is offline   Reply With Quote
Old 20-12-2011, 13:21   #1132
Registered User
 
HappySeagull's Avatar

Join Date: Dec 2010
Location: B.C.,Canada
Boat: 29'
Posts: 2,395
Re: Charts II: NGA - 2700 Charts

Hi,WD...yes Tiabui and recri and well, we we all are thinking of a revisions repository..but there's still Plans to do...

To edit the header,just open A COPY of the kap in "Notepad++"(portable is best for me) or anything that's half-decent....Not Windows' notepad- it's a little difficult-It adds characters to the beginning... and ,well,don't bother is my advice.

Then,save.Then drag-and-drop onto bsbfix.exe.It's a utility from the libbsb folder.PM me if you can't find it online.Opensource....if you don't have this,you should.bsbfix can be anywhere on your computer.Dragging the file onto a desktop shortcut to it should work too.
If bsbfix spews a ream of gobbledeegook.You failed.Try again.you just need to pay attention to how things are done in there.Don't add spaces unless it's in the name ...stuff like that.
__________________
HappySeagull is offline   Reply With Quote
Old 20-12-2011, 13:23   #1133
Registered User

Join Date: Dec 2005
Location: WNA
Boat: Dufour 35
Posts: 3,249
Re: Charts II: NGA - 2700 Charts

@wdkester

I agree with your views.

To change the DTM is not difficult.
Let's take 83495 Wallis Island as an example. It is way off compared to CM93 which is said to be WGS 84. I applied
Quote:
DTM/-5.0,-131.0
To bring it into reasonable agreement.

As you said open a hex editor (work with some text editors as well), edit the DTM line. Save. Then, as a general recommendation, run "bsbfix" from libbsb on the changed file.
Sometimes you can get away with changes without doing this, and some recommend aginst using "bsbfix".

Then go to the chart tab and run "Scan charts, and update database" in 2.6 or "Force full Database rebuild" in earlier versions.

The correction should now be applied.

I have a longstanding feature request aimed at making this process easier. Check FS#309 .

Thomas
__________________
cagney is offline   Reply With Quote
Old 20-12-2011, 13:32   #1134
Registered User
 
Viking Sailor's Avatar

Join Date: Nov 2006
Location: San Francisco Bay
Boat: Fantasia 35 - s/v Feeling Good
Posts: 1,074
Re: Charts II: NGA - 2700 Charts

Happy,

Be careful with using GE as a reference. I have seen a couple of spots where two over lapping photos did not align correctly. I wish I could remember where these spots are so that you could see them for yourself but I'm sorry I can't.

__________________
Viking Sailor is online now   Reply With Quote
Old 20-12-2011, 13:45   #1135
Registered User
 
HappySeagull's Avatar

Join Date: Dec 2010
Location: B.C.,Canada
Boat: 29'
Posts: 2,395
Re: Charts II: NGA - 2700 Charts

HxD is a freebie hex editor...It's a little tricky,to edit with it,but you do get the feeling you are doing serious things in there!...Surveyor's recommend of Notepad++ has been good though.It's a lot less "crucial-feeling".
Thomas is right,you can just try without bsbfix

For windozers,A good idea is to just quickly check your new kap (or any kap)in Thomas' "Kapview" which won't often load it if the kap is no good...This can save firing up Opencpn and rebuilding database and all that.It also spits out a handy tif!
I finally made a dragndrop script for irfanview or xnview if anbody wants to dragndrop a kap to View ,edit and rebuild with mapcal +imgkap....
__________________
HappySeagull is offline   Reply With Quote
Old 20-12-2011, 14:02   #1136
Registered User
 
HappySeagull's Avatar

Join Date: Dec 2010
Location: B.C.,Canada
Boat: 29'
Posts: 2,395
Re: Charts II: NGA - 2700 Charts

Quote:
Originally Posted by Viking Sailor View Post
Happy,

Be careful with using GE as a reference. I have seen a couple of spots where two over lapping photos did not align correctly. I wish I could remember where these spots are so that you could see them for yourself but I'm sorry I can't.

Thanks Viking..yes,I think you're right.I'd end up with 2 bad charts!Trusting neither.No substitute for somebody there with local knowledge.But it's still something to keep in mind.People upthread have said the oldest of these charts are just plain bad in the Philippines ,and other places.Checking them out in GE is at least a start to see if they are within seeing distance!
__________________
HappySeagull is offline   Reply With Quote
Old 20-12-2011, 15:10   #1137
Registered User

Join Date: Jan 2007
Location: Minneapolis
Boat: Irwin 37 CC
Posts: 660
Re: Charts II: NGA - 2700 Charts

Thomas,

Just a little more help and I'll have it
1. What are the units for DTM? deg or min or sec or ?
2. Which is entered first N/S or E/W?
3. Which way do positive DTM values move the chart?

Thanks
__________________
David Kester
Pegasus IV
wdkester is offline   Reply With Quote
Old 20-12-2011, 15:16   #1138
Registered User

Join Date: Jan 2007
Location: Minneapolis
Boat: Irwin 37 CC
Posts: 660
Re: Charts II: NGA - 2700 Charts

Viking,
The only GE problem that I've seen is a beaut. Look at Eluthera in the Bahamas about the N/S mid-point. It may have been reported and fixed by now tho.
__________________
David Kester
Pegasus IV
wdkester is offline   Reply With Quote
Old 21-12-2011, 09:46   #1139
Registered User

Join Date: Dec 2005
Location: WNA
Boat: Dufour 35
Posts: 3,249
Re: Charts II: NGA - 2700 Charts

Quote:
Originally Posted by wdkester View Post
Thomas,

Just a little more help and I'll have it
1. What are the units for DTM? deg or min or sec or ?
2. Which is entered first N/S or E/W?
3. Which way do positive DTM values move the chart?

Thanks
David

1 Seconds of arc.
2 First lat then long corrections.
3 N and E are positive and moves the chart in just those directions. So in the example I used, the chart is moved 5" to the South and 131" to the west.

Thomas
__________________
cagney is offline   Reply With Quote
Old 28-12-2011, 09:54   #1140
Registered User
 
manimaul's Avatar

Join Date: Feb 2008
Location: Seattle, WA
Posts: 416
Re: Charts II: NGA - 2700 Charts

Wow, I guess I've been out of the loop for a while. I didn't know this was going on. You guys have been busy .I created a similar scraper a while back to pull chart images from NGA: ngaRasterScraper.py

Not sure if it helps... but feel free to use this script. Note: requires python (obviously) and imagemagick

Grabs jpg files and pieces them together into a png chart image. By default it grabs zoomify level 6. Edit zoom = "6" to grab whatever zoom you want.

Also, this script will grab every available chart from NGA: so it may take a few days to run .

PS ~ Is there a central repository for user calibrated KAP header files?

Will
__________________

__________________
Marine Navigation for Android:
http://mxmariner.com
manimaul is offline   Reply With Quote
Reply

Tags
charts

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 01:41.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.