Potential users of LCRA GPS Sub-HARN control data
Terry L. Nygaard / RPLS - Supervisor Surveying Operations / LCRA
Information on data provided
Terry Nygaard -
1-800-776-5272 ext 4528
Or Chris Holland -
In August 2008, LCRA readjusted its Sub-HARN network based on the newer NGS NAD 83 (NSRS2007) coordinate values. The previous LCRA network was based on HARN/CORS 2003 values. LCRA has archived the last adjustment values based on the 2003 epoch in an Excel 97 spreadsheet. This archive information is available on request.
HORZ DATUM :
NAD 83 (NSRS2007) (NGS adjustment epoch May 2007)
NAVD 88 (NGS adjustment epoch Sept 1994)
NGVD 29 (NGS legacy datum - values modeled)
TEXAS CENTRAL 4203
US Survey Feet
LCRA 2008 Sub-HARN Re-Adjustment
In 2007, the NGS (National Geodetic Survey) developed a new adjustment and in May 2007 published new NAD 83(NSRS2007) positions for all HARN stations which correlated these stations closer to CORS Continuously Operating Reference Stations) positions.
LCRA utilized more than 140 NAVD88 benchmarks and heavy reliance on the Geoid2003 geoid model to develop orthometric heights which will hopefully relate consistently with local datum elevations.
By constraining the new 2007 NAD 83 (NSRS2007) positions, LCRA's Sub-HARN network became even more consistent with the CORS positions. The CORS sites are being used more and more by local surveyors to help minimize the cost of observing static GPS sessions. The free processing service OPUS provided by NGS is getting a tremendous amount of use and the base for ALL its processing are the CORS sites.
The new adjustment of the LCRA Sub-HARN network using the combination of 2007 NAD 83 (NSRS2007) positions and new current CORS positions changed coordinate values in relation to the 2003 epoch adjustment on average the following amounts:
Y(N) coordinate values by less than < 1cm (0.03 ft)
X(E) coordinate values by less than < 1cm (0.03 ft)
Ortho height values by about +- 0.5cm (0.01 ft)
While the differences may be significant for geodesy applications, it is the opinion of the author that these differences are insignificant for all but the most precise surveying applications. These positional variances are within the tolerances required for the great majority of LCRA survey projects.
For more information:
Currently there are three major components provided.
AutoCad LCRA_SUB_HARN.dwg. The dwg is in Texas Central Zone, US Survey Feet units. This dwg also contains 7.5 minute topographic boundaries and names in addition to County boundaries and names and the Colorado River.
Excel spreadsheet named LCRA_Sub_HARN_XX-XX-XXXX_USfeet.xls with coord values in US survey feet. This xls file contains the final four character station ID's, adjusted coordinates, geographic positions, NAVD 88 elevations, NGVD 29 Elevations (modeled using NGS Vertcon), County, 7.5 minute topo map, etc in Texas Central Zone.
PDF images of most all the Station ID's. For example, for GPS station A034, a PDF document named A034.pdf would represent a multipage sketch document. Typically the first image is a local area sketch with ties and the second more of a vicinity sketch with mileages to road intersections etc. PLEASE NOTE that there will NOT be PDF images for all GPS point ID's. In these cases the best tool to locate a point will be the AutoCad dwg supplied or a handheld GPS receiver. Any PDF image viewer can be used to print or view these files.
How was the data collected and how accurate is this data?
All GPS data was collected with a typical occupation time of 45 minutes to 2 hours. About 5% of the data was collected with L1 receivers, the remainder with L1/L2 receivers. About 75% of the vectors were observed multiple times, especially on longer lines of about 25km and up. Loop closures were performed on all triangles. No trivial vectors were used in the final adjustment. Only fixed solutions were used in the final adjustment using STARNET. NGS Geoid2003 geoid model was used to derive orthometric heights. Typical accuracy of these control points is about 1.5cm (0.05 foot) horizontally and about 4cm (0.13 foot) vertically according to adjustment results.
How many points are in this network and why was it created?
Currently there are more than 1500 control points that cover about 38 counties in Central Texas. Back before more densified CORS stations and OPUS, this network was created to allow LCRA to map all of its projects to a highly accurate and consistent GPS network. NGS spacing on GPS HARN control points in Texas is 100km and we needed something more dense to help facilitate what we believe in the future will be a large amount of RTK GPS surveying. This GPS network was built incrementally over about a eight year time span. Our GIS base mapping components also utilize this system. Most all LCRA surveying projects are currently mapped to Texas Central Zone using US Survey Feet units.