Digital Ground Survey Guideline

 

1. General

This document shall be used as a guideline to achieve the minimum survey requirements necessary for the collection of suitable Topographical field data in a variety of survey types.

Survey Project requirements can be complex and thus any queries should be directed to the Senior Engineering Surveyor.

Topographic Surveying involves measurement of the Earth's surface to determine the three-dimensional location of man-made and naturally occurring physical features for the purpose of Digital Ground Modelling.

Topographic surveys are often referred to as Feature Surveys,Digital Ground Surveys or Detail Surveys.

Digital Ground Modelling can be defined as a mathematical representation of the earth's surface upon which Planimetric Mapping, Geometrical Design, Analysis and Computation are based.

The "Digital Ground Survey Standard 67-08-43" documents the standard requirements and outputs for performing a Digital Ground Survey which ensures MRWA has continuity of standard, accuracy, quality and procedure.

The MRWA measurement accuracies for point and linear feature classifications documented in the “67-08-43” Standard ultimately determines the appropriate survey methodology.

Surveyors undertaking field work for Main Roads WA shall be eligible for Membership of the Surveying & Spatial Sciences Institute of Australia and shall be on-site at all times during survey work.

2. Guideline for Digital Ground Surveys

2.1 Introduction

Digital Ground Surveys are performed to capture and model the physical geometric characteristics of the Earth's surface to facilitate subsequent design of appropriate structures e.g. roads, bridges, culverts and flood-ways. The Digital Terrain Model created, forms the basis upon which all planning, design, spatial computation and analysis is based.

Digital Surveys are also carried out for Utility/Service location, for as constructed compliance or verification, for earthwork measurement for area, volume and geometric shape determination, for Waterway/drainage investigation and analysis.

2.2 Survey Requirements and methods

The survey methodology, equipment, duration and accuracy requirements needed in performing a Digital Ground Survey can vary significantly depending on the purpose of the data i.e. planning, preliminary design, final design, overlay, realignment (greenfields), full reconstruction, waterway analysis, earthwork quantities etc.

The purpose of the survey must be clearly defined in the Project Scope in order  to meet the appropriate digital ground model accuracy, degree of feature definition and coverage, and subsequently the most suitable survey methodology to achieve these criteria.

Generally, the higher the degree of accuracy (particularly vertical accuracy), feature definition and coverage, the greater the survey complexity and duration and hence the higher the survey costs.

When planning for some form of Digital Ground survey to be carried out, the following factors need to be considered.

• What is the purpose and extent of survey?
• Will the road be re-aligned?
• Does MRWA already have any existing suitable spatial data of the project area?
• Do tie ins need to be captured so the designer can extrapolate the existing road geometry and blend/tie-in adjoining roads?
• Is there a suitable Geodetic Survey Control Network in the close vicinity?
• Will additional Road Reference Mark control need to be installed for construction?
• What is the required delivery timeframe and milestones?
• What are the extents of roads, drainage and other hard surface structures to be included in the design?
• What are the extents, of roads, drainage and other hard surface structures to be located as general topography?
• Does underground, piped drainage need to be captured?
• Is a Underground Services model required? eg. To show Power cables; Gas Mains; Sewerage network etc.
• Are there any below ground services/utilities to be included in design?
• Are there any bridge or drainage structures to be captured?
• Are there railways to be captured?
• Are there significant natural watercourses contributing to the project area which need to be included in the design?
• Does the survey require access into private property and will “notice of entry” be required?
• Are there any site restrictions which could hinder survey access: ie. protected flora, fauna and waterways, quarantined areas, dense canopy/vegetation or inaccessible terrain, Aboriginal Heritage Sites, railways etc
• Are there environmental restrictions which could hinder survey measurement? ie. dense impenetrable canopy/vegetation and steep terrain.

Generally, the different types of data capture survey techniques available to MRWA are:

1. Survey using a Total Station Theodolite.
2. Survey using Real Time Kinematic (RTK) GPS or other post processed data methods.
3. Combination of techniques 1 and 2.
4. Digital Photogrammetric Mapping
5. Terrestrial Laser scanning
6. Mobile Laser Scanning

LiDar-Laser scanning from an aircraft

2.3 Data Capture methods

2.3.1 Surveys using conventional Total Station equipment

The Total Station represents the most accurate and common method of data capture available, able to achieve better than +/-20mm for horizontal position and +/-15mm for vertical. It is the only single method currently capable of achieving all the MRWA Survey & Mapping Vertical Accuracy Standards for A, B and C feature classifications (see Digital Ground Survey 67-08-43).

