char** argv can contain multiple "strings". (I know C/C++ doesn't technically have a string type, but I hope you get conceptually what i mean). argv is an array of char * (strings) including the name of the executable and all parameters;
For example: myfile.exe param1 param 2
would yield
argv[0] = myfile.exe
argv[1] = param1
argv[2] = param2
and argc = 3
You can do this a couple of different ways, but the easiest:
float feet = atof(argv[1]);
You should do some error checking to make sure that argc is at least 2 or that will blow up on you. There could also be more than one parameter argv[2], 3 etc.. (If argc == 1 there is no parameters supplied)
Hope that helps.