Surveying 3 875 km of railroad track in South Africa
[Headnote]
Because Spoornet's historical records are often Inaccurate and incomplete, it became necesary to re-surveg its mainlines. As conventional land survey methods are more expensive and time consuming, It was decided to use a laser altimetry system, a new and Innovative technology. This article by Willem Ebersnhn. Professor in Rallway Engineering at the University of Pretoria, and P B Venter. Senior Engineer. Spooenet Infrastructure.
Spoomet allocated an international laser altimetry tender and Fugro-Inpark with its FLI-MAP, a mobile laser altimetry system, was selected to conduct the survey. At the beginning of November 1999 FLI-MAP was mobilised to South Africa where Spoornet, the South Africa railway division of Transnet, awarded Fugro-Inpark BV a major contract. This project consists of surveying 3 875 km of railway track covering all fixed assets in the right-ofway. The point density covering the assets had to be at least 10 points per m2 integrated with video images. All tools and software to process the points and video to identify assets and allocate attributes had to be provided.
The purpose is to define the accurate geographic position and attributes of all fixed assets in order to build an integrated information system to manage and maintain the fixed railway assets. The first step in establishing such a centralised maintenance management system is to identify where all assets are located. The foundation for the Spoornet infrastructure applied maintenance management (IAMM) system is a relational database with the geographic location of all fixed infrastructure as the referencing system for the database.
What is FLI-MAP?
Fast Laser Imaging and Mapping Airborne Platform (FLI-MAP) is a helicopter-based, high-accuracy airborne mapping and profiling system that can cover on average 200 km per day. The basic concept is that a helicopter flies over the corridor to be surveyed, collecting precise GPS measurements, platform attitude, laser ranges, and imagery data (figure 1).
Flying at 40-50 m altitude with a speed of 40-70 km/h, the system scans the surface area and objects directly below the helicopter at a rate of 11 000 points a second, which results in approximately 10-ZO points per M2. This high point density is required to differentiate between railway assets such as rails, kilometre posts, signals, switches, electrification wires and masts.
The FLI-MAP system integrates kinematic GPS, a reflectorless scanning laser, a solid state inertial navigation system, and digital video images into a complete remote sensing survey platform. By using advanced kinematic GPS technology, an absolute accuracy of 5-10 cm. can be achieved without compromising the environmental conditions or the necessity of permits to have access to every property. FLI-MAP is equipped with two high-resolution, broadcast-quality, digital S-VHS colour video cameras and the precise UTC time is encoded on each frame of video, providing an accurate record correlation with the laser data.
The final post-processed output, including the video from the FLI-MAP system, includes XYZ positions of the laser returns. The identification of each asset is done by recognising patterns of points with spatial relationships. Thus, the FLI-MAP system integrates the latest altimetry technology into a high-tech survey tool, which can compete with conventional survey techniques both commercially and in accuracy.
Survey operations
A normal survey flight usually consists of 5-7 base stations at known locations spaced along the flight line continuously logging GPS position during the survey flight. Survey analysis after each flight consists of checking and determining base station co-ordinates, calculating helicopter flight line and co-ordinates of surface area laser returns.
Project preparations
After signing the contract with Spoornet in Johannesburg, an agreement was made with a local helicopter firm to hire a Bell 206 Jetranger including pilot, technician and fuel support. A memorandum of understanding was signed between FugroInpark and Omega Scientific Research (Pty) Ltd, a black empowerment company in South Africa, to ensure technology transfer and training of local South African personnel. This training can be divided into the following items:
basic survey techniques introduction in GPS technology and training in GPS operations LiDAR data processing altimetry project management
* basic survey techniques
* introduction in GPS technology and training in GPS operations
* LiDAR data processing
* altimetry project management
All logistic support (hotels, transport and communications) and the employment of two armed security guards were also arranged by Omega.
At the same time, all the arrangements were made in the Netherlands to mobilise personnel and equipment to Johannesburg. Fugro's Supervisor Field Operations was send to South Africa two weeks prior to the start of the project to carry out a reconnaissance and scouting survey in collaboration with the Spoornet team. The objective of this was to locate the intended locations of the base stations and lines to be surveyed before the start of the actual survey:
To ensure that the accuracy requirements of the client are met, base stations had to be spaced at approximately 25 km so that the heli copter is never more than 10-15 km away from a base station. The reconnaissance survey ensured that the base stations were correctly spaced, taking terrain constraints and local conditions, such as hazardous surroundings, into account.
Verification of accuracy compliance
Mobilisation of the equipment, including the acceptance of the installation and flight tests by a representative of the South African Civil Aviation Authorities, took half a day. Generally time should also be allowed for clearing customs and calibrating equipment for local conditions.
To validate that accuracy requirements were met, Spoornet prepared and surveyed several concrete markers along a 100-km test section. This section had to be surveyed using the FLI-MAP system before starting the altimetry survey. The differences between the static and FLI-MAP determined positions of the targets had to comply with
Spoornet's specifications: relative accuracy per scan (50 mm) and flight (70 mm) as well as the absolute accuracy between flights (150 mm).