Applications include:

• Project areas which contain A, B or C feature classifications.
• Final design applications where positional accuracy is critical.
• Project Areas containing D and E feature classifications where dense ground cover and/or tree canopy restrict the use of RTK GPS and Aerial Photography.
• Large scale urban design applications ie. Typically road intersections, built up areas, and earthwork volume computations.
• Audit of digital photogrammetric mapping models.

2.3.2 Surveys using RTK GPS

RTK GPS surveying uses satellite technology to capture three dimensional position.
RTK GPS Accuracy tolerances are generally +/- 20 mm for horizontal and +/- 30 mm for height, provided it is operated in accordance with the MRWA quality controlled procedures and standards.

In certain situations significant cost and time reductions can be achieved by using this method relative to the total station method.

However, RTK GPS is only suitable for the acquisition of D and E feature classifications.
Variations to this standard are permissible depending upon the purpose, location and constraints of the project. All GPS queries should be directed to the Senior Geodetic Surveyor to determine whether a method of GPS surveying can be used efficiently and effectively on your project.

Applications include:

• "Greenfield" road alignment surveys.
• Waterways surveys
• Embankment and foundation surveysStockpile surveys.

2.3.3 Digital Photogrammetric Mapping

Digital Mapping created from aerial photography and satellite imagery have many applications within Main Roads and are a non obtrusive method of viewing and capturing information on the earth's surface for planimetric mapping and Digital Terrain Modelling.

Aerial Photography is generally used for mapping wide corridors (up to 2km) or large areas.

This method requires minimal entry onto land causing minimal disruption to land owners. On a per kilometre rate, for covering large areas, it is generally the cheapest method but sacrifices accuracy to some degree.

Mapping can be ideal for Waterways analysis surveys, to aid in route analysis for planning purposes, and for creating maps for preliminary and final road design.

Main Roads WA Standard for Mapping "67-08 44" details 3 standards depending on the output:

Standard A is the highest accuracy and can be suitable for final design in a greenfield situation or complete road  re-construction but is not suitable for Class A, B and C detail capture where tie in to existing hard surfaces is required.

Standard B and C - suitable for Planning and concept design.

The Senior Mapping Surveyor can assist you by providing advice on products, procedures and quotes gathered from mapping sources.

2.3.4 Terrestrial Laser Scanning

This is an emerging technology which can capture very dense datasets of information. Laser scanners are particularly useful in obtaining detailed survey information of surfaces or structures where access is difficult or dangerous or where a more complete image is required. These instruments obtain measurements without requiring physical access and are often used in conjunction with Total Stations and are capable of achieving surface accuracies better than +/- 10mm. Examples would be underside of bridges above electric rail and Freeway road surfaces.  Consideration should be made for appropriate spacing of instrument setups to ensure complete capture coverage and elimination of missing data due to obstructions.  The distance range of capture of a horizontal surface is generally less than is possible with total station – more instrument setups may be required.

2.3.5 LiDAR-airborne laser ranging

Where a large scale terrain model is required for planning or design purposes, LiDAR surveys can provide a detailed DTM of the earth's surface where accuracies of between 5cm and 15cm in height are required. The need for access on the ground is minimal so these surveys can be very useful in built up areas or where sensitive landowners exist.

2.3.6 Mobile Laser Scanning

Like Terrestrial Laser Scanning, Mobile laser Scanning is a developing technology which captures huge data sets in the form of “point clouds”. Industry continues to develop software to process this data into a format that meets Main Roads Standards and fit for purpose

Main Roads encourages the use of Mobile Scanning in certain circumstances, as it has huge advantages in minimising Traffic Management requirements and lane closures on Freeways and major highways. When using this technique, verification that the survey meets Main Roads standard for Digital Ground Survey “67-08-43” is a requirement. This may require the use of Spirit levelling and RTK or Total Station methods to provide the basis for improved accuracy.

2.4 Field Technique examples

2.4.1 Survey for complete road replacement and design and improvement along existing casement   

All above ground features and existing utility services will need to be located as well as all drainage structures or waterways which would impact on the new construction. The existing road surface would only need to be located at regular intervals and features such as line markings would not be required. A 7 point cross section across the road would be sufficient to locate the road features necessary.

