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CompassTools provides Free Base Station Data for the GPS Community in Colorado

Durango CORS GNSS Reference Station (CTI4)

Installed in September of 2012 – CompassTools Inc. 4 Corners Reference station provides Real-time and Post processed reference data to improve accuracy for Mappers, Surveyors and anyone within a 150 mile radius of the Durango LaPlata AirPort.

The Trimble NetR9 was installed with the help of the Southwest Chapter Professional Land Surveyors of Colorado.

In December of 2012 CompassTools Inc. 4 Corners Reference station was accepted by the National Geodetic Survey as a Continuously Operating Reference Station (CORS).  CTI4 is the NGS CORS site id.

Real-time corrections can be accessed through the Mesa County Real Time Virtual Reference Network (RTVRN) – http://emap.mesacounty.us/GPS_Survey/GPS_Survey.htm
Base Station Specifics:
Receiver: Trimble NETR9 Dual Frequency GPS + GLONASS
Antenna: Trimble Zephyr Geodetic II with Ground Plane (TRM55971.00)
Logging Interval: 5 sec correction measurements
File Format: Zipped RINEX v2.11 (Observable and Ephemeris)
E-Mail Address
| Computed in Dec 2012 using 7 days of data.
|     X =  -1552692.273 m     latitude    =  37 09 10.50868 N
|     Y =  -4848834.557 m     longitude   = 107 45 21.91940 W
|     Z =   3832151.762 m     ellipsoid height = 2017.983   m
NAD_83 (2011) POSITION (EPOCH 2010.0)

| Transformed from IGS08 (epoch 2005.0) position in Dec 2012.
|     X =  -1552691.591 m     latitude    =  37 09 10.48965 N
|     Y =  -4848835.887 m     longitude   = 107 45 21.87666 W
|     Z =   3832151.828 m     ellipsoid height = 2018.866   m

CompassTools provides free Base Station Data for the GPS community

For more than a decade, CompassTools has provided free Base Station Data for the GPS community in Colorado, through a Base Station located at the University of Denver. This “accuracy enhancer” provides differential GPS corrections to provide a level of accuracy that otherwise would not be possible for data collectors in the field. Anyone within a 150-mile radius of Denver, Colorado, should be able to access this Base Station.

In the summer of 2011, we upgraded the technology to a new Trimble NetR9 receiver. With this new receiver, we can provide corrections not only for GPS but also for GLONASS, the Russian satellite system. This receiver will still be helpful for all GPS, and now provides data for those with GLONASS capable equipment.

The University of Denver has been the long-time host of the CompassTools Base Station. In upcoming months, CompassTools will install a new NetR9 receiver to provide free Base Station data for the Four-Corners region of Colorado, Utah, Arizona and New Mexico. We are also planning to install a GPS Base Station in South Dakota. We will let you know when those are available for your field data collection use.

Base Station Facts:

Receiver: Trimble NETR9 Dual Frequency GPS + GLONASS
Antenna: Trimble Zephyr Geodetic II with Ground Plane (TRM55971.00)
Logging Interval: 5 sec correction measurements
File Format: Zipped RINEX v2.11 (Observable and Ephemeris)
DAT File Naming: CTIDU_____201108160000.zip (CTIDU_____YYYYMMDD00HH.zip)
Contact: Mitch Tweedy
E-Mail Address:*protected email*
File Archiving: Files are posted to the FTP site every hour. Files will be kept on the system for a period of at least one year.

Reference Coordinates

NAD 83 (CORS96) (Epoch:2002.0000)
39 40 28.29716 N
104 57 47.70014 W
1630.971 m (HAE) – bottom of antenna mount
1647.281 m (NAVD88 – GEOID09) – bottom of antenna mount

WSG84(G1150) (Epoch:2011.486)
39 40 28.31661 N
104 57 47.74533 W
1630.077 m (HAE) – bottom of antenna mount
1647.381 m (MSL – USGG09) – bottom of antenna mount

Coordinate Discussions:

The Coordinates above represent an updated survey from July 6th, 2011. 8 separate dual frequency GPS data files were post-processed using the NGS OPUS site. Each file was a 10 hour AM or PM logging session spanning June 25th through June 28th, 2011. The results were geometrically averaged using Trimble Pathfinder Office. The coordinates encoded as the reference location in the base files themselves use the NAD83 (CORS96 – Ep:2002.0000) coordinate system. However, if you use Trimble’s Pathfinder Office software for post processing and keep the default “Use reference position from base provider” the coordinates will reference WGS84(G1150).

The WGS84(G1150) (2011 Epoch) coordinates are useful if you’re interested in true global geodetics. The International Terrestrial Reference Frame is updated continuously from GPS sites all over the world. As plate tectonic action moves the continents around by a few centimeters per year, the ITRF folks update the whole coordinate system.

It is often asked what the relationship between NAD83, ITRF, and WGS84 really is. The NGS WEB Site has a good explanation: http://www.ngs.noaa.gov/faq.shtml

It doesn’t really make sense to talk about coordinates on the ground without specifying the time epoch involved. The surface of the earth moves due to plate tectonics – several centimeters per year in all different directions. WGS84, as defined by the broadcast ephemeris of the GPS, is from a 2002 Epoch which translates to WGS84 (G1150). To move coordinates forward and backward in time (should you wish to get really technical about your geodetics), use the NGS Horizontal Time Dependant Positioning (HTDP) at http://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml


Many Trimble GPS Pathfinder Office or GPS analyst users have noticed duplicate NGS CORS base stations with odd messages “offline” or “derived from IGS08.” Many base stations have also lost their integrity index. This Trimble support note explains. This may take until June for resolution, as Trimble is re-computing the Integrity Index for all CORS stations. Until then, the integrity index may be displayed as ‘?’.

For More Information on the CompassTools Base Station using the internal Base Provider Selection options of Pathfinder Office

Learn more about the Trimble NetR9 GNSS Reference Receiver Series

CompassTools plans to install two more base stations:
· The Four-corners region of Colorado will receive a new Net R9
· Spearfish, South Dakota, will receive a Trimble 5700 dual-frequency receiver