I am getting an error when I run ntop with the command 
'ntop -P /usr/local/var/ntop', 
and below is the output:
  
Wed May 10 18:55:48 2006  NOTE: Interface merge enabled by default
Wed May 10 18:55:48 2006  Initializing gdbm databases
Wed May 10 18:55:49 2006  ntop will be started as user nobody
Wed May 10 18:55:49 2006  ntop v.3.2 SourceForge .tgz
Wed May 10 18:55:49 2006  Configured on May 10 2006 18:29:31, built on May 10 2006 18:30:40.
Wed May 10 18:55:49 2006  Copyright 1998-2005 by Luca Deri <deri@ntop.org>
Wed May 10 18:55:49 2006  Get the freshest ntop from http://www.ntop.org/
Wed May 10 18:55:49 2006  NOTE: ntop is running from 'ntop'
Wed May 10 18:55:49 2006  NOTE: (but see warning on man page for the --instance parameter)
Wed May 10 18:55:49 2006  NOTE: ntop libraries are in '/usr/local/lib'
Wed May 10 18:55:49 2006  Initializing ntop
Wed May 10 18:55:49 2006  Checking eth0 for additional devices
Wed May 10 18:55:49 2006  Resetting traffic statistics for device eth0
Wed May 10 18:55:49 2006  DLT: Device 0 [eth0] is 1, mtu 1514, header 14
Wed May 10 18:55:49 2006  Initializing gdbm databases
Wed May 10 18:55:49 2006  VENDOR: Loading MAC address table.
Wed May 10 18:55:49 2006  VENDOR: Checking for MAC address table file
Wed May 10 18:55:49 2006  VENDOR: File './specialMAC.txt.gz' does not need to be reloaded
Wed May 10 18:55:49 2006  VENDOR: ntop continues ok
Wed May 10 18:55:49 2006  VENDOR: Checking for MAC address table file
Wed May 10 18:55:49 2006  VENDOR: File './oui.txt.gz' does not need to be reloaded
Wed May 10 18:55:49 2006  VENDOR: ntop continues ok
Wed May 10 18:55:49 2006  Fingeprint: Loading signature file.
Wed May 10 18:55:49 2006  Fingeprint: ...loaded 1697 records
Wed May 10 18:55:49 2006  ASN: Checking for Autonomous System Number table file
Wed May 10 18:55:49 2006  ASN: Loading file './AS-list.txt.gz'
Wed May 10 18:55:49 2006  ASN: ...found 111435 lines
Wed May 10 18:55:49 2006  ASN: ....Used 3780 KB of memory (12 per entry)
Wed May 10 18:55:49 2006  I18N: This instance of ntop does not support multiple languages
Wed May 10 18:55:49 2006  IP2CC: Checking for IP address <-> Country Code mapping file
Wed May 10 18:55:49 2006  IP2CC: Loading file './p2c.opt.table.gz'
Wed May 10 18:55:49 2006  IP2CC: ...found 52395 lines
Wed May 10 18:55:49 2006  GDVERCHK: Guessing at libgd version
Wed May 10 18:55:49 2006  GDVERCHK: ... as 2.0.21+
Wed May 10 18:55:49 2006  Initializing external applications
Wed May 10 18:55:49 2006  THREADMGMT[t3083836336]: NPA: Started thread for network packet analyzer
Wed May 10 18:55:49 2006  THREADMGMT[t3073346480]: SFP: Started thread for fingerprinting
Wed May 10 18:55:49 2006  THREADMGMT[t3062856624]: SIH: Started thread for idle hosts detection
Wed May 10 18:55:49 2006  THREADMGMT[t3052366768]: DNSAR(1): Started thread for DNS address resolution
Wed May 10 18:55:49 2006  Calling plugin start functions (if any)
Wed May 10 18:55:49 2006  SSL is present but https is disabled: use -W <https port> for enabling it
Wed May 10 18:55:49 2006  INITWEB: Initializing web server
Wed May 10 18:55:49 2006  INITWEB: Initializing tcp/ip socket connections for web server
Wed May 10 18:55:49 2006  INITWEB: Initialized socket, port 3000, address (any)
Wed May 10 18:55:49 2006  INITWEB: Waiting for HTTP connections on port 3000
Wed May 10 18:55:49 2006  INITWEB: Starting web server
Wed May 10 18:55:49 2006  THREADMGMT[t3041876912]: INITWEB: Started thread for web server
Wed May 10 18:55:49 2006  Listening on [eth0]
Wed May 10 18:55:49 2006  Loading Plugins
Wed May 10 18:55:49 2006  Searching for plugins in ./plugins
Wed May 10 18:55:49 2006  LASTSEEN: Welcome to Host Last Seen. (C) 1999 by Andrea Marangoni
Wed May 10 18:55:49 2006  RRD: Welcome to Round-Robin Databases. (C) 2002-04 by Luca Deri.
