DGPS Survey

Differential GNSS positioning for control points, coordinate transfer, alignments and ground control, delivered in the coordinate reference system and height datum your project uses.

Overview

A DGPS survey uses satellite positioning with differential corrections to determine the coordinates of points on the ground. DGPS stands for Differential Global Positioning System. Because modern receivers track several satellite constellations, the same technique is often called DGNSS (Differential Global Navigation Satellite System). In everyday use on Indian projects, "DGPS survey" usually means any survey in which a reference station with known coordinates is used to correct the positions measured by a roving receiver.

A single stand-alone receiver is affected by errors in satellite orbits and clocks and by the delay that signals suffer while passing through the atmosphere. When two receivers observe the same satellites at the same time and one of them sits on a known point, most of those errors are common to both. Measuring them at the reference point and removing them from the rover is what turns a navigation-grade position into a survey-grade coordinate.

DGPS is a positioning method rather than a complete deliverable. It answers the question "where exactly is this point?" and is typically used to set up the control framework that other survey work, design and construction depend on.

How it works

A base, or reference, receiver is set up on a point whose coordinates are known or have been determined for the project. A second receiver, the rover, occupies the points to be surveyed. Both record signals from the same satellites. Differences between the known and observed position at the base are used to compute corrections, which are then applied to the rover observations. Depending on the method, the corrections are applied in real time or later during processing.

In Real-Time Kinematic (RTK) surveying, corrections are sent from the base to the rover over a radio or mobile data link, and the rover displays corrected coordinates within seconds. RTK suits detail capture, stake-out and situations where results are needed on site. The corrections may also come from a network of permanent reference stations, where such a service is available and appropriate. In post-processed methods, the base and rover simply log raw observations. Static surveys keep receivers on each point for an extended session and are used where the highest reliability is needed, such as primary control. Post-Processed Kinematic (PPK) works like RTK in the field but computes positions afterwards, which is useful where a reliable correction link cannot be maintained.

Receivers may track several Global Navigation Satellite Systems: GPS (United States), GLONASS (Russia), Galileo (European Union) and BeiDou (China), along with India's regional NavIC system (IRNSS). Tracking more satellites generally improves geometry and availability, particularly where part of the sky is blocked, although the systems a given receiver can use depend on its hardware and firmware.

Satellite positions are computed in a global earth-centred reference frame, commonly expressed as WGS 84 latitude, longitude and ellipsoidal height. Most engineering work needs plane coordinates, so results are usually projected to a grid such as Universal Transverse Mercator (UTM). Mainland India spans UTM zones 42N to 47N, and a project may also use a local grid or a client-specified system. Agreeing the coordinate reference system before fieldwork avoids costly conversions later.

Heights need particular care. GNSS gives heights above the reference ellipsoid, a mathematical surface, whereas design levels and existing benchmarks are normally orthometric heights related to mean sea level. The two differ by the geoid separation, which varies from place to place. Converting between them uses a geoid model, a connection to known benchmarks, or both, and the approach used should be stated with the results.

Applications

Project survey control

Establishing a network of control points with known coordinates so that topographical, setting-out and as-built work all refer to the same framework.

Road and highway alignments

Positioning centrelines, chainage marks and control along long corridors, where satellite methods avoid the long traverses a total station would need.

Pipeline and transmission line routes

Coordinating route points, tower or angle-point locations and crossing positions for linear infrastructure across open country.

Land development control

Fixing control and key points for plotting, layout planning and site grading on development land, tied to the project coordinate system.

Ground control for aerial mapping

Coordinating ground control points and check points used to georeference and verify drone imagery or LiDAR data, where such mapping forms part of a project.

Boundary coordinate support

Recording coordinates of existing boundary marks or proposed corners to support planning. Legal or cadastral boundary determination depends on the authorisation of the relevant authority and licensed professionals.

Support for topographical mapping

Providing control and capturing open-ground detail and levels as one part of a broader topographical survey.

Industries

Infrastructure
Roads and highways
Power
Rail and metro
Construction
Government
Engineering consultants

Methodology

Requirement and reference system

Confirm the purpose, the points or area to be coordinated, the required tolerances, the coordinate reference system, the height datum and any existing control or benchmarks to connect to.

