AERIAL SURVEY OF FIX ASSETS IN THE RIGHT-OF WAY
Huug HAASNOOT, The Netherlands
Key words: LiDAR, Corridor Mapping, 3D-geometry,
Ortho-rectified, geo-referenced video images, Automatic Filtering
Routines, Synchronised video imagery
1.
Abstract
High density airborne LiDAR, being an innovation in remote sensing
techniques, has now taken away the barriers of the traditional survey
techniques. Especially for long corridors like roads, railway and
electricity lines, laser alitimetry provides a new survey method to
collect remotely sensed data in a quick way. Particularly the LiDAR
systems which operate with the latest technology at low altitude
(50-70 m) and low speed (50 km/hour) present a professional survey
tool that can compete with traditional survey methods as it is precise
(5-8cm absolute X,Y,Z accuracy), reliable, fast (150 km per day) and
cost-effective.
The advantages of aerial surveys are twofold: obviously the safety
aspect is very important as surveyors are not required to physically
occupy the survey area and secondly for the transportation industries
it is essential that the survey will not interrupt the schedule of
trains / road transport. Further aerial surveys are not compromising
the environmental conditions, nor any permits are required to have
access to every property.
Fugro, a multinational on the Dutch stockmarket with more than 200
offices worldwide, has developed the Fast Laser Imaging and Mapping
Airborne Platform (FLI-MAP®) system in 1996, fully based on the
reason that long corridors do not lend themselves well to conventional
land survey techniques. Since 1996 this new high-detail aerial survey
method has been used successfully world-wide in more than 150
projects.
The high point density (10-20 points per m2), high accurate data of
the FLI-MAP system enables the surveyor to map all assets along
corridor like railway tracks and powerlines to details of signals,
tracks, junction boxes, sag of conductor lines, the location and
height of existing buildings, etc. This information can also play an
important role in the determination of the optimum route for a new
line.
A dedicated processing package supplies filter algorithms and CAD
functionality "on top of" the laser data as well as fully
synchronised video imagery, providing additional capabilities to the
operator in extracting valuable information from the LiDAR data. This
extracted information can easily be incorporated in external specific
GIS or CAD software packages. Recently software development has been
concentrated on automatic filtering routines and the possibility to
generate otho-rectified, geo-referenced images.
Bell 206 Helicopter with FLI-MAP® system
As of August 1999, Fugro-Inpark operates the third FLI-MAP system
from the Netherlands to execute projects in the Eastern Hemisphere.
During the last 1.5 years projects have been completed in various
industries with a variety of endproducts, e.g. detailed Digital
Terrain Models with more than 10 points per m2 for flood control in
the Netherlands, planning of new roads for the expansion of tourism on
French Polynesia, high-accuracy cross-profiles of the Expressway near
Kuala Lumpur, planning of noise protection walls for the German
railroad, coast line protection survey in Portugal, etc.
2. TECHNICAL INTRODUCTION
The integration of numerous latest innovations such as the high
frequency scanning laser, the solid state Inertial Navigation System,
kinematic GPS technology, automatic filter routines and mosaic of
digital ortho-rectified video images has provided numerous industries
a new survey tool and a near continuous 3D-geometry coverage of
terrain features. This, in combination with dedicated software, has
resulted in an outstanding tool to inventorise corridors for the
railway, pipeline, electricity, and tele-communication industry.
Installed on a helicopter, the FLI-MAP system is navigated at a low
altitude along the corridor collecting GPS, scanning laser, helicopter
attitude, and video information. The helicopter gives the system a
unique perspective to efficiently gather sufficient data to determine
characteristics such as powerlines attachment points, sag of
conductors, signals, junction boxes in the railroad industry and land
use to aid in the engineering process without occupying the actual ROW
or touching the conductor.
FLI-MAP's scanning laser is a custom designed eye safe,
reflectorless rangefinder capable of measuring first return ranges
from 20 to 200 meters. Every scan has a width of 60 degrees and
contains 200 range measurements. Each scan record contains timing,
laser attitude, and intensity of the reflection and data
verification/error detection information.
