This is true, it is not easy to dierctly and accurately correlate geographical location to IP address, much less large blocks of them.
Aesources which may help a lot though:
http://www.iana.org/numbersMore specifically:
http://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xmland
https://www.arin.net/knowledge/ip_blocks.html