Section 4: Coordinate Systems
Anchor: #i1000347Overview
Many spatial activities, such as navigation, mapping, and surveying, use geographic coordinates to describe the position of objects. Whenever two activities share a common coordinate system, their data can be more readily compared and exchanged.
For this reason, federal and state mapping products are referenced to two standard coordinate systems: the North American Datum of 1983 (NAD 83) for horizontal positions and ellipsoid heights, and the North American Vertical Datum of 1988 (NAVD 88) for orthometric heights. Surveys are referenced to these datums through measurements to control points of the National Spatial Reference System (NSRS).
Anchor: #i1000362National Spatial Reference System (NSRS) and Continuously Operating Reference Stations (CORS)
The NSRS is a set of geographic point attributes that provides a consistent framework to coordinate all spatial activities. The NSRS includes a nationwide network of Continuously Operating Reference Stations (National CORS), statewide Federal & Cooperative Base Networks (FBN/CBN), regional User Densification Networks (UDN), and other historic vertical and horizontal control. Figure 2-2 illustrates the CORS Network stations in Texas and some nearby stations in adjoining states. In Texas, TxDOT operates the majority of CORS stations.
Anchor: #i1000372Cooperative CORS Stations
Also noteworthy is the rapidly growing system of Cooperative CORS stations. Links to the data from these stations are available on the NGS Web site. Because of the reduced quality control (QC), limited hours of operation and less permanent nature of these stations, it is important that the surveyor be thoroughly familiar with those stations in their own area before depending on them. Also, note that Cooperative CORS coordinates are less accurate than CORS coordinates.
Figure 2-2. CORS Stations.
Anchor: #i1000391FBN and CBN
Federal Base Network stations (FBN) (75 to 125 km spacing) or Cooperative Base Network (CBN) stations (25 to 30 km spacings) are B order accuracy and make up the HARN network. These HARN stations have been observed using GPS and have been either used previously as reference stations in the adjustment of the old conventionally surveyed federal monuments or they are newly placed monuments. There are about four hundred of these listed by NGS in Texas.
This manual uses and/or references specific information from the following publications:
- 1989 Federal Geodetic Control Subcommittee (FGCS) document “DRAFT Geometric Geodetic Accuracy Standards and Specifications for Using GPS Relative Positioning Techniques”
- the 1998 NOAA Technical Memorandum “NGS-58, Guidelines for Establishing GPS-Derived Ellipsoid Heights,”
- the May 15, 2000 “Preliminary DRAFT Guidelines for Geodetic Network Surveys Using GPS,”
- and numerous other federal/state guidelines and specifications listed in Appendix A, References.