This plan of the siege is interesting for the fact that is shows no troop positions, on either side, unlike other 'plans' produced at and after the event by the likes of Lieutenant Patrick Finnegan, JFW Des Barres, George Sproule and William Faden. The plan does show a great deal of detail however including British redoubt and batteries including the 1st, 2nd and 3rd parallels just north of the American 'Horn works' or defences (marked 'D').
It is more than likely that Blaskowitz was working directly for Sir Henry Clinton at the time but he is careful to acknowledge the importance of the leading Naval commander at the engagement, Vice Admiral Mariot Arbuthnot: The success of the siege was, afterall, in no little measure due to the co-ordination of land and naval forces which effectively sealed-off the city from all hope of relief thus causing Maj Gen Lincoln to capitulate after 45 days.
Interestingly, on the 21 April, Lincoln had requested a surrender with "honours of war", which was rejected by Clinton, a rejection which was later repaid in-full by the Americans at the British surrender at the Siege of Yorktown two years later where Washington said, "The same Honors will be granted to the Surrendering Army as were granted to the Garrison of Charles Town."
Fort Moultrie surrendered without a fight on 7 May, as a consequence of the naval blockade imposed by Arbuthnot and shortly thereafter other American troops in the South Carolina backwaters such as Camden also surrendered to the British.
Blaskowitz's plan contains all of his trademark attention to detail with detailed pen and watercolor work. He carefully depicts the Charleston wet-lands, rivers, streams and shorelines as well, of course, as the roads and lay-out of the city itself. Individual buildings of note are highlighted in traditional red and city blocks in pink. Blaskowitz provides a scale for his plan of 400 feet to an inch and a table of notable features is also included.
This is more than a plan of a siege. Blaskowitz has also produced a survey of the town as may be used by the British for when the siege is finished. There remains a question as to whether this plan was used by JFW Des Barres, back in London, as a template for his later 'A Sketch of the Operations before Charleston' which is a possibility as he would likely have had access to it.
Blaskowitz's plan of Charleston is dated the 12th May 1780 but includes information surveyed in the weeks and days before the surrender of the American troops under Major General Benjamin Lincoln. In fact the British siege, under General Henry Clinton started on the 1st of April and Blaskowitz was present from the outset.