The FLI-MAP helicopter with the two eye-safe lasers
Operationally, the laser scans at a rate of 55 times per second and
has a coverage width that is 1.15 times the helicopter's altitude
above ground level. A military classified Solid State Inertial
Navigation System (INS) measures heading, pitch, roll information as
well as 3D velocities, maximum 200 times per second.
By matching the GPS derived position with the INS data, a position
of the laser is determined for every scan. This information, with the
scanned ranges, produces the XYZ positions of the laser returns on the
terrain along the flight path.
FLI-MAP can be configured to operate with two scanning lasers
simultaneously. The primary laser is tilted 7 degrees forward to
ensure enough reflections will be collected on the front of buildings.
To ensure that also the rear of objects are surveyed, the secondary
laser is tilted 7 degrees backwards.
Two colour S-VHS video cameras, one downward looking and one at 45
degrees forward looking, are collecting digital video images of the
survey corridor. In addition to being used to show terrain conditions
and identify objects, the S-VHS video can provide captured digital
images which can be used in data processing.
Flying at 20-225m attitude with a speed of 50-70 km/hr the FLI-MAP
system makes between 11,000 to 22,000 range measurements per second,
resulting on average to 10-15 points per m˛. This high point density
allows accurate and reliable position and height of all objects in the
corridor.
FLI-MAP® equipment inside helicopter
Based on those numbers the system collects on average between
1,000,000 and 2,500,000 3D data points per kilometer of Right Of Way.
This volume of data can be handled and manipulated by Fugro's FLIP7™
software which allows an operator to easily access the information
that is required. This package has been developed for processing
airborne LiDAR data and has tools designed to streamline the
extraction and formatting of specific information such as conductor
clearances, centerline profiles, rails positions, etc. FLIP7 is used
in the process of preparing traditional deliverables such as plan and
profile sheets and digital input files for engineering design software
packages. In addition, the software gives the engineer the ability to
confirm measurements or conditions at his or her desktop, without the
need to send survey crews into the field.
3. FLI-MAP SURVEY OPERATIONS
Reconnaissance
Four to five reference locations (every + 20-30 km) are planned
each flight session which will guarantee the necessary accuracy,
quality control and back-up in case of problems on a base station,
assuring that the distance between the helicopter and a base station
never exceeds 10-15 km.
The reconnaissance survey ensures that the base stations are
correctly spaced taking terrain constraints and local conditions, like
hazardous surroundings, into account.
During the preparation phase also local helicopter companies are
visited to check availablility and prices. The FLI-MAP system is
worldwide certified by the American Federal Aviation Authorities on 5
different type of helicopters: Bell Jetranger, Bell Longranger
McDonnell-Douglas 500, Eurocopter AS350 (A-star) and Eurocopter AS355
(Twinstar). Independent of the country (e.g. South Africa, Portugal,
Germany), the mobilization of a helicopter has taken a maximum of half
a day.
GPS planning software is used to determine the time schedule for
the flight sessions. Those survey windows are generated such that at
least 6 GPS satellites can be observed at the same time. This is
essential for an accurate and reliable positioning of the helicopter
and thus the co-ordinates of the laser points.
Operations
The total FLI-MAP system is mobilised on a Bell 206 Jetranger
within 4-5 hours. After a project briefing and specific instructions
to the pilot, the survey can be started in the afternoon of the first
day.
Fugro Omnistar® satellite receivers are used to collect standard
RTCM-104 differential corrections, which provide a sufficient accuracy
for real-time navigation to guide the pilot along predetermined survey
flight lines or to waypoints. The pilot and operator can view both the
down-looking and forward-looking video on a flat screen colour
display, while the operator has a specially designed interface for
control and status monitoring of the system during operations.
The Fugro crew consists of a Pilot, a System Operator, a Supervisor
Field Operations, two Data Processors and a field crew of on average
four to man the base stations.
An average of two to three flights are completed per day, each
lasting + 2-2˝ hours. During each repose period for the pilot and
operator, the field crew travels approx. 100km to the next reference
locations along the corridor.