Wed May 10 18:55:49 2006  SNMP: Welcome to SNMP. (C) 2004 by F.Fusco and G.Giardina
Wed May 10 18:55:49 2006  PDA: Welcome to PDA. (C) 2001-2005 by L.Deri and W.Brock
Wed May 10 18:55:49 2006  XMLDUMP: Welcome to XML data dump. (C) 2003-2004 by Burton Strauss
Wed May 10 18:55:49 2006  NETFLOW: Welcome to NetFlow.(C) 2002-05 by Luca Deri
Wed May 10 18:55:49 2006  SFLOW: Welcome to sFlow.(C) 2002-04 by Luca Deri
Wed May 10 18:55:49 2006  ICMP: Welcome to ICMP Watch. (C) 1999-2005 by Luca Deri
Wed May 10 18:55:49 2006  Calling plugin start functions (if any)
Wed May 10 18:55:49 2006  RRD: Welcome to the RRD plugin
Wed May 10 18:55:49 2006  RRD: Mask for new directories is 0700
Wed May 10 18:55:49 2006  RRD: Mask for new files is 0066
Wed May 10 18:55:49 2006  THREADMGMT: RRD: Started thread (t3031387056) for data collection
Wed May 10 18:55:49 2006  THREADMGMT[t3083836336]: NPA: network packet analyzer (packet processor) thread running [p20314]
Wed May 10 18:55:49 2006  THREADMGMT[t3073346480]: SFP: Fingerprint scan thread starting [p20314]
Wed May 10 18:55:49 2006  THREADMGMT[t3062856624]: SIH: Idle host scan thread starting [p20314]
Wed May 10 18:55:49 2006  THREADMGMT[t3052366768]: DNSAR(1): Address resolution thread running [p20314]
Wed May 10 18:55:49 2006  THREADMGMT[t3041876912]: WEB: Server connection thread starting [p20314]
Wed May 10 18:55:49 2006  Note: SIGPIPE handler set (ignore)
Wed May 10 18:55:49 2006  THREADMGMT[t3041876912]: WEB: Server connection thread running [p20314]
Wed May 10 18:55:49 2006  WEB: ntop's web server is now processing requests
Wed May 10 18:55:50 2006  THREADMGMT[t3086269216]: ntop RUNSTATE: INITNONROOT(3)
Wed May 10 18:55:50 2006  Now running as requested user 'nobody' (99:99)
Wed May 10 18:55:50 2006  **WARNING** INIT: Unable to create pid file (/usr/local/var/ntop/ntop.pid)
Wed May 10 18:55:50 2006  Note: Reporting device initally set to 0 [eth0] (merged)
Wed May 10 18:55:50 2006  THREADMGMT[t3086269216]: ntop RUNSTATE: RUN(4)
Wed May 10 18:55:50 2006  THREADMGMT[t3020897200]: NPS(1): Started thread for network packet sniffing
Wed May 10 18:55:50 2006  THREADMGMT[t3031387056]: RRD: Data collection thread starting [p20314]
Wed May 10 18:55:50 2006  THREADMGMT[t3020897200]: NPS(1,eth0): pcapDispatch thread starting [p20314]
Wed May 10 18:55:50 2006  THREADMGMT[t3020897200]: NPS(1,eth0): pcapDispatch thread running [p20314]
Wed May 10 18:55:50 2006  THREADMGMT[t3062856624]: SIH: Idle host scan thread running [p20314]
Wed May 10 18:55:50 2006  THREADMGMT[t3073346480]: SFP: Fingerprint scan thread running [p20314]
Wed May 10 18:56:00 2006  **ERROR** RRD: Disabled - unable to create base directory (err 13, /usr/local/var/ntop/rrd)

any ideas?