Land Surveying

Applications
Title	Land Surveying
Author(s)	GMV.
Level	Medium
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 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 an centimeter level accuracy is enough. For detail surveying that requires more accurate measurements or when there isn't a clear view to the sky traditional surveying is still used.

Application Architecture

Land surveying

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 WAAS or EGNOS 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 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 correction (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 costs of equipment become irrelevant given that 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 traditional methods are still preferred.

Application Characterization

There are two methods by which station positions in the GNSS reference frame can be derived: relative positioning and point positioning.

In relative positioning, two or more GNSS receivers receive signals simultaneously from the same set of satellites. These observations are then processed in one of two ways.

In the first way, the components of the baseline vectors between observing stations are determined. Once the coordinates for one or more base stations are known, new rover stations can be determined with an accuracy relative to the known coordinates.

The other processing technique uses a single GNSS receiver located at a known point or base station which compares observed satellite ranges with known ranges. These corrections are then made available to other receivers in the vicinity through DGNSS.

Relative positioning can only be used in situations where there are source control stations with known coordinates. When there is no national continuously operating GNSS receivers (COGRs) to be used as source control, the surveyor has the choice of using data from one or more of the International GNSS Service (IGS) COGRs or establishing a point position fix as the source control for the survey.

In point positioning method, data from a single station is processed to determine three-dimensional cartesian coordinates (X, Y, Z) referenced to the WGS 84 earth-centered reference frame (datum). The present accuracy for GNSS point position determinations ranges from 0.3m to 30m. This is the standard method used for stand-alone navigation receivers.

Survey Techniques

GNSS survey techniques can be separated into the following methods:

• Static Surveys,
• Dynamic Surveys,
• Real-time dynamic surveys,

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 10m to 6h, 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 that 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 by post-processing the collected data.

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.

Application Examples

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:

• Ashtech,
• Hemisphere GPS,
• Nikon Optical Instruments, produces Robotic and Optical instruments for land surveying.
• Sokkia,
• Spectra Precision,
• Trimble.