NAD27 is named for the earth model that it’s referenced to, the North American Datum of 1927. That makes it pretty old, right? Amazingly enough, the NAD27 datum utilizes the Clarke Spheroid of 1866.
Like its successors, NAD27 is optimized for North America at the expense of its usefulness in other parts of the world. We run into it frequently in our work because it’s the GCS that many legacy coordinate systems are built on.
- UTM 27 was the coordinate system of choice for many Federal agencies, particularly the US Forest Service
- State Plane 27 was used by a number of southeastern Wisconsin counties. They didn’t upgrade to NAD83(2011) based systems until January of 2019.