TO: Potential users of LCRA GPS Sub-HARN control data
FROM: Terry L. Nygaard / RPLS - Supervisor Surveying Operations / LCRA
CONCERNING: Information on data provided
LAST EDITED: 03-31-2014

QUESTIONS: Terry Nygaard - tnygaard@lcra.org
Ph# 512-578-4528
Ph# 1-800-776-5272 ext 4528
Or Chris Holland - christopher.holland@lcra.org

PLEASE NOTE: In March 2014, LCRA readjusted its Sub-HARN network based on the newer NGS NAD 83/2011 Epoch 2010 coordinate values. The previous LCRA network was based on NSRS 2007 values. LCRA has archived the last adjustment values based on the NSRS 2007 epoch in an Excel 97 spreadsheet. This archive information is available on request.

METADATA

HORZ DATUM : NAD 83/2011 Epoch 2010 (NGS adjustment epoch October 2012)
VERT DATUM1: NAVD 88 (NGS adjustment epoch Sept 1994)
VERT DATUM2: NGVD 29 (NGS legacy datum - No longer supported)
PROJECTION: LAMBERT GRID
SPC ZONE: TEXAS CENTRAL 4203
COORDINATE UNITS: US Survey Feet
GEOID MODEL: NGS GEOID2012A

LCRA Sub-HARN Re-Adjustment
In 2012, the NGS (National Geodetic Survey) developed a new adjustment and in October 2013 published new NAD 83/2011 Epoch 2010 positions for all HARN stations which correlated these stations to CORS Continuously Operating Reference Stations) positions as well as existing Leica and TXDOT RTN's.

LCRA utilized CORS and Leica RTN ellipsoid heights and the newer Geoid2012A geoid model to develop orthometric heights for the LCRA Sub-HARN readjustment. This should allow LCRA control to relate consistently with all systems that correlate with current CORS positions and elevations. Reliance on CORS to develop orthometric heights will also mean that local datum legacy NGS benchmark elevations may NOT correlate with elevations derived with the newer adjustment.

By constraining the newer NAD 83/2011 Epoch 2010 positions, LCRA's Sub-HARN network became even more consistent with the CORS positions and those positions developed from OPUS solutions.

The new adjustment of the LCRA Sub-HARN network using NAD 83/2011 Epoch 2010 positions and ellipsoid heights changed coordinate values in relation to the NSRS 2007 epoch adjustment on average the following amounts:

Y(N) coordinate values by less than < 1cm (0.02 ft)
X(E) coordinate values by less than < 1cm (0.03 ft)
Orthomethic height values varied significantly up to about +- 0.10cm (0.30 ft) in some cases

For more information: NGS website.

Currently there are three major components provided.

1) 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.
2) 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.
3) 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 Geoid2012A 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 (0.14 foot) vertically according to adjustment results.

How many points are in this network and why was it created?

Currently there are more than 1700 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 RTK and RTN 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.


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