As of autumn 1999 more than 13,000 km has been surveyed in various
countries and industries of which below a summary is included:
- Mapping of 1000 km high voltage electricity lines for Nuon, the
largest Electricity Company in the Netherlands. The survey was
completed within 4˝ days and after 4 weeks the processing was
finalized with the delivery of almost 4000 mast and sub-stations
X,Y,Z- co-ordinates (incl. attributes like mast and line number)
and the longitudinal profile.
- 600 km of flood defenses in the Netherlands have been surveyed
at the beginning of October for the Water Management Authorities.
The goal of this project was to obtain a high-density (more than 9
points per m˛), high-precision digital terrain model for analysis
of the flood defenses.
- In South Africa the FLI-MAP system has surveyed 3875 km of
railroad track for Spoornet, the South Africa railroad division of
Transnet.
- A total of 700 km has been surveyed in Portugal. The project
targeted a range of industries with various small missions
throughout the whole country, like new planned roads and
electricity lines, coastlines and existing electricity lines.
- Detailed topographic survey of 220 km of a 36" and 24"
oil production pipeline in the Netherlands. The objective of the
survey was to locate all objects in the ROW and to obtain a high
accuracy longitudinal profile of the ground level above the
pipeline. This in combination with a detailed (internal) pipeline
survey using an intelligent scout scan, resulted in a very
accurate cover determination.
- DTMs have been provided to the cities of Nijmegen and Leeuwarden,
The Netherlands, for areas of new house developments.
- 105 km of route corridor has been surveyed in Germany to provide
the German Railroad Authorities with a precise DTM, which will be
used to plan the most optimum route for a high-speed railroad line
between Frankfurt and Mannheim.
- 30 km of highway 4 in Belgium has been surveyed to provide
detailed geographic information on road surface, traffic signs
etc. The goal of this project was to obtain a high-density (more
than 10 points per m˛), high-precision digital terrain model.
- A Condition, Asset Inventory and Maintenance Survey of 2700 km
of railroad on behalf of Railtrack Scotland.
- Recently corridors for new to be developed roads have been
measured in Malaysia and French Polynesia with the FLI-MAP system.
- More than 1900 km has been surveyed along both riversides of the
Rhine valley to determine the optimum position for noise
protection walls.
Quality Control
Daily, the GPS data is downloaded from all base stations and the
two GPS receivers on the helicopter. This data is processed into
accurate positions of the laser returns and reviewed during night to
assure full coverage and required quality.
Pre-processing comprises of the following control measurements to
check the final quality of the surveyed data:
- the overlap of joining flight lines and coverage of
right-of-way;
- the quality of the laser points and video images;
- the point density verification is done by taking a 10x10m2
section randomness in the FLI-MAP® data and calculate the number
of laser points;
- during each flight session, the helicopter will pass over at
least one of the base stations. The calculated heights of the base
stations will be compared with the actual published co-ordinates
which will ensure that the accuracy requirements are met;
Set-up of computer and video recorders in a hotel
room in South Africa during pre-processing operations
4. DATA DELIVERABLES AND ORTHO-RECTIFIED IMAGES
General
The FLIP7 software package is used to process the billions of X,Y,Z-points,
into various standard endproducts:
- Various Digital Terrain Models
- X,Y,Z raw data
- X,Y,Z filtered data (e.g. ground or buildings dataset)
- Cross and Longitudinal Profiles
- Contour lines
- Digital video images fully integrated with the 3D laser points
- Volume calculations
- GIS Database
- Object inventory (polylines /points) including attribute
information
- Intensity figures for classification of the terrain
- Exchange of data with other CAD/GIS packages
FLI-MAP® video capture of forward oblique
perspective
FLIP7 is a Microsoft Windows NT 4.0 true 32 bit Windows application
that merges the GPS position and INS attitude information with the
LiDAR sensor data and video imagery. The package provides full CAD
(Computer Aided Drafting) capabilities "on top of" the LiDAR
data, providing additional intelligence to the CAD drawing.
Each layer contains drawing objects, which include points,
polylines, and "groups", or collections of points and
polylines. Every class of object can have its own user-defined set of
unlimited attributes. Each attribute defined can have an unlimited
number of pre-defined values. When the user extracts a point on a
given layer, and has a user defined flag set, a dialog box appears
with a data base entry table to be filled out. The fields have the
predefined values available to be selected. A default value for each
attribute can also be defined, as well as simply editing the field
with a new value. This data base entry scheme greatly increases the
reliability of the data captured by reducing simple mistakes like
spelling and redundant naming of attributes.
