Exporting from QGIS to process in IMC

[archjeb]

Hi Folks,

I just joined the group. Been using IMC for a while and I like to create custom charts from various sources. Typically, I have been getting PDF maps of the ocean floor from various organizations that has bottom composition. I then load them into QGIS as a raster image after loading into the Georeferencing tool and creating lots of GCP points to convert it into a GeoTiff.

Then when I get all my layers I want I like to export it as an image with 1200dpi.

If it is only one single GeoTiff that I converted, I've used the 'Raster->Conversion->Translate' functions to convert to KML format.


My question to those that have experience; what method of exporting layers have you found the best for importing into IMC?
When I go to export to image, and then have to specify a canvas size and then the DPI...I notice that my font size of my labels will be different and sometimes everything gets skewed.
But if I have a single layer that I save as a GeoTiff and then go through the conversion utility to convert to KML, it seems to keep all my aspect ratios, and detail correctly.

A couple of years ago, I created a how-to video on doing these conversions of maps and then how to process in IMC. Hopefully this is useful for others that are starting.
https://youtu.be/3t7rL8aBw3c?feature=shared

In the meantime, if you have some suggestions on better ways to export from QGIS for IMC, please let me know.

Thank you.

Jeremy

[fjordfischer]

Conversion is not necessary, the GeoTiffs are processed by the IMC, the most important thing is the correct coordinate reference system and the highest possible dpi with good quality of the source map and overlapping pictures.

The result is particularly good and above all zoomable if you can create GeoTiffs in several layers lavels, which works very well with aerial photographs, for example. The attached image illustrates this procedure with 3 ... 5 fictitious layers, but there can also be more if you accept the increase effort.

[archjeb]

So are you saying I don't have to convert the GeoTiff's to KML files for IMC to process them?

Have I been doing that extra step for no reason?


I tried to process some PNG files with the beta version of IMC but it didn't do anything. It says it processed them...but nothing in the working directory. I was trying PNG for compression reasons....easier to work with size wise.

[archjeb]

@fjordfischer

Maybe I'm doing something wrong, but IMC will not process my GeoTiff files. I have to convert them to KML...which is easy enough. I just use gdal_translate
'gdal_translate -of KMLSUPEROVERLAY MyTiffImage.tif   MyKMLFiles/MyKML.kml'

So if there is a way to get a native GeoTiff image processed by IMC that would be great...similar to transport the files around.

On another note, I need to get my AT file size down on a new project. 25MB of KML files turned into 32GB of AT Files with Zoom level 1 to 16.  Got to figure out why its so big.

-J

[fjordfischer]

Yes, exactly.

I've been doing this for years, but a tif file has to be combined with a tfw file (georeferencing).

If you want, I can create an example for you.

I myself mainly work with ArcGIS, but as you can find out on the Internet, tif + tfw also works in QGIS.

You have little influence on the amount of data for rasters from the IMC because they are generated in blocks. Of course, this also depends on the structure of an initial raster, but ultimately a certain block size is not exceeded. Therefore, it is always better to try to generate vector data from large-scale rasters or to obtain the original vector data. In the USA, the authorities seem to be more cooperative in this regard, quite unlike in the EU.

[archjeb]

Thanks for the reply.

I'll try the GeoTiff with a Worldfile.

I tried PNG with a worldfile (as you can create one with gdal_translate)...but IMC still didn't like it for some reason.

example: "gdal_translate -of png -co WORLDFILE=YES NorthCoast.tif NorthCoast.png"

#ls  NorthCoast.*
NorthCoast.png      NorthCoast.png.aux.xml   NorthCoast.tif      NorthCoast.wld


So the Worldfile (.wld) gets created with the same prefix as the png file and I place that in the same directory for IMC processing.  I thought that is all that was needed for Raster processing in IMC...but maybe I'm missing something. I even tried renaming the world file to .pgw to be more standard; and still IMC doesn't process.

If I convert to KML, then it works.

-J

[fjordfischer]

I think you have a misunderstanding about the world file:

https://www.tacklefever.de/index.php?topic=6182.0

https://en.wikipedia.org/wiki/World_file

[archjeb]

Sorry, maybe I didn't have enough coffee this morning...
The world file format looks correct to me. Am I missing something? The default extension with gdal_translate is 'wld'....so that is why I even tried renaming to pgw suffix. Still IMC doesn't process.

j@MacBook-Pro TestMaps % gdal_translate -of PNG -co WORLDFILE=YES NorthCoast.tif NorthCoast.png
Input file size is 7167, 13750
0...10...20...30...40...50...60...70...80...90...100 - done.

j@MacBook-Pro TestMaps % cat NorthCoast.wld
22.7559822520
0.0000000000
0.0000000000
-22.7559822520
-13929033.6805088725
5910014.9399412917