Table I presents the results of the comparison between the FLI-MAPdetermined positions and the co-ordinates verified by the survey division of Spoornet. The co-ordinates the in table are presented in South African Lo-system (zone 29).
The accuracy of the FLI-MAP system was accepted by Spoornet and on 4 November 1999 the actual survey of the 3 875 km started.
Daily survey operations
The data acquisition was done by two to three flight passes per day, depending on environmental conditions. Before the start of the survey, base stations were set-up on top of the 'scouted' trig-beacons (figure 2). All base stations start recording their data at a predetermined time, once all have reported their readiness to the field operation manager. During the execution of the project the accuracy of the FLI-MAP survey was monitored by flying over a base station each flight session. With the high point density of FLI-MAP, several reflections will be registered on top of the base station. Because of the accurately known position and height of the base station, the accuracy and reliability of the gathered data can be checked and verified after each flight.
As high point density is one of the requirements to be able to locate all assets, the helicopter flew at an altitude of approximately 50 rn above ground level at a speed of 60 km/h, which provided a point density of 13 to 15 per m2. This density is sufficient to clearly distinguish smaller assets such as rails. The project was completed in 25 flying and 15 transport and processing days. A total of 128 base stations were used.
Quality control
As soon as the helicopter flights were completed, the collected data from all the base stations and the helicopter were delivered to the hotel where the data processor had set up two complete processing suites. Obviously the first action of the processing team was to back up all laser files and video tapes. Once the data was backed up the information was pre-processed. Pre-processing consisted 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
point density verification by taking a 10x10 mz section randomness in the FLI-MAP data and calculating the number of laser points
comparison of the calculated heights of the base stations with the actual published co-ordinates, which ensured that the accuracy requirements were met
As all these checks needed to be completed before the next section of the survey could start, it was not unusual for the processing team to work till very late at night and in the event of problems with the data, for the survey programme of the following day to be adjusted.
Data deliverables and ortho-rectified images
Because Spoornet personnel have knowledge of the railway infrastructure in South Africa, it was decided to do the final processing in-house. As part of the contract, Fugro-Inpark provided the client with three processing systems, including the FLI-MAP processing software, and special training on the processing.
Using the geographically referenced drawing objects created for all railway assets and the FLIP7 software, the linear distance along a base track and the perpendicular offset from this track can be calculated. This provides the dual referencing system information for each asset. The asset location and attribute information is then exported to the infrastructure database. For the delivery of ortho-rectified images a special recently added feature of the FLIP7 processing software is used. This feature captures a frame of the downward video at the desired location, and the synchronisation of the video frames with the laser data ensures the correct frame is selected. 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 orthorectified image. This not only provides a better view for the operator on the video images and thus optimises the recognition of objects and attributes, but also 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 of generating automatic seamless mosaics of rectified video images along the line of flight.
This process can be highly optimised by digitising the video images in MPEG I or 2 format and storing these digital images on hard disk, CD or DVD. The big advantage of this approach is direct and instantaneous access to every video frame without waiting for the video recorder to wind the tape to the desired location. Besides saving time it also preserves the original video tapes from damage due to excessive use.
Conclusions
The barriers of traditional techniques for corridor mapping have disappeared now that FLI-MAP can provide a method to survey long corridors by collecting remotely sensed data in a precise, reliable, cost-effective and quick way without needing to physically occupy them.
With the experience gained on the Spoornet Project from the FLI-MAP system, it is now possible to survey railway lines and take an inventory of all the infrastructure components in the right-of-way in at least half the time taken before. An additional advantage is that the survey provides an accurate visualised electronic asbuild record of the right-of-way with considerable reduction in cost and with no disruption of traffic.
The high point density provides more detailed infrastructure asset component dentification and makes the FLI-MAP survey the first important step in setting up a maintenance management system.
The extracted information can be used for asset audits (inventory), depreciation, condition management and maintenance budgeting. In addition, the data of the FLI-MAP survey lends itself well to engineering applications such as planning, design, construction and operational control of train movement.
