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.

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.

Survey Types

When using GNSS techniques, there are essentially three types of survey, which can be split conveniently into different accuracy bands:

• Control surveys – high accuracy
• Detail surveys – medium accuracy
• Positioning – low accuracy.

Control Survey

A GNSS control survey is used to form the main coordinate framework for a project, as in a classical survey. Control surveys are typically at sub-centimeter accuracy. The number of stations, their location and spacing will be determined by the purpose of the control, the accuracy of the eventual survey and the type of GNSS equipment available for the project.

Detail Survey

Detail GNSS survey provides an excellent tool to quickly, accurately and reliably position points of detail. For instance the points or features which may need to be mapped as part of the survey, within the confines of an area surrounded by the control survey. Detail surveys typically have a requirement for accuracy of between one to ten centimeters. Some applications, such as utility asset mapping, require accuracies in the 10 to 30 centimeter range, and hence form a middle ground between detail and positioning GNSS surveying.

The surveyor shall decide which type of GNSS data capture technique is most suitable to their locale and/or survey specification, for instance, Real Time Kinematic (RTK) network, single baseline or own base station. There may be many instances of smaller areas of detail in a mapping project where traditional survey methods are more appropriate. Such methods can be quicker and more accurate.

Positioning

GNSS positioning frequently uses a single receiver, possibly receiving real-time DGNSS corrections or logging data for later post-processing, contrariwise to control and detail surveying. As a result, the accuracy for positioning is generally at the level of one to a few meters, rather than at a few centimeters. This type of survey would normally be accepted for precise navigation or for surveying features at the meter level to input into a CAD package or geographic information system.

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

This description was partially adapted from the Guidelines for the use of GNSS in land surveying and mapping.

GNSS survey techniques can be separated into the following methods:

• Static Surveys,
  • Low,
  • Medium,
  • High,
• Dynamic Surveys,
  • High precision,
  • Low/medium precision,
• Real-time dynamic surveys,
  • High precision,
  • Low/medium precision.

Static Surveys

In a static GNSS survey, the antennas and receivers remain fixed during the period of observation. Static survey is grouped in levels of, low, medium and high precision.

GNSS surveys operate closely to the technology limit when surveying height, where the precision is often more important than the one obtained in the plan. In cases where GNSS observations are to be used solely to fix plan position, the site chosen for the control station doesn't need to be perfect. However, when the accuracy of the height component is required to be better than 40mm, site selection is clearly important.

Static survey is grouped in three levels of precision, high, medium and low precision.

In high precision static surveys method a baseline vector is computed after the observations have been logged, using a differencing technique. Dual-frequency static methods are most suitable for control surveys and achieve the highest precision, of less than a centimeter.

The medium precision static surveys method is similar to high precision static surveys, but occupation times are reduced, hence a smaller amount of data is used to estimate and obtain the correct baseline solution. The differences between high-precision static and medium-precision static are the following:

• Dual-frequency data must be used,
• The processing software must allow computation of the baselines,
• The survey data must be virtually free of cycle slips, multipath and interference,
• Good satellite geometry is critical,
• Baselines are limited to a maximum of 40km.

The low precision static surveys is a point positioning method rather than relative positioning method. The procedure consists in fixing a GNSS receiver in a certain location and logging data for a period of time. The average precision obtained is around 0.3metres, however, the use of dual-frequency data can improve the position obtained.

Dynamic Surveys

In dynamic GNSS survey techniques many random measurement and GNSS system errors are absorbed in the coordinates, providing the highest production rate for all the GNSS methods. Although, it generates coordinates very quickly, the precision obtained is not as high as by static techniques.

Dynamic survey is grouped in two levels of precision, high and low/medium precision.

In high precision dynamic surveys, the methods of kinematic and on-the-fly kinematic can be used keeping one receiver fixed at a known control base station base, whilst one or more other receivers (rovers) move around the site using the same satellites.

In the kinematic technique, both receivers must initialize on a known baseline and then maintain lock on at least four satellites throughout the session. The on-the-fly method, on the other hand, does not require an initialization on a known baseline.

In low/medium precision dynamic surveys the same general technique as the high precision method id used, but it can be based on L1 code, dual-frequency float or phase-smoothed code solutions, giving positioning up to the decimeter level.

Real Time Dynamic Surveys

With suitable communications link and proper processing software and firmware, the dynamic survey types can be carried out in real time. These techniques can deliver corrections to the user in the field.

Real-time dynamic survey is grouped in two levels of precision, high and medium/low precision.

In high precision base and user real-time (RTK) surveys, high-speed communications links are used to transmit the base data to the rover using UHF or VHF, integrated in base and rover devices, with antenna, power, GNSS receiver, radio, modem and cables all mounted on or inside it.

A RTK network can also be used if there is mobile coverage available on the site. The procedure consists in subscribing to a RTK network correction service which provides data to the base station, via cellular network, instead of having a base instrument set up over a known point sending data to the rover.

In low or medium real time dynamic surveys, differential GNSS techniques can be used, achieving a precision that can vary from one decimeter to a few meters. Dual-frequency data techniques can be used, granting a precision of one to three decimeter.

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.