Charts II: NGA - 2700 Charts

[sinbad7]

I use an old 2.18 version (trial) which allows a max of 6 files to be converted simultaneously,but that's enough for me, and it's free!!

Tore

[sinbad7]

David..

No,my version doesn't have a percentage reduction so you need to calculate the reduction from the width of each chart manually. A real pain if you are hoping to batch convert many charts of unequal size.

Tore

[dpons]

Originally Posted by phiggins

Tia Bu,

Yes I realize that the Meractor projection is the problem. I googled it to death and tried a dozen solutions that I found for what is called the Gudermannian formula, but none gave the answer.

Anyway I think I found the code in the OpenCPN source so I'm going to steal/borrow it and try it.

To me it looks like the OpenCPN code handles many different projections.

Thanks for your help.

There a linux package named proj that does a large number of conversions:

Cartographic projection filter and library
Proj and invproj perform respective forward and inverse transformation
of cartographic data to or from Cartesian data with a wide range of
selectable projection functions (over 100 projections).

Geod and invgeod perform geodesic (Great Circle) computations for
determining latitude, longitude and back azimuth of a terminus point given
a initial point latitude, longitude, azimuth and distance (direct) or the
forward and back azimuths and distance between an initial and terminus
point latitudes and longitudes (inverse).

The proj package now provides builtin datum conversion.

[cagney]

Originally Posted by paulr

Good find.

There appear to be at least 4 other incorrect files on the ftp server:

26247, 57082, 61200, 92220

I say "at least" because I found these by checking duplicate file sizes and it is possible for there to be duplicates with different file sizes, due to the various download and stitch scripts in use.

I am getting the 4 files above from NOOA, but others with faster connections are very welcome to step in and upload them to the ftp server first!

Paul

The five charts mentioned - fixed now!
Thomas

[cagney]

All the chart pictures sitting in "incoming" , and that were mirrored in "misc", are now in their proper place. "Misc" is gone. I left "incoming" unchanged, as it works as a reference for the new chart pictures. The new chart pictures are not yet visible on opencpn.info, but are available via normal ftp download.

Thomas

[phiggins]

Thanks dpons I'll take a look at them.

[wdkester]

I've been using a set of scanned NGA charts for the Western Caribbean. The originals were in grey not color. I have downloaded the Mexico, Belize, and Honduras NGA charts to update my non-color charts. Some observations :
1. There are problems: e.g. 28006 is an East/West mirror image, 28168 is missing, 28201 Puerto Morales is 2 nm West of where it belongs, but the updates (added lights) seem to be inthe right place.
2. Several charts are skewed; I assume from being scanned in. Some are very slight like 1 deg, others are more like 5 deg.

Please consider the following as questions. One artifact of this skewing is that when you set borders in Mapcal you end up with something that's not a true rectangle? When Opencpn quilts the chart you get long skinny triangles that are outside the border that you drew in Mapcal or am I just doing a bad job of border setting?

Some times, with unskewed charts, I get the long skinny triangles and this is definitely operator error.

I really would like to have a Opencpn map calibrator. After messing with a few charts, I would rather have this than the scripts to automate the chart creation. I'm sure that this would save a 1000 man-hours on this project and have better charts as a bonus. For my own charts it's too painful to trim the borders again so I'm just leaving them til later. Much later.

[phiggins]

Has anybody found NGA charts that are not MERCATOR projection? I'd like to test TRANSVERSE_MERCATOR and/or POLYCONIC. If so what are the chart numbers?... thanks

[nohal]

Paul...
for example:
TMERC: 43342
POLY: 14202 and 17421

Pavel

[phiggins]

Thanks Pavel,

Did you find any other projections?

[nohal]

Paul...
I would wait to see how many such charts we encounter - there will probably be some using Gnomonic projection - Wikipedia, the free encyclopedia and maybe Lambert conformal conic projection - Wikipedia, the free encyclopedia
For now I would leave them aside and handle them when they appear. I don't expect too many of them.
Did you manage to get to reasonable precision with the calculations for Merc and TMerc?

Pavel

[Tia Bu]

A couple of examples if you want to test them out: Chart #56 is gnomonic projection. Chart #80 is an azimuthal equidistant polar projection. I agree with Nohal that these will probably be very few in number. I looked around for a Lambert projection chart, but couldn't find one in a quick search. If it helps...

[cagney]

Example of Lamberts projection 38380 & 38320. Anyone knows of a chartplotter, non ECDIS, that supports this projection?
Small scale Gnomonic Great Circle charts are not that interesting, as great circle routes can easily be imported as gpx files.
What is interesting is if there are any large scale, harbor plans etc in the Gnomonic projection. BA used to issue such charts and plans, but now they are replaced by tmercs in newer editions.
The challenge here is to see how well the default merc projection works with these charts, as OpenCPN hasn't any native support for gnomonic charts.

Thomas

[Tia Bu]

Well, here's a doozy for you: 41000. Polar stereographic projection. I guess some of these Arctic charts just aren't going to be adaptable for use on OpenCPN? The key would be to make sure somebody doesn't accidentally calibrate them with the wrong projection, I suppose.

[phiggins]

Pavel,

Yes I got my code working for Meractor projection after stealing most of the OpenCPN code and putting it a DLL. The high level code I translated to REXX so I could use with GE2KAP. I'm now testing the Transvere Mercator.

wdkester,

Chart 28006 seemed to calibrate OK one I rotated it 180 degrees.

Chart 28201 is that famous chart that I think has put many sailors on the reef. In 2005 I tried to help a Swiss sailor who ran aground there and eventually lost his boat. I noticed at the time that he had 28201 on his navigation table before we abondonded the boat to the sea. Most of the chart guide books say not to use it.

I managed to calibrate it myself by comparing it to Google Earth. I used it in 2009 to go in there and it was right on. The problem is that chart was made in 1905 so trying to align it up with the GE picture is a bit hard. You could take the NGA png and calibrate with MapCal then run GE2KAP against it and don' check the "Create from overlay..". When it gets overlaid on GE move the chart until is aligned with GE. The current El Cid marina, is about where the cemetery was on the paper chart.