Complex sentence: The two islands where photographed by U.S. Navy Operation Highjump (1946--47) and U.S. Navy Operation Windmill (1947--48), and though rather clearly shown in the photography were not shown on the resulting charts.
Split into simpler sentences: The two islands were photographed by U.S. Navy Operation Highjump (1946--47) and U.S. Navy Operation Windmill (1947--48). Though rather clearly shown in the photography, they were not shown on the resulting charts.