Read a text file - output to a text file

[tblake3]

My C++ knowledge is from a half semester in college 5 years ago and I'm a more than a bit rusty on even that limited education. I've pulled out my text book and can't find anything that seems to solve my problem.
I want to write a program that will read a user specified text file (actually a .gpx), a format of coordinates of GPS waypoints and descriptions. Each coordinate is preceded by a standard tag. When the program gets to the coordinate, I want to take the lat and long, alter them and write the new coordinates to a separate file. Each .gpx file will have several coordinates and I want the program to continue to the next coordinate tag, do the alterations and then continue again until the end of the file is reached. Help!