Hello,
Using my Laptop/tablet & team viewer I tried to continue a game I had started on my other computer at home. My tablet has that square surface on the keyboard and two buttons to emulate a mouse it does not have a scroll wheel.
I was unable to find an option to zoom in / out on the map without the scroll wheel. I suggest you add such as feature as not every computer user will have a mouse with scroll wheel.
Some possibilities: + (plus): To zoom in, - (minus) to zoom out. Also: Ctrl + Left mouse button to zoom in, Ctrl + right mouse button to zoom out.
I did see a +/- on the upper/rightmost corner of the map (which shows the reduced map) which I think were intended for this but it did not function. Having a +/- key or ctrl + mouse key is faster than having to move the pointer on that area to click anyways.