Adjusting perspective of a lower layer

[Stargazer]

Hi, forum newbie here (although I've been using GIMP for a few years on and off)

I have a specific task that I'm struggling with...

I'm working on producing an overlay of two old maps. One of them is rendered accurately to correspond with modern mapping (this is the one I want to use as a reference), the other a photo of an older map which is distorted.

I need to have the 'modern' map on top and have made it transparent so the older map can easily be seen through it when the modern map is overlaid on top of it. The modern map is more or less monochrome with a white background so is the obvious choice for the top layer (the older map is multicoloured and dirty so would be a poor choice for the top layer).

I now need to adjust the perspective of the older map, which is in the lower layer, to remove the distortion and line it up precisely with the modern map. This is possible, but when I try to do it I can't see the modern map on top, which I need to get the registration between the layers correct - it disappears when I select the perspective adjustment option (Shift-P).

Does anyone have any suggestions?

Many thanks, David.

[Ofnuts]

In the transform tools optoins, there is an Opacity slider to reduce the opacity of the preview image:

   

[Stargazer]

Many thanks - that worked It was a bit hard to get the opacity right so I could see both layers clearly as the lower layer is rather indistinct generally, but I managed to get them to align then upped the opacity again to get the overlay.