I have a lake that has a weir in it. The water level in the top part of the lake above the weir is about 2 metres higher than below the weir.

How do people manage mapping the weir in Reefmaster?

My first thought was to map the two areas above and below the weir as separate lakes. So I created a shoreline for each. But then I discovered that you can only have one shoreline.

So how do I define the weir wall on the map?

John,
Probably better to make two separate map projects, with a shoreline each, and then combine the results in a single user map. If exporting to AT5 you could skip the user map and just export each map separately, renaming the "large.at5" files as you go so that they do not clash. You can then place multiple at5 files on your card and they will all load on your unit. Note that you can only do this with vector maps - there are issues with multiple at5 map files and raster backgrounds/overlaid vector areas.

Ok, thanks Matt, I'll have a play and see if I can work it out. It's good to know that multiple AT5 maps can be loaded on the one one map card.

In general, multiple at5 maps are fine but just be aware that RM always generates using the default name (large.at5), so you will need to change that to prevent overwriting the first map with subsequent ones. Also, as mentioned before, things get a little tricky with multiple maps over raster, especially if transparency is involved. There are ways to do it, but it takes a bit more work (export sets of shapefiles, plus the required raster files, and generate everything together in the IMC).