Just looking at the OP, my two cents' on this:
While the shape of the land is preset (which is a big loss to cartographers, in my opinion), size alone is not what determines a "big" discovery. Mapping the pass between seemingly impassible mountains separating two sections of the landscape is a pretty big discovery. Mapping the vacant spaces within a heavily forested area and the high/low points of elevation through lands rich in resources are vital for the establishment of efficient spatial networking in a kingdom, and can mean the difference between a wagonload of ore taking days to reach its destination or hours. Being the first cartographer to map a lost city (potentailly home to artifacts or unique resources) could make you a very rich and famous person if the map is sold to someone who knows its worth.
Cartographers miss out on the biggest discovery of all, but that doesn't mean there's nothing big to discover. Anoch Sun lays nested somewhere within that desert in the middle of the map, along with the key to humanity's survival. You know the desert's there, but can you honestly say there's nothing mind-blowing to map within it?