1. How often should I send data? Any packet rate that you are comfortable with is fine with us as long as it is not faster than one data packet every 9 or 10 minutes. Right now data are sent from findu.com to NOAA every 15 minutes, so one packet every 10 minutes is adequate. We recommend that you set your software so that the first packet of the hour is sent in the minute that matches the last number in your CW ID or ham callsign. The purpose of this is to even out the data flow received by the APRS-IS servers to reduce the number of connects at multiples of 5 minutes after top of the hour. If your application does not let you set the start time or if your last number is 0 or 5, select a time interval of 9 or 11 minutes.
2. How can I change items in the CWOP database? Just send an e-mail. If you're sending latitude/longitude changes, use decimal degrees and if elevation change, use meters above sea level. If your zip code has changed, note that. Also, if you'd like to have a personal web page added, send the URL. Note that only you can change the location on findU maps by entering new values of latitude and longitude into your local software.
3. What is the APRS latitude/longitude position format? In an APRS weather data packet, the part between "z" and "_" is the latitude and longitude of the station position. The particular format used is from the LORAN position format and is capable of about 60 feet resolution on the findu.com maps. The format is "ddmm.hhN/dddmm.hhW" without the quotation marks, where d is degrees, m is minutes and h is hundredths of minutes. The first eight characters represent latitude (north in this case), and the last nine characters represent longitude (west in this case). The hemispheres are represented by capital letters; N, E, S, W and remove the need for plus or minus signs on the latitude and longitude. Note that leading zeros are required, that there must be only two numbers to the right of the decimal point in the latitude or longitude, and that colons are not allowed. Many examples of correct format positions are contained here, and here and there is a web-based DMS, LORAN, decimal conversion tool. Any lat/lon changes for the CWOP database should be in decimal format.
4. How can I find my latitude, longitude and elevation? While there are several different ways, one is to go to ACME mapper and enter your nearby town and your state at the bottom and press "find". Then change to a high resolution image and double click on your actual location. Your latitude and longitude is then given in the "Lat/Long" box. Once you have your latitude and longitude in decimal degrees, copy and paste (without the quotes) "http://www.earthtools.org/height/40.0545/-105.20783" into your browser and then change the first number to your latitude and the second number to your longitude and then press enter. In the resulting text, you'll see your elevation in feet and in meters which is what is used in the CWOP database.
5. What is the APRS time format? In an APRS weather data packet, the part between "@" and "z" is the time of the weather station clock when the packet was sent. However, many computer clocks frequently have large errors and to avoid these errors, the data time is taken as the time that the packet arrives at the findu.com server. This does introduce an error of the time that it took for the packet to travel from the station to the server, but that is usually not significant for weather uses. Marc, CW0009, operates a CWOP station in Antwerp, Belgium and has a time synced computer clock accurate to within one second. Marc sends four packets per hour on the quarter hour exactly. You can see Marc's data here. The last two numbers in the time column are the time in seconds that it took to connect, send the packet through his ISP, across the Atlantic to the APRS-IS server, and to findu.com. This total time is usually between 10 and 20 seconds and is not a problem for our uses.
6. Why do I have two IDs for CWOP? Actually, you have three IDs because there are three different databases involved. To reduce confusion, we've tried to make the IDs the same or similar where possible. The first database is the findu.com mySQL database of received weather packets. Ham radio stations use the callsign of the station, and CW stations use CWxxxx in the findu.com database. The second database is the MADIS set of netCDF files where the ID is called the NWS ID and is a maximum of 5 characters. Ham radio weather stations are identified by APxxx or ARxxx or ASxxx (two upper case letters followed by 3 numbers) and CW weather stations are identified by Cxxxx (upper case C followed by 4 numbers). The MADIS netCDF files are used to send the data on to users and to store the quality checking information. The third database is the CWOP database. Ham stations have a Provider ID of apxxx or arxxx or asxxx (two lower case letters followed by 3 numbers) and CW weather stations have a Provider ID of CWxxxx (2 upper case letters followed by 4 numbers). The key things to remember are that the ham callsign or CWxxxx goes in with your data packet and is what you look for on findu.com while the quality checking results are identified by the five character NWS ID of APxxx or ARxxx or ASxxx or Cxxxx. The members list provides the cross reference.
7. How is the weather data coded into the packet? When you look at examples of APRS position weather packets here, or here, the part after the longitude "E" or "W" carries the weather data as symbols followed by numbers. The underscore "_" followed by 3 numbers represents wind direction in degrees from true north. This is the direction that the wind is blowing from. The slash "/" followed by 3 numbers represents the average wind speed in miles per hour. The letter "g" followed by 3 numbers represents the peak instaneous value of wind in miles per hour. The letter "t" followed by 3 characters (numbers and minus sign) represents the temperature in degrees F. The letter "r" followed by 3 numbers represents the amount of rain in hundredths of inches that fell the past hour. The letter "p" followed by 3 numbers represents the amount of rain in hundredths of inches that fell in the past 24 hours. Only these two precipitation values are accepted by MADIS. The letter "P" followed by 3 numbers represents the amount of rain in hundredths of inches that fell since local midnight. The letter "b" followed by 5 numbers represents the barometric pressure in tenths of a millibar. The letter "h" followed by 2 numbers represents the relative humidity in percent, where "h00" implies 100% RH. The first four fields (wind direction, wind speed, temperature and gust) are required, in that order, and if a particular measurement is not present, the three numbers should be replaced by "..." to indicate no data available.
8. How can I check my data flow through APRSWXNET/CWOP? The CWOP search tool shows data at various points. Your local app sends your packets to the APRS-IS (Internet Service) and, for each of the past ten days, your last packet seen on APRS-IS is shown along with the number of packets in the last hour of that day, total number of packets in that day, the number of duplicates, and the number of bytes. Your data packets flow from the APRS-IS to the findU server and you can check your weather data on findU by clicking on "WxGraph". Note that you can view your data over the past 12 hours out to 10 days. Every 15 minutes the findU server sends your data to MADIS if your station has been registered. While it is not easy to check the output of MADIS, you can check your data at three places that get your data from MADIS. The first, "NOAA Meso Map" will return a map (after a delay) that should have your location in the center and by double clicking there, your data will be listed. The second, "Town/City/Meta" will show the location and other information used in CWOP Quality Checking along with your data for the past day. The third, "MesoWest" will show your data that were sent from MADIS to MesoWest. If these links are not there, then your station is not registered.
Discussion of CWOP weather data quality checking with sign-up here.
Discussion of the weather and the WxSolution software here.
Discussion of the weather and CWOP/APRSWXNET here.
Discussion of the weather and related topics on the TAPR Weather Special Interest Group e-mail reflector. To join, go here, then use the rightmost slider bar to go to the bottom of the frame and follow the directions under WXSIG.
| Page generated on May 29, 2008. Site maintained by Russ Chadwick, KB0TVJ. Send any comments to russ@wxqa.com. |
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