I think with a raster background you are kind of stuck. I am not sure exactly how AT5 raster layers are compressed, but since you would have a raster here for most of the lake area already (mostly empty, of course), it's possible that the raster itself wouldn't take up a lot more space when the map is complete. The only way to reduce the raster size (that I know of) is to choose a higher minimum metres-per-pixel. What are you using now?
Vector, of course, would be much smaller. Any reason not to use an all vector map?

I always use Vector for export to at5. The enclosed map was just to show the size of this particular project. :)

No ideas?

How big are the vector maps? I would expect those to be relatively small. What format are you sharing them in?

Lake Mjøsa is Norways largest lake with 362 km2... I have the shoreline for the whole lake in my map, but only about 3% has contours for the time beeing.

I have not checked size of the at5 and xml, but it is the total content of the Shadedrelief-folder that is the problem. It seems to me that the total size of this folder increased after the lprevious update of IMC and RM. Our Tyrifjorden-project (that I have shared with you before, back in the days when it was half the GB of logfiles it is now) ends up in a Shadedrelief-folder folder of 3,4 GB after export with IMC and Randsfjorden is around 6 GB somewhere. These are all large lakes, to some extent very large.

I use 5 meters for major contours, minor down to 15 meters. Colours 12 (red) and 9 (blue) and shading feom 255 to 10. We share our maps in PNG,KMZ,AT5 for all maps, and a few in formats for Humminbird as well. We make one at5-version for HDS/Simrad and one for Elite HDI/ older units. The only differnece is the use of colours and shading.

I guess my real question is; what is driving the total size of the Shadedrelief-folder? What would be the best way to keep it's size down? I understand the total km2 size of the lake plays a large role, but are there other factors as well?

Why do you even need a ShadedRelief folder if you are vector only?
The biggest factor is going to be physical size of map, and, empty or not, that whole area is going to filled with multi-scale rasters if you are exporting a vector map. I doubt there's a lot you can do about it, but if you must have a raster layer (why?) you could generate the raster by hand in the IMC and specify a higher pixels-per-metre value at the lower end of the zoom. For every doubling of this value, you should see a 4x decrease in raster size.

Seems I'm missing something here. :D
I am using Vector, but I still need the Shadedrelief-folder to get the colours in Lowrance-units right?

Not if you are creating vector maps with isobaths, but there are some caveats. Units won't show transparency without a raster background, so if you are using transparency in your User Map, then a "dummy" raster background gets created. This is simply a rectangle with land colour. From memory, the we have to make this with a relatively high pixels-per-metre because otherwise the raster disappears at high zoom, and the transparency is lost.
If you want a vector only map, then create one with full opacity colours. This will be much, much smaller (as big as your current large.at5 file).

Why do you share your maps in at5 raster format (Shaded Relief)?

My home lake has about 80 km2 and the whole lake as at5 vector is only 1.3 mb! This lake has a lot of structure, some islands and the map is containing some aditional POIs and markerlines.

Thanks for the info. I'll do some more testing on this to see how it turns out in HDS Gen 1 and Gen 2.

Was out on the water today, fantastic weather and even a few fish. :)

I also got to take a look at what happens in my HDS without the shadedrealief-folder, and I have to say that I don`t like it. As expected the shading disappeared, leaving the map in its two original colours.

I guess the solution to this is to use A LOT of colours to get the same effect. Fully the same effect on the HDS but a lot more work when updating the 25 or so maps I have. The HDS did work better without the folder though, even my Gen 1 was faster then with the folder when zooming in and out.

HDS Gen 1 should work with vector much smoother than with (big) raster maps.

HDS Gen1 has less memory and a slower processor ...

Months ago I had the same problem, when working with DrDepth and switching over to RM. I had perfected my maps in raster format, but I am very happy to have changed and learned to create vector only and do a few little changes with QGIS or a simple editor. Give it a try ;)

If you like, you could have my color table, that works from 0 - 75 meters with every meter in shallow areas. If this one is not perfect, it is easily manipulated to your own needs.