Final CAD drawing of a transmission line
Ortho-rectified, geo-referenced video images
FLIP7, in conjunction with the Windows' application, DVController,
controls special time coded MPEG video's allowing the user to
co-ordinate the video images with the processed LiDAR data to get a
multi-media presentation of the surveyed area.
This new development within FLIP7 is capable of capturing frame(s)
of the downward video at the desired location The pixels of the
captured image are then corrected for position, heading and height and
'fused' with the laser data to present the image as a geo-referenced
ortho-rectified image. This provides not only better view for the
operator on the video images and thus optimises the recognition of
objects and attributes, also it enables the operator to correctly
determine the position of small objects as the resolution of the video
images is higher than the laser point density.
Not only single images can be ortho-rectified this way, the
software also offers the possibility to generate automatic seamless
mosaics of rectified video images along the line of flight.
Example of FLI-MAP laser data fused with a mosaic
of ortho-rectified, geo-referenced video images
Automatic filtering routines
To enable automatic classification and feature extraction, the
FLIP7 Processing Package has been expanded with various filtering
routines. The filters can be developed as fairly simple modules for
specific tasks (e.g. find ground points, or calculate average height),
but the ability of filters to communicate with each other also makes
it possible to build complex algorithms. Such filters have already
been successfully used to automate the classification of terrain
features and, in so doing, have reduced the task of manual
classification and interpretation from several weeks to only days.
Over 25 filters are currently available, the most important of which
include:
- A sophisticated filter to determine ground level and non-ground
level points
- Depending on data intensity, stripes, arrows and other road
markings can be automatically determined
- A filter to visualise break lines in terrain
- A 3D alignment filter for automatically detecting danger areas
(e.g. where points are too close to a power line)
Combination of ground filter (green points) and
filter that detects areas of danger close to a power line. Yellow
points that identify trees at a safe distance (approx. 10m) change to
red when they are within 2-3m of the power line.
One result of this is that automatic filtering routines that
operate independently of FLIP7 can now be written by third parties.
This will be aided by a Visual C++ Filter Wizard to be included in a
software development kit.
5. CONCLUSION
More and more clients realise that the barriers of the traditional
techniques for corridor mapping have disappeared now that aerial laser
altimetry can provide a unique method to survey long corridors by
collecting remotely sensed data in a precise, reliable, cost-effective
and quick way without physically occupying the Right Of Ways. The
laser imaging and ranging data, integrated with the information from
kinematic GPS techniques, INS systems and digital ortho-rectified
video images, provide clients a fast, save and cost-efficient
alternative for traditional surveys.
The accuracy of the advanced LiDAR surveys is comparable to the
conventional land survey methods and the qualitative and quantitative
nature of the FLI-MAP data has allowed engineers to analyse drainage
conditions, measure clearances between overhead power lines, or model
any area along the survey corridor.
Cross Profile of Electricity Line
BIOGRAPHICAL NOTE
Huug Haasnoot has more than 12 years experience in the
survey industry after graduating in 1988 from the Technical University
of Delft, The Netherlands, with a MSc. in Geodesy.
He is currently working for Fugro-Inpark B.V. in the Netherlands and
as Director of the division FLI-MAP® responsible for all corridor
mapping projects with the laser altimetry system FLI-MAP® in the
Eastern Hemisphere.
Fugro-Inpark B.V. is an engineering and consultancy company, offering
expertise and project support in the areas of geographic information
systems, and their applications to photogrammetry, subterranean
infrastructure, telecommunications, railroad industry, electricity
utility industry and civil engineering.
CONTACT
Huug Haasnoot
Director
Fugro-Inpark B.V.
Division FLI-MAP
P.O. Box 3000
2260 DA Leidschendam
THE NETHERLANDS
Tel. + 31 70 31 70 700
Fax + 31 70 31 70 750
Email: h.haasnoot@fugro-inpark.nl
15 April 2001
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