This archive contains answers to questions sent to Unidata support through mid-2025. Note that the archive is no longer being updated. We provide the archive for reference; many of the answers presented here remain technically correct, even if somewhat outdated. For the most up-to-date information on the use of NSF Unidata software and data services, please consult the Software Documentation first.
>From: "ORAM, TIM (JSC-ZS)" <address@hidden> >Organization: NASA/JSFC >Keywords: 200101031713.f03HDFo26562 GEMPAK NOAAPORT NIDS Tim, >You've done quite a bit of work with the radar data over NOAAPort, so I was >hoping you could help or point me in the right direction. When I compare >the radar data for the same time on AWIPS versus the data at the new NWS >"NIDS" ftp site, the ftp site has a WMO header embedded in the file and the >file is longer. The images on the FTP site are probably the zlib compressed images. The current AWIPS products should be decrypted copies of the encrypted products that have been flowing in NOAAPORT for awhile. For reference, the form of the zlib compressed files is: ** Section Product Example End ** +--------------------------------+ ** | | ** 1 | start of product | CTRL-A \r\r\n ** | | ** +--------------------------------+ ** | | ** 2 | sequence number | 237 \r\r\n ** | | ** +--------------------------------+ ** | | ** 3 | WMO header | SDUS53 KARX 062213 \r\r\n ** | | ** +--------------------------------+ ** | | ** 4 | PIL | N0RARX \r\r\n ** | | ** +--------------------------------+ ** | | ** 5 | | AAO130006R2 CH-1 ** | | Interface Control ** | CCB | Document (ICD) ** | | for the NWS NWSTG ** | | Figure 7-1 p 38 ** | | ** +--------------------------------+ ** | | ** 6 | WMO header | SDUS53 KARX 062213 \r\r\n ** | | ** +--------------------------------+ ** | | ** 7 | PIL | N0RARX \r\r\n ** | | ** +--------------------------------+ ** | | ** | | ** | | ** | | ** 8 | image | ** | | ** | | ** | | ** | | ** +--------------------------------+ ** | | ** 9 | trailer | \r\r\nETX ** | | ** +--------------------------------+ ** | | ** 10 | Unidata floater trailer | \0\0 ** | | ** +--------------------------------+ ** ** Sections 5-8 are zlib compressed. They must be uncompressed and ** read to find out where the image begins. When this is done, sections ** 5-7 are thrown away and 8 is returned immediately following 4. ** Section 9 and, if it is there, section 10 are also thrown away. >I compared hex dumps of both and there appear some >differences there as well even when you account for the WMO header. I also >tried displaying the data from the ftp site in GARP&NMAP and it failed >miserably, but the AWIPS file displays fine. This really does tell me that the AWIPS files are not _yet_ the zlib compressed ones. Unidata (Steve Chiswell) has added support for the zlib compressed images soon to be freely available in NOAAPORT to our distribution of GEMPAK/GARP/NMAP. We have been successfully displaying the _one_ unencrypted NEXRAD site (ARX) products in both GEMPAK/GARP/NMAP and McIDAS. Steve has made his modifications to GEMPAK/GARP/NMAP available to Mary DesJardin's group and they will be releasing it in their 5.6b release. >It was my impression that the data at the ftp site was supposed to be >identical to the AWIPS data (we've referred to this as Level III data). The FTP site I believe is already in the zlib compressed format. The stuff in NOAAPORT will be in the same format come January 10. >Since this assumption was wrong, do you know what I need to do to the data >from the ftp site to make it usable in GARP? Get the 5.6b GEMPAK distribution from NCEP. Tom >From address@hidden Wed Jan 3 11:34:41 2001 >Subject: RE: 20010103: NIDS FTP Radar data versus AWIPS products Thanks.