During the latter part of the Spanish rule, he was appointed Surveyor General of the District of Natchez. He also served as a representative of the Spanish government in locating the 31° of North Latitude, which was established as the boundary line between the United States and the Spanish possessions east of the Mississippi. As he was never a subject of his Catholic Majesty,[41] these services were strictly professional. The relationship between him and Governor Gayoso was, however, very cordial, as is shown by their correspondence.[42] Upon one occasion,[43] Dunbar presented Gayoso with a costly sextant, which the latter needed in order to complete a course of astronomical observations upon which he was engaged. At another time[44] Gayoso had cause to thank Dunbar for the use of a "famous astronomical circle" belonging to the latter. Gayoso says of this instrument, "it surpassed my expectation,____. Every part is so delicately finished and solidly supported & so well prepared to be adjusted that it would give me courage to make an observation myself. If the instrument was not your own property I would have advised you to make a voyage to admire it. Now I think with your assistance I may with confidence and decency proceed to the demarcation of the Line as soon as I receive orders for the purpose." The sickness of Mr. Dunbar about this time was a source of great concern to Gayoso. In a third letter[45] upon this subject, written two weeks before the work upon the line began, Gayoso expressed some apprehension that, for his sake, Dunbar might imprudently expose himself. When Dunbar was at work upon the line, Gayoso wrote to him as follows: "I congratulate myself for having had the opportunity of meeting with a person so well calculated to fulfill so important a charge for which is required science with every other quality worthy of public trust; you possess them all in a degree to do honor to any country; these are my sincere sentiments." Dunbar's services on the line of demarcation extended from May 26 to August 28, 1798, the time consumed in surveying the first eighteen miles of the boundary.[46] The preliminary observations leading to the location of the 31° were made in his private observatory on Union Hill.[47] An inundation of the Mississippi prevented the survey from beginning at the bank of the river. The water having receded by the 28th of July, Dunbar began to extend the line to the river from the point of starting, while Ellicott, the American Commissioner, continued his survey to the east.[48] Through this swamp, which was found to be 2111.42 French toises or 2 miles and 186 perches English measure, a trace sixty feet wide was cut to designate the boundary, and posts were put at intervals of a mile.