It seems that many early engineers had an important role in creating maps. Ramelli, for example, was captured by surveying and mapping the fortifications of La Rochelle. The importance of mapping probably extended beyond just military applications.
Buisseret notes four different ways that mapping was an important part of the early modern era. It was used for 1) military and naval purposes; 2) political and judicial reasons such demarcating various jurisdictions; 3) economic and financial reasons; and 4) ecclesiastical structure. In an era when the church held such tremendous power, the borders of different diocese was an important consideration.
Maps and Italian engineers became popular at approximately the same time. Henri II, for example, used maps as a military tool. He made a suggestion to the city hall of Paris in September of 1550 that Girolamo Bellarmato could map the extents of Paris. This map could then serve as an important input for ongoing planning activities.
The other Valois had different views of maps. Charles IX, for example, imprisoned a cartographer who presented him with a map of his country. He considered the map to be a crucial instrument of war and deemed the cartographer's actions as a dangerous breach of intelligence.
Henry IV was perhaps the first great admirer of maps in France. He collected them and even executed few (along with fortification designs) himself. Buisseret notes that creating maps was an important job for the Ingeniurs du Roi. He notes that 14 of Errard's plans of the towns of Picardy are maintained by the British Library. He also notes the relative limited number of engineers in France at the time. In 1597 they number four; In 1611 they were six. Goodman notes that there were only six royal engineers in Iberian Spain during the same era.
Merriman notes a similar trend among the Engineers of England. Like France, England imported a large number of Italian engineers. They were engaged in tasks such as the creation of fortifications and with mapping various military, political, economic, and ecclesiastical sites. Merriman points to the 1540s as the era when true maps emerged in England. They were created from a bird's eye view, were measurable, and included a scale. Not all maps of the era met these standards. There were still a large number of maps created from some sort of artificial vantage point.
There seems to be some tension here in the creation of maps. A similar sort of tension is evident in the machine books. Critics have argued that the designs are somehow inarticulate or insincere since the depicted machines couldn't actually be built. They lacked scales or the type of working drawings that a craftsman would require (provided that a craftsman of the era could actually interpret the thing!). The machine designs of Ramelli and Besson were still stuck in an older style of representation that depended on artificial views and linear perspective. They had not yet explored the potential of the measured view. This same sort of confrontation was going on with mapping at about the same time. This theme is also explored by Svetlana Alpers.
I am struck by something in this discussion of both maps and machine books. Everything was there to create good maps. There was the mathematical theory, the instruments (e.g., Besson's Cosmographe), and a diversified work force able to create them (e.g., the Ingenieurs du Roi). So why didn't we see measured maps and descriptive construction drawings. I guess the representational media just hadn't developed yet. I'm reminded of Galison's concept of the trading zone where Experimentalists, Theorists, and those guys who build apparatus all interact to create some kind of truth. The case of mapping is different. There was the apparatus, the theory, and the workers. But there was only slow closure. What was missing was a standard visual media or genre to describe the findings and structure the work of these engineers.
Buisseret. 1992. Monarchs, Ministers, and Maps in France before the Accession of Louis XIV. In Buisseret (Ed.) Monarchs, ministers, and maps : the emergence of cartography as a tool of government in early modern Europe. Pg. 99-123.
Merriman, Marcus. 1983. Italian military engineers in England in the 1540s. In Tyacke (Ed.) English Map Making.