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|Year of Publication||2011|
Land Surveying is a technique and science of accurately measuring the distances and angles between different points, on the surface of Earth. GNSS has been used by land surveyors since the late 1980s, primarily for geodetic control networks and for photo control.
Nowadays, GNSS is used to determine precise locations all over the globe, in any weather conditions and at any time of the day. GNSS geodetic surveying equipment has become smaller and easier to use being faster to use than other surveying methods. GNSS is specially used for large topographic surveys where a centimeter level accuracy is enough.
Land surveying usually relies on geodetic control networks that will be used as reference points and surveys are made in relation to these reference points. In detail surveys the traditional survey techniques rely on measurements from other known locations, such as the edge of properties, landmarks, or even a surveyor's stake. These land references, are subject to change over the time. With the use GNSS the coordinates can be located precisely on a worldwide reference frame and the GNSS land surveying tools produce measurements that do not rely on what happens to the surrounding land, constructions or landmarks.
Usually GNSS survey equipment use GNSS augmentation techniques to achieve the required level of accuracy. These techniques can range from the use of satellite based augmentation systems such as EGNOS or WAAS to dual frequency receivers using Real Time Kinematic (RTK). The augmentation technique is chosen depending on the required accuracy of the survey, the available equipment resources, the time required for the survey and the environmental characteristics of the surveyed site.
Typically survey grade receivers use DGNSS or Real Time Kinematic (RTK). These techniques require data from a base station with accurate and known coordinates. The base station data can be obtained from a base station network, a single public station or a own base station setup by the surveyor. Also the surveyor can chose between realtime corrections (requiring a communication link between the station and the rover) or post-processing correction.
In general terms GNSS high-end surveying equipment is more expensive than high-end traditional surveying equipment but when used for large topographic surveys where centimeter level accuracy is enough the added cost of the equipment becomes irrelevant given that it can be much faster than traditional methods. Traditional methods are still able to achieve better accuracies and are still the best option for surveys where: sub-centimeter accuracies are required, in situations where a clear view of the sky is not available or if vertical accuracy is important. In general, for detail surveying for construction, the traditional methods are still preferred.
GNSS survey techniques can be separated into the following methods:
- Static Surveys - In a static GNSS survey, the receiver remain fixed during the period of observation, that is usually called occupation time. The occupation time refers to the time for how long the rover unit should be kept static to achieve the desired level of accuracy. Longer occupation times will lead to higher accuracy. Occupation times range normally from 10 minutes to 6 hours, although higher values may be used. Carrier-phase techniques are usually used in this kind of surveys.
- Dynamic Surveys - In dynamic surveys, the rover unit will move around the site observing the same satellites as the base station. This type of survey provides a high rate of coordinate generation at the cost of having a lower accuracy than static methods. Kinematic methods are usually used for this kind of surveys. The algorithms used in dynamic surveys rely on the fact that while the rover can move around the site should never loose lock on the satellites signal. The techniques and algorithms used in dynamic surveys can be used in post-processing.
- Real-time dynamic surveys - Real Time Dynamic Surveys use the same techniques and algorithms than dynamic surveys except that these algorithms are run on realtime on the rover units. This type of survey requires a permanent communication link between the base station and the rover.
Land surveying makes use of several equipments such as transits, tape, theodolites and GNSS receivers. Modern instruments rely on GNSS and lasers for measurements.
The following companies are prominent in GNSS land surveying equipments market:
- Hemisphere GPS,
- Nikon Optical Instruments, produces Robotic and Optical instruments for land surveying.
- Spectra Precision,