“What’s New With CRA” Blog: Horizontal Positioning | 01/16/2015
By: Laci Theriot
The Global Positioning System (GPS) is used to display exact positions on land and over water. The accuracy of a position derived by GPS is determined by several factors, including tall obstructions near the antenna length of data collection time but most of all by the type of data acquisition. There are actually three types of data in common use today. The first is static or rapid static positions from measurements made at base stations on land and post-processed against data from other base receivers located over known points. GPS positioning systems used during hydrographic surveys fall into one of two other categories, Real Time Differential or Real Time Kinematic. Again, in both methodologies a base station must be set up somewhere over a known point which then measures a number of parameters from each satellite in view. This unit compares its computed location from the satellites to its known (true) coordinates and computes range corrections to each satellite in view. The latter are then transmitted over a telemetry link of some kind to the receiver on the boat where they are applied to the ranges to the same satellites. This brings the accuracy down to 1 to 2 feet for differential mode and 2 to 3 inches for kinematic mode. Differential corrections are usually obtained from the U.S. Coast Guard “Navbeacon” system which has receivers operating on towers along the coast and inland along the largest rivers. CRA typically uses either a dedicated base station, or a Virtual Reference Station (VRS) network put up and maintained by the State or private industry for kinematic corrections. For more information on horizontal positioning, please visit NOAA’s website: http://www.nauticalcharts.noaa.gov/hsd/control.html.