Section 3: Internet Resources
Anchor: #i1001471The CORS Site
Using the CORS reference stations insures that all project control points are on a recognized network. It may not seem important at the time, but it also puts the TxDOT project on the NSRS at no extra cost. In fact, it saves sending extra people to the field and buying or renting extra GPS receivers.
For the extra few minutes it takes to download data, the office technician may as well include several additional CORS stations beyond the one or two required by the specifications. Refer to Figure 2-2 for a map of CORS stations as of this writing or visit the NGS Web site www.ngs.noaa.gov under the CORS/OPUS heading for an all-inclusive and current map.
Anchor: #i1001491Retrieving Data from the CORS Site
Retrieve CORS data from NGS from the NGS Web site at: www.ngs.noaa.gov/CORS.
The User Friendly CORS site (UFCORS) allows the user to download all the data desired in just one file containing the number of hours needed from a start time entered on the online form and, at the user’s request, can include the coordinates and ephemeris in one simple zipped download. It is not necessary to convert to Universal Time Coordinate (UTC) time or sort through the coded file names. All files available from NGS are in RINEX format.
Should users need to use individual hourly files, a typical NGS RINEX data file from a CORS station appears with this naming convention: {SSSS} {DDD} {H} . {YY} {T}
Where SSSS is the four (4)-character site identifier:
DDD is the day of year
H is a letter that corresponds to an hour-long UTC time block
YY is the year
T is the file type
Example: txan3350.01o
For daily files, the format would be {SSSS}{DDD}{H}0.{YY}{T}.
Anchor: #i1001540Obtaining Coordinates from the CORS Site
Coordinates for the station needed can also be found on the Web site by clicking the appropriate site on the map then choosing “coordinates” from the left hand menu. The coordinate data sheet has two sections.
The top section contains the position information for the antenna reference point (ARP) and the bottom section, the information for the L1 phase center of the GPS antenna. It is important to use the ARP coordinates for the held position in processing.
The antenna type at the CORS station (needed during processing) is included in the header of the downloaded file. In addition, each section of the coordinate listing contains the ITRF position and the NAD83 position. Be sure to use the NAD83 position information.
If the PID of a station is known, the coordinates can also be found in the NGS database for the National Spatial Reference System. By starting from the NGS home page, click on “datasheets” in the five selections at the top. Click on the DATASHEETS retrieval link and click on the PIDs. To retrieve the appropriate data sheet, key in the PID.
Remember, there may be as many as three (3) separate data sheets associated with a station:
- a data sheet for the monument on the ground (if one exists)
- a data sheet for the L1 phase center of the antenna (the point at which data is actually collected) and finally,
- the ARP (the mounting surface of the antenna).
Each of these points has its own PID.
Anchor: #i1001596OPUS
The Online Positioning User Service (OPUS) is the newest addition to NGS’ Geodetic Tool Kit. OPUS allows users to submit their GPS data files in RINEX format to NGS, where the data will be processed to determine a position using NGS computers and software. Each RINEX file submitted is processed with respect to three (3) National CORS sites or Cooperative CORS sites.
Any stations in a TxDOT network that contain two (2) or more hours of raw GPS data can be processed at this Web site. The tie sites selected may not be the nearest to the users’ site but are selected by distance, number of observations, site stability, etc. Users have the option to select their own CORS sites. The ITRF and NAD83 coordinates, as well as Universal Transverse Mercator (UTM) and SPC Northing and Easting report position data back to the user via e-mail.
TxDOT recommends that users check the results of the processing and adjustment on new points with this NGS service. As a minimum, at least one station should be checked in every network. OPUS positions are usually within one or two tenths of a foot. However, they may be less precise in areas of one-direction-only ties.
Use this to verify the NAD83 HARN position and ellipsoid height. Orthometric heights may not tie, based on whether the elevation was established before or after the original Texas HARN network was created.
Anchor: #i1001626NGS Description of OPUS
The National Geodetic Survey operates the Online Positioning User Service (OPUS) as a means to provide GPS its user’s easier access to the National Spatial Reference System (NSRS).
OPUS allows users to submit their GPS data files in RINEX format to NGS, where the data is processed to determine a position using NGS computers and software. Each RINEX file submitted is processed with respect to three (3) CORS sites.
The sites selected may not be the nearest to the users’ site but are selected by distance, number of observations, site stability, etc. Users have the option to select their own CORS sites. The position for data will be reported back to the user via e-mail in both ITRF and NAD83 coordinates, as well as UTM and SPC Northing and Easting coordinates.
Anchor: #i1001651NGS OPUS Requirements
OPUS is completely automatic. Users are required to enter only a minimal amount of information. OPUS requires the following:
- e-mail address to receive results
- RINEX file that the user wants to process (which may be selected using the browse feature)
- antenna type used to collect this RINEX file (selected from a list of calibrated GPS antennas)
- height of the Antenna Reference Point (ARP) above the monument or mark that user is positioning
- as an option, users may also enter the state plane coordinate code if they want SPC Northing and Easting.
Once the information is completed, click the upload button to send the data to NGS. User’s results will be e-mailed in a few minutes. Upload one RINEX file at a time.
Read through each of the OPUS Help Links. It is important that users understand how to correctly submit their data and how to interpret the results. For inquires or comments, use the OPUS e-mail button.
OPUS is intended for use in the conterminous U.S., Alaska, and Hawaii. It is NGS policy not to publish geodetic coordinates outside the U.S. without the agreement of the affected countries.
Anchor: #i1001703Useful Web Sites
Below is a list of Web site addresses which may be accessed for further information regarding GPS-related activities. Click the hyperlink to access the Web site for the following:
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GPS Antenna Information |
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GPS Orbital Data |
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GPS Overview |
http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html |
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National Geodetic Survey |
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NGS Data Sheets |
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NGS CORS Data |
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NGS PC Software |
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OPUS |
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Space Weather |
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TxDOT RRP Data |
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USCG |
Anchor: #i1001712
Retrieving Data from TxDOT
Retrieving CORS data from the original TxDOT maintained stations (RPS’s) can be done from the Internet using the following TxDOT Web address: http://www.dot.state.tx.us/isd/gps/gps.htm.
TxDOT posts files in both RINEX and Trimble formats. RINEX file naming convention is standard but a TxDOT data file in Trimble format will have a different naming convention. TxDOT is in the transition of updating the GPS data distribution web page. The “FileFormat.txt” file included on the Web site will contain the latest information for naming convention and the “Position_update.doc” contains the original TxDOT coordinates as well as antenna heights (for use of the actual monument rather than the ARP). Always refer to these documents when processing data from the TxDOT Web site.
Files in Trimble format are available in six (6) hour increments or four (4) files per day and files in RINEX format are available hourly. Note that Julian dates are based on Universal Time Coordinate (UTC). The is no provision for combining files before download. Data is kept for six (6) months and data older than six (6) months must be retrieved from NGS.
If all the CORS reference stations in the network the user is building are original TxDOT RRP’S, and if users have need for maintaining the TxDOT position used in the mid 1990’s, TxDOT still adjusts some of the newer RRP’s to the original unchanged coordinates of that time and makes them available on the “Position_update.doc” file above. Some RRP location at the outer edges of the state may differ from the NGS positions (epoch 2002) by a couple of centimeters. Remember to us H.I.’s when holding the actual monument rather than the usual ARP position. Also, note that these H.I.’s are to the phase center of the antenna.