Experiences With Kinematic GPS Surveys in Developing Countries Dr Christopher Bennett email@example.com
An Introduction • Consultant to World Bank • Preparing Terms of Reference • Executing Surveys • Developer of the ROMDAS road measurement system • Used for kinematic GPS and other data collection • 130+ systems in over 30 developed and developing countries
Overview • A road engineer’s view of centreline surveys • Some experiences • Lao PDR • Samoa
Role of GPS? • Interface with data • GIS the most intuitive method • Referencing the base network • GPS vs Linear • Referencing data • GPS not suitable for all attributes
Interfacing With Data • GIS offers the most intuitive way of interfacing with data • Layers can be used which reflect the desired level of detail
Referencing the Road Network • Historically roads referenced linearly • In adopting GPS must provide link to existing data • Use of GPS for referencing not practical for many types of data • Need to carefully consider field procedures and staff capabilities
For Each Data Item Must Consider: • PRECISION • Positional error using different technologies • ACCURACY • The tolerance for the required position • RESOLUTION • Level of detail • EXTENT • Physical characteristics in terms of length, breadth and depth
NZ Example • Level 1: (+/- 3 m) • Reference stations • Level 2: (+/- 5 m) • Other referencing (section start) • High speed data (roughness, texture, skid) • Traffic facilities • Level 3: (+/- 10 m) • Visual condition • Signs, roadmarkings • Structures • Accidents
Not Every Item Should Be Referenced With GPS • General rule: Only those data which are most suited to spatial referencing should be spatially referenced • Examples of spatially referenced data: • Reference Stations and other key referencing features • The road centreline • Off road objects, such as signs, which cannot be referenced using linear system
Remote Sensing vs Driving? • Each method have a role • For developing countries driving preferable because: • Often do not know which roads belong to the agency • Can collect additional data at same time • Can easily locate intermediate referencing markers (eg km markers)
Summary • For most countries linear referencing will continue to be used by road engineers • Need to adopt GPS technology to enhance road management • Cannot underestimate: • Effort required to collect the data • Effort required to maintain the data • Effort required to use the data
Opportunities and Challenges of Centreline Surveys In Developing Countries
The Situation • Developing countries often do not have a proper record of their roads • Maps are usually incomplete and out of date (if available at all!) • Centreline surveys provide this information as well as other key data
Technology • Depends upon accuracy requirements • Users have done surveys have been done with: • Garmin 12 channel consumer • Trimble Pro-XRS • Pro-XRS with Base Station • Pro-XRS with RTCM • OmniStar
Accuracy? • Comparison of Garmin GPS distance with DMI distance from Lao PDR
Inertial Navigation • Gyroscopes are useful when loss of satellites • Typically can lose 5-20% of your data but depends on location
Lao Peoples Democratic Republic • Laos the Country • South East Asia • 236,800 sq km • Pop. ~5.5 Million • National & Provincial Roads • 15,600 km • 3,600 paved
The Project • Component of WB Third Highway Improvement Project • Survey of All National and Provincial Roads including: • One location reference point survey (LRS) • Road roughness survey (IRI) • Visual assessment of surface integrity (SII) • Inventory (surface type, width, bridges) • Digital photos of the start of links • GPS record of the road centreline • Contract Sum US$208,000
Used Garmin - Appropriate Accuracy for Project • Two hour 100% within ±22m, 85% within ±10m withoutdifferential correction • Extreme terrain impaired satellite availability (PDOP) for < 5-10% of survey • No gyroscope or differential correction obtained maps for 100% of network • 95% spatial to ±20m • 5% spatial to ±20-300m
Appropriate Accuracy 0m 100m • System recorded point every second. • Post-processed to one point approximately every 10m Same Road surveyed by different teams 3 weeks apart
The Survey Teams • Four locally staffed survey teams • Completed 14,000 km in 8 weeks • Ex-pat training • 3 staff per vehicle • Driver • Computer Operator • Condition Assessor • (plus up to 4 observers) • “Support” vehicles in Special Zones
Not all Plain Sailing Roads often became impassable with rain Time-out on one of the “support” vehicles used through the Special Zones
Unexpected Delays: the Photo that Carried a 2-day Sentence... Vietnam Border Post
Outcomes of Project • Using low-cost technology a centreline was established for the entire country • Additional data on pavement type and condition collected at the same time • Local teams used for data collection - successful transfer of technology • Data now in use by consultants, Ministry and World Bank.
Samoa • South Pacific • 5,000 sq km • Pop. ~ 80,000 • ~ 850 km of roads
The Project • Part of Samoa Asset Management System project • Funded by World Bank • Centreline survey conducted to: • establish the extent of the network • identify nodes and location reference points • create videolog of roads • Contract $USD 20,000
Equipment • Used Trimble Pro-XRS • Differentially corrected using base station established for project • Data overlaid onto aerial photographs
The Problem • Aerial photos and centreline data didn’t match!
Systematic Difference • Data shifted 13 m E/W; 4 m N/S
Possible Sources of Error • ROMDAS Data Processing: • There was an error in the processing of the ROMDAS data. • Eliminated by testing process against data collected and aerial photos for city in NZ • Position of Base Station: • There was an error in the position of the base station. • Took average position over 12 days, measurements within 1 m
Likely Source of Error... • Projections: • The two sets of data were prepared using different projection parameters when converting to WSIG. • Aerial Photograph Rectification: • There was an error in the rectification of the aerial photographs.
Implications • Trimble Pathfinder Office gave closest results for projections • Appears that aerial photos were incorrectly projected or rectified • Subsequent investigations showed that the national grid has errors up to +100 m • Solution (temporary!): processed data using Trimble, moving position of base station by 4/13 m.
Conclusions • Centreline surveys are an important management tool for road agencies in developing countries • Low cost technology makes it possible for any country to collect the data • Need to come up with improved procedures for projections and reconciling with existing data sources