Anyone know what I can use to convert a geoid surface stored as , BYN into .GTX for use with PROJ4?
The CCG2013A vertical datum and geoid model from NRCan is only available, as far as I can find, in BYN format.
You can download a demo verion of the Hydromagic software from https://www.eye4software.com/download/
It includes a geoid converter freeware: https://www.eye4software.com/hydromagic/documentation/manual/utilities/geoid-file-conversion/
Supported file types are
.geo, .ggf, .gsf, .byn, .bin, .grd, .txt, .gtx as source and destination.
They offer a bunch of geoid files in their .geo format at https://www.eye4software.com/hydromagic/documentation/geoid-models/
Disclosure: I'm not affiliated with the software company.