Future Location of Chatham County, 1735
The area enclosed in red represents the future boundaries of Chatham County. The above map shows what is labeled the "County of Savannah" and is commonly attributed to having been prepared in 1740 in conjunction with the Trustees creating the "County of Savannah" in 1741. However, stronger evidence suggests that the map was based on a sketch James Oglethorpe carried to England in 1734 and was subsequently published in a 1735 report on Georgia's Salzburger immigrants.
Why this map is entitled the "County of Savannah" is not known, though it may have been based on the assumption that the English system of counties would be applied in Georgia. The Trustees had debated a new plan for administering the colony of Georgia for some time, and in April 1741 they divided Georgia into two counties named Savannah and Frederica. The County of Savannah included settlements on both banks of the Ogeechee River, plus all lands northward to the Savannah River.
Source: T.F. Lotter, "A Map of the County of Savannah," 1735