The term utilities or services in this context, refers to communication, electrical, drainage and sewer, gas and water lines many of which may occur above and/or below ground. The three dimensional capture of services may be required for preliminary design purposes to determine if service relocation is required prior to construction.

Survey and Mapping Branch have developed an Underground Utilities Survey Standard to detail the technical outputs required.

2.4.2 Survey for pavement overlay

Typically, an 11 point cross section every 15 metres would be suitable (batter to batter) using survey techniques appropriate to capture Class B features (i.e. total station or appropriate scanning). A utility services limited to spanning between the batter extents would typically suffice, but a more detailed survey of the existing hard surface would be necessary including all linework.

2.4.3 Survey for passing lanes or widening:

Undertaken where the location and extent of the extra lane or need for widening has been determined. In this case a more detailed ground surface survey would be required and should extend at least 5 metres beyond the batter hinge point. Hard surfaces would require survey by Total Station or Scanning techniques with cross sections generally at 15 metre intervals along the road section, but off-road surfaces could be surveyed by RTK GPS methods. All tie-in sections would require survey by Total Station / Scanning .Only significant trees would need to be located unless it was a requirement to identify all re-useable trees(for timber, firewood etc) or vegetation of environmental significance.

2.4.4 Survey for structures(Construction Survey Standard)

Survey of existing structures is usually completed using Total Station and or Terrestrial Scanner instruments. Convenient and accurately located Survey Control is critical to these surveys.

2.4.5 Survey for Earthworks(Construction Survey Standard)

Earthwork measurement surveys (Foundation Surveys) are performed to capture the surface upon which all subsequent earthwork volume and area calculations can be based. These surveys are typically completed using Total Station methods for the best accuracy but can utilise RTK GPS up to Sub-grade level.

2.4.6 Surveys for Waterway Investigations

Waterway Investigation Surveys are required for the modelling of major stream crossings for waterway analysis and investigation which may then require construction of significant structures.

MRWA has developed a Guideline "Waterway Investigation Surveys" which provides current guidance on the recommended field practices to complete these valuable surveys.
RTK GPS techniques are generally suitable for these surveys, but some features (eg. pipe inverts) may require higher accuracy survey.

2.4.7 As constructed surveys(Compliance surveys)

These surveys are performed to accurately determine if a feature has been constructed in accordance with its design requirements. Typical features requiring this type of survey include: Alignment profile and grade of all pavement layers, Culvert Inverts, Pipe diameters and alignment, relative and absolute dimensions of structures, orientation of structures, Utility location(new services)and Earthwork quantities for payment .

2.4.8 Survey Datum

Unless otherwise directed, all Digital Ground Surveys shall be supplied in the applicable Main Roads approved project zone for that location. These grids are available from the Main Roads Senior Geodetic Surveyor or online via the Survey Portal website.

All project zones are based on the Geocentric Datum of Australia 2020 (GDA2020) and Australian Height Datum (AHD) unless otherwise specified.

2.5 Geodetic Survey Control

2.5.1 Horizontal Survey Control

Most large Digital Ground surveys will require the installation of a minimum of three (3) Road Reference Marks (RRMs) to form the basis of the Survey. This is to ensure that future survey work can be completed using the same datum and that check measurements can be made, regardless of instrumentation. Care will be required in the placement of these marks so that they will generally be safe from disturbance during the construction. Thus the proposed alignment centreline would need to be known to within 3 metres of its final position.

New survey control for Digital Ground Surveys shall be established, surveyed and lodged in accordance with the Geodetic Control Survey standard.

Minor Control can also be established on smaller projects or between RRMs on larger projects to enable Total Station surveys to proceed. Refer to the Geodetic Control Survey standard for the details, placement and accuracy required for these marks.

2.5.2 Vertical Survey Control

All vertical control for Digital Ground surveys shall be established to Third Order standard. When a Digital Ground survey is undertaken more than 10km from an established Benchmark or Road Reference Mark with Spirit levelled height value, it may be necessary to establish height on the new RRMs using GPS methodology. It will however still be necessary to 3rd Order Spirit level between the newly established RRMs on site and indicate this on the control summaries and Metadata statement. This statement will summarise the method used to establish the height and the expected absolute and relative accuracies obtained for the control. Any variation in vertical standards will need approval from the Senior Geodetic Surveyor.