Desk study and reconnaissance

Review maps and imagery, identify open-sky locations, check access and note obstructions, power lines and other sources of interference before observation.

Reference station setup

Establish the base on a known or newly determined point, record the antenna height carefully and confirm the correction link or logging settings.

Observation

Occupy each point using the agreed method (static, RTK or PPK), observing for long enough and repeating occupations on key points so that results can be compared.

Processing and adjustment

Process baselines where required, check misclosures and repeat measurements, apply the coordinate transformation and height model, and flag any points that do not meet tolerance.

Reporting and handover

Issue coordinates with point descriptions, the reference system, the height method and the checks carried out, so others can reuse the control with confidence.

Equipment and technology

A DGPS survey uses survey-grade GNSS receivers with antennas, tripods or poles, tribrachs for accurate centring and a field controller running data collection software. For RTK, a radio or mobile data link carries corrections; for static and PPK work, raw data are logged for office processing.

Processing software computes baselines, checks the consistency of repeated observations and applies the transformation from the satellite reference frame to the project grid and height datum. The specific receivers and software used on a project are confirmed in the scope of work.

Benefits and considerations

Benefits

Points several kilometres apart can be coordinated without line of sight between them.
Works efficiently over long corridors and large open sites.
Results are tied to a recognised coordinate reference system.
Provides a common control framework for other survey methods.

Considerations

Requires a clear view of the sky at each point.
Heights depend on the geoid model or benchmark connection used.
Not suited to points under dense canopy, inside buildings or close to tall structures.
Reliability depends on observation time, satellite geometry and checks.

What to prepare

Describe what the coordinates will be used for and the accuracy you need.
Share the required coordinate system and height datum, or tell us if it is still to be decided.
Provide details of existing control points, benchmarks or previous survey reports.
Mark or describe the points, corridor or area to be surveyed on a map or drawing.
Arrange access to all points, including any permissions needed for private land.
Tell us about tree cover, tall buildings or other obstructions at the site.

Limitations

Satellite signals can be blocked or reflected by buildings, trees, terrain and vehicles. Points in urban canyons, under tree cover or beside walls may need a different method.
Multipath, where signals reflect off nearby surfaces before reaching the antenna, can bias positions and is not always obvious from the field display.
Poor satellite geometry and disturbed ionospheric conditions reduce reliability; observation planning and repeat measurements help to manage this.
Achievable accuracy depends on the equipment, method, observation time, baseline length and site conditions, and is assessed for each project rather than assumed.
Converted orthometric heights are only as reliable as the geoid model or benchmark connection used.
Coordinates do not by themselves establish legal boundaries.

DGPS Survey vs Topographical Survey

A DGPS survey is a positioning method. Its output is a set of coordinates and heights for selected points, such as control stations, alignment marks, ground control points or boundary marks, determined from satellite observations corrected against a reference station.

A topographical survey is a survey objective: a complete record of the ground surface and the features on it, with levels, contours and a terrain model. It can be carried out with DGPS/GNSS, a total station, LiDAR or a combination. DGPS often provides the control framework and some of the open-ground detail, while a total station picks up features under trees or close to buildings.

If you need a limited number of accurately positioned points or a control framework for your project, request a DGPS survey. If you need a measured drawing of the site for planning or design, request a topographical survey.

Read about topographical surveys

Frequently asked questions

What is the difference between DGPS and RTK?

RTK is one way of carrying out a differential GNSS survey. It applies corrections in real time over a data link. Static and post-processed kinematic methods are also differential, but the corrections are applied during office processing.

Can DGPS be used under trees or between tall buildings?

Usually not reliably. Satellite signals need a clear view of the sky. Points in those locations are normally coordinated with a total station from nearby GNSS control.

Which coordinate system will the results be in?

The system is agreed before fieldwork. Common choices are WGS 84 geographic coordinates, a UTM zone between 42N and 47N, or a local project grid specified by the client.

Will the heights match the existing site benchmarks?

Only if the survey is connected to them. GNSS measures ellipsoidal heights; relating them to orthometric levels needs a geoid model, a benchmark connection, or both, and the approach is stated in the report.

Is a DGPS survey the same as a topographical survey?

No. DGPS is a positioning method that produces coordinates. A topographical survey maps the terrain and features of a site and may use DGPS alongside a total station or LiDAR.