Updated advice for calibrating/georeferencing raster files

[Jammer]

I've undertaken a project to add the MN DNR's water trails maps to OpenCPN. These are the best available charts for non-navigable rivers in Minnesota, and offer some detail not readily available in other sources for the Mississippi, Minnesota, and St. Croix:

1. River mile.
2. Dangerous obstructions, particularly dams and rapids.
3. Bridges, although there is no clearance information.
4. Public lands adjoining the rivers, including information on services available, such as restrooms, water, and picnic areas.
5. Boat launches.

The information in the OpenCPN on how to georeference a raster file is badly out of date, and depends on weak tools like SeaClear that have been unsupported for years. Here is the path I am using:

1) Use ghostscript to rasterize the PDF files from the state. Though not strictly necessary, this allows control of the resolution, where a direct PDF import does not. Ghostscript is in all the major Linux distros, and is available as part of cygwin for Windows users. I created BMP files although other formats will work.

2) Use QGIS to perform the georeferencing. I find that it works best to download the OpenStreetMap plugin and use it for a base map. The georeferencer is included in the QGIS package, but has to be enabled in the "plugins' page in order to work. It is important to add an OpenStreetMap layer (or other base layer) before doing any georeferencing or the coordinate system will be wrong, even if you specify it manually (bug). The georeferencer will produce a georeferenced .tiff file, and save your points separately also.

3) The thin plate spline transformation setting works best for most cases. It is necessary to have at least 3 reference points. 4 will work better, and 5 will give you useful residual error data that will identify any inaccurate points. Unlike older tools, QGIS will properly adjust maps that are skewed. If you include enough points, it will correct distortions that result from a bad scan or even a hand-drawn map.

I was not able to get GRASS to do what I need.
I found SeaClear to be buggy on Windows 10 and almost impossible to use.
I was not able to get ImageCal to work on Windows 10.

[Jammer]

I'm working on the BSB/KAP export now.

Libbsb appears to be orphaned. There's a working mirror at https://github.com/nohal/libbsb - no binaries, you have to build it from source.

On cygwin you have to have libtiff-devel installed before building it in order to get the build to produce tif2bsb.

[Jammer]

Looking at imgkap source code now. It doesn't look like there's anything out there that can convert a georeferenced TIFF file into a BSB/KAP file. Is there? Anyone? I may have to modify imgkap to be able to read the TIFF geo header. Yay.

[nohal]

Originally Posted by Jammer

Looking at imgkap source code now. It doesn't look like there's anything out there that can convert a georeferenced TIFF file into a BSB/KAP file. Is there? Anyone? I may have to modify imgkap to be able to read the TIFF geo header. Yay.

libgeotiff
I will gladly accept a pull request implementing it directly to https://github.com/nohal/imgkap of course...

Pavel

[seandepagnier]

which sources of imgkap did you pull?

[nohal]

BTW, for plain manual conversion of GeoTIFF to KAP, the new photolayer OpenCPN plugin (https://github.com/Rasbats/photolayer_pi) seems to do it quite fine.

[Jammer]

Originally Posted by boat_alexandra

which sources of imgkap did you pull?

The ones from Pavel's git repo. (nohal)

[rgleason]

Jammer wrote:

The information in the OpenCPN on how to georeference a raster file is badly out of date, and depends on weak tools like SeaClear that have been unsupported for years. Here is the path I am using:

It appears you are in the process of rewriting this old section. When you have perfected the process please update this, so we can update the wiki.

Also btw which pages in the wiki exactly are outdated?

[Jammer]

Originally Posted by nohal

libgeotiff
I will gladly accept a pull request implementing it directly to https://github.com/nohal/imgkap of course...

Pavel

I'm working on a getting a shell script together that will do all the steps using existing tools.

Modifying imgkap is problematic because it relies on FreeImage, which has not been actively maintained for several years and will no longer compile, at least on cygwin, which is what I have to work with at the moment. I doubt if the outcome would be different on more widely accepted platforms. FreeImage also uses nonstandard license terms, fwiw.

Anyway, to make the chain work cleanly without extensive scripting, I would have to modify FreeImage so that it will compile, and then modify it to use the geotiff library and add APIs to pass those coordinates up. A separate problem is that imgkap will not read the image data from the tiff files produced by QGIS, apparently due to a limitation of FreeImage, so that would also have to be fixed.

All that aside, QGIS works well for the georeferencing itself, nice interface, will rotate the input rasters as needed, and uses a spline algorithm that can compensate for nonlinearities in the input.

[Jammer]

Originally Posted by rgleason

It appears you are in the process of rewriting this old section. When you have perfected the process please update this, so we can update the wiki.

Also btw which pages in the wiki exactly are outdated?

I will do this.

There are several pages on the wiki that document individual paths of conversion that have worked for people over time, such as https://opencpn.org/wiki/dokuwiki/do...manual:pdf2kap

The whole section, IMO, should be updated to focus on modern tools.

[nohal]

Originally Posted by Jammer

I'm working on a getting a shell script together that will do all the steps using existing tools.

Modifying imgkap is problematic because it relies on FreeImage, which has not been actively maintained for several years and will no longer compile, at least on cygwin, which is what I have to work with at the moment. I doubt if the outcome would be different on more widely accepted platforms.

I remember some very trivial stuff when compiling on Windows with VS2017 some time ago, took few minutes to fix. Can we look at what cygwin complains about exactly?

FreeImage also uses nonstandard license terms, fwiw.

What is nonstandard on "FreeImage is licensed under the GNU General Public License, version 2.0 (GPLv2) or version 3.0 (GPLv3)"? It is certainly good enough to be included in Debian. And as we know nothing is good enough for Debian license wise

Anyway, to make the chain work cleanly without extensive scripting, I would have to modify FreeImage so that it will compile, and then modify it to use the geotiff library and add APIs to pass those coordinates up. A separate problem is that imgkap will not read the image data from the tiff files produced by QGIS, apparently due to a limitation of FreeImage, so that would also have to be fixed.

As far as I can say FreeImage is based on libtiff, do you want to say libtiff, the standard used by everything, does not read the QGIS tiffs? Or is there some other problem? Which?

All that aside, QGIS works well for the georeferencing itself, nice interface, will rotate the input rasters as needed, and uses a spline algorithm that can compensate for nonlinearities in the input.

So to conclude, you will script QGIS and don't need any other tools? That would of course be a solution as well.

Pavel

[Rasbats]

MAP2CMAPC (15GBP) will accept GeoPDF files and output .tif and .kap files. If you want to use the tifs in the PhotoLayer plugin (transparency) use tif latitude/longitude not mercator. The loading of the initial pdf file takes time as these are large files.

cando

Mike

[rgleason]

Jammer wrote:

I will do this.
There are several pages on the wiki that document individual paths of conversion that have worked for people over time, such as https://opencpn.org/wiki/dokuwiki/do...manualdf2kap
The whole section, IMO, should be updated to focus on modern tools.

Believe it or not Contributed by [egodsey] on Cruisers Forum contributed this fairly recently. I am pleased that you would like to update it! We always appreciate better methods.

Jammer if you'd like to become a Opencpn Wiki Editor/Manager just PM me with your email address and preferred user name.