GNSS Surveys – How does it work
An overview of GPS (Global Positioning System)/ GNSS (Global Navigation Satellite System) technology and how it works to deliver precise measurements.
GNSS is a satellite-based navigation system that provides users all over the world with precise location and time information. There are a handful of constellations that are used in modern-day GNSS survey equipment, the most well-known is the GPS or North American Global Positioning System which is operated by the United States government. In addition to GPS, survey field crews in the United States are utilizing GLONASS (Russia), GALILEO (European Union), BEIDOU (China), and QZSS (Japan).
GNSS operates using a network of satellites that circle the planet and broadcast signals back to the receiver. GNSS receivers receive these signals and use the information contained in them to calculate the receiver’s precise location. The receiver compares the time a signal was transmitted by a satellite to the time it was received and then utilizes this information, coupled with the satellite’s known positions, to compute its location.
The role of GNSS in boundary surveys, and how it is used to locate and mark the boundaries of a piece of property.
GNSS technology is an essential tool used by land surveyors and is helpful in conducting boundary surveys since it can be used to quickly establish Survey Control on a project that is referenced to a specific Projection/Datum. It can also be used to locate and mark the corners of a piece of property. GNSS technology is very valuable in boundary surveys because they offer highly accurate position information more quickly than other survey methods and without the need to traverse a tract of land or have a visual line of sight in between points.
A surveyor will use a GNSS receiver to collect data at various spots along the property’s boundaries to conduct a GNSS survey for a boundary survey. Typically, the surveyor will install the receiver at each place and capture the location data supplied by the GPS satellites. The surveyor will then utilize this information, as well as other sources of information such as property records and deeds, to identify the exact location of the property’s boundaries.
After determining the boundary lines, the surveyor will usually use survey stakes or other markers to physically mark the limits on the ground. These markers can be put at specific spots along the boundary lines, such as corners or other landmarks, or intervals along the boundary lines. The surveyor will next create a map or other written report indicating the placement of the boundary lines and markers.
One thing to consider is that the GNSS receiver must have a clear sky to receive data from the various satellite constellations. The canopy of large trees can interfere with reception and can make it impossible in some instances to utilize this technology. Other interference can occur from tall buildings, weather, and atmospheric conditions can also affect the quality of GNSS data.
Three GNSS survey applications.
Static GPS/GNSS Control
Static GPS observations consist of a surveyor setting up a GPS receiver over a point and letting it “cook” for a minimum of two (2) hours. This data is then exported from the receiver as a RINEX file and sent to the National Oceanic and Atmospheric Administration’s (NOAA) Online Positioning User Service (OPUS). OPUS will use the RINEX file from your receiver and post-process it against continuing operating reference stations (CORS) to apply the necessary corrections to the single receiver’s position and determine a Static Position. https://geodesy.noaa.gov/OPUS/
RTK (Real-Time Kinematic) GNSS Surveying
RTK surveying is a technology that utilizes two GNSS receivers at the same time. The first receiver is known as the “Base” and it is placed by the surveyor over a survey control point with known coordinates (X,Y,Z) values. The base receiver is connected to a radio which broadcasts the necessary corrections at a specific frequency to the second GPS receiver which is known as the “Rover”. Using this method, a surveyor must use two receivers simultaneously on the same job site and the surveyor is limited by the range of the radio broadcasts (1-10 miles).
RTN (Real-Time Network) GNSS Surveying
RTN surveying is one of the latest technologies. It is similar to RTK, but instead of using a radio and two receivers, it utilizes one receiver “Rover” and a base station that is maintained by a network, and which publishes GNSS corrections via the internet. Networks are usually private and require the surveyor to pay a subscription to access the network corrections. The RTN method of GNSS surveying requires that the field crew has cell phone service to take measurements and get the corrections from the base. This method is quick to set up but if you are working in very rural areas that do not have cell phone coverage then you will struggle to complete the survey.
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