u-blox's ZED-F9P chip is capable of 10 mm (0.39 inch) accuracy [0]. AFAIK, high accuracy requires being relatively close to a terrestrial correction station. So

u-blox's ZED-F9P chip is capable of 10 mm (0.39 inch) accuracy [0]. AFAIK, high accuracy requires being relatively close to a terrestrial correction station. Some states, such as Iowa, run a network of these stations and provide free access to the public [1]. 0 - https://www.sparkfun.com/products/15136 https://www.sparkfun.com/products/15136 1 - https://iowadot.gov/rtn/IaRTN-resources/About-the-IaRTN https://iowadot.gov/rtn/IaRTN-resources/About-the-IaRTN