If you are importing point files into C3D and you are getting an error message like below:
Your point file might be in one of the typical formats (PENZD) when you open it and look at it but C3D still doesn't appear to recognise it. This usually happens because there is something in the file that C3D doesn't understand. Typically this will be a number in a column where it is expecting alpha characters or vice versa. Check your point file to make sure that, for example, the 'P' column contains only numbers. Looking at my file I see that the first point was a base station and this point does not have a number, rather it uses the STN code in the 'P' column.
C3D doesn't know what to do with the STN as it is expecting a number. Edit the point file and try reimporting and it should work ok.