中文翻译
在南非测量3875 m的铁路轨道
引言：
因为南非铁路以前的历史纪录不是很完全并且不是很精确，所以重新进行测量这条路的主线是很必要的。因为普通的路线测量太耗费财力并且太浪费时间，所以决定采用一种新创新的技术—激光测量系统。这篇文章是由皮瑞尔大学的高级工程师瓦利埃博森教授所作。
正文
这条铁路起用了激光测高法和利用FLI地图进行环测的方法，一个流动激光导航系统用于这次测量。在1999年11月上旬开始在南非进行FLI地图的测定，对南非的铁路进行区分，政府与福格公司签订了一份重要的合约。这个工程包括测量3875m长的铁路线，这几乎包括了南非铁路公司的所有固定资产。测量的点密度必须达到图像处理要求的每米至少10个点，福格公司必须要提供为了识别点和录像带以便与识别资产并对财产的属性进行标定所需要的工具和软件。
这次测量的主要目的是要确定所有的铁路固定资产的位置和属性并且建立一个完整的数据库系统以便于对铁路资产进行保护。建立的第一步是建立一个可以随时调阅各部分财产位置的财产维护管理系统。这样做的基础是管理系统内部的应用管理系统（IAMM）是由各部分的地理位置来表示关系的数据库参考系统组成的。
FLI—地图是什么？
快速激光成像和成图飞机空降平台（FLI-MAP）是一个直升机基地，高精度的成图和成像系统能在一天之内完成200km的任务。基本的概念就是一架飞机飞过所需要测量路线并同时利用全球定位系统测量和收集空间数据，空间角度，测量范围和属性数据。
飞机以40-70km/h的速度在40-50m的高度飞行，系统将以11000点每秒的比率对直升机下面的区域和物体进行扫描，大约每平方米10个点。这样高的点密度主要是为了区分铁路标示，如铁轨，栏杆，里程碑，信号灯，电线和天线等铁路资产之间的分别。
FLI-地图系统把全球定位系统，激光扫描系统，固定路线的惯性导航系统和数据录像技术结合到一个技术平台下面来。采用高级的全球定位系统，精度可以达到5-10cm。这样在不考虑环境和不需要接近目标的情况下就对所有的铁路财产进行掌握的目的就可以轻易达到了。FLI-地图技术要求的装备有两个高度固定点，高质量的微波传输系统，数码相机，并且把精确的UTC时间附加在数据上以便使所提供的数据更加精确。
最后的处理输出, 包括来自 FLI- 地图系统的录像带,包括激光返回的 XYZ 位置。通过各个点之间的空间关系可以确认各个财产的信息。因此，FLI地图系统把最近的测高法技术结合到高科技的测量工具中来，在精度和商业性两方面向传统的测量提出了挑战。
测量操作
在进行测量期间一次正常的飞行一般是通过在5-7个基础站之间不断地沿着飞行路线通过全球定位系统进行测量。在每次通过基础站的飞行进行测量并且决定了车站的共纵线之后，通过计算，即可得到直升飞机飞行线路和激光测量区域表面的共纵线。
工程准备
在签约以后，从一个地方性的直升飞机公司雇请了飞行员，技术人员和租了一架满油的比尔206飞机。亚米茄公司和弗仪柯公司签订了一个协议。亚米茄公司负责南非地方人员的技术训练。训练主要分为下列训练科目：
全球定位系统基本测量技术的训练和全球定位系统在LIDAR测高法数据处理中的应用。
*基本测量技术
*全球定位系统技术和全球定位系统操作的介绍
* LiDAR 数据处理
* 测高法工程管理
所有的后勤人员（服务，运输和联系）和两个武装的保安人员的雇佣都由亚米茄公司来安排。
同时，所有的仪器和人事安排被确定了下来。弗仪柯在开始计划之前，派出了技术人员去南非和当地的人员进行合作进行了为期两周的侦察和大体调查工作。目的是为了使所经过的车站和路线进行大致的了解以便于在真正测量之前了解大致的情况。
为了保证精度，基础站必须相隔大约25km以使直升飞机飞行不超过10-15km。一定要确定车站位置的正确性，考虑到环境的限制和每个地方的具体情况，例如：危险的环境就要在考虑的范围之内。
精度检验
仪器的检验，包括对装置的验收和通过南非航空局的飞行测试，这花费了一天半时间。平常的时间要对仪器刻度进行改正和扫清关税的问题。
为了使精确度达到要求，公司沿着100km的区域准备并且测量了许多钢筋混凝土标记。这个区域，这个区域必须在整个测量之前进行FLI地图测量，在基本点和FLI地图控制目标之间的差异必须被考虑到。
测量规格：相对精度每扫描50mm ,70mm,绝对精度每扫描150mm
确定出在FLI地图确定点和扫描纵线确定点比较的结果，共纵线在已知的计算结果中是已知的。
FLI-地图系统的精确性经过检验之后，真正的调查从 1999年11月4日开始了。
每日工作：
经过两到三天进行一次航空测量工作，这完全取决于天气的情况。在开始测量之前，基础站开始进行信号处理。一旦信号稳定，所有的基础站在一个预定的时间开始数据的纪录。在测量进行期间，FLI地图的精度通过在每一个测段之间进行不断的检测来保证。为了保证点的密度，在基础站附近要进行一定的映射测量。因为基础站的位置和高度都是已知的，这样被测量的数据的准确度和可信度在每次测量之后就可以得到检验。
之所以要求点的密度较高，是为了确定所有资产的信息，直升飞机以60km/h的速度在离地面50m高的高度上飞行，这样点密度大约就是13-15点/m2。这样才可保证测图的清楚程度，例如栏杆等较小的铁路资产才可以被测量到。这次工程计划飞行25次，并同时进行数据传送以保证每天可以进行数据的处理。使用了128个基础站。
质量保证
直升飞机飞行完之后，从各个基础站采来的数据两个飞机不同的数据被同时送到旅馆以便与进行比较和监测。很明显，处理的第一部分主要就是所有的激光扫描文件和录像带。返回的信息要进行预先的处理，预先的处理包括如下的措施以监测测量数据的最后精度：
参加飞行的飞机排成一行以保证覆盖率；
激光和录制图像的精度；
计算在FLI地图的任意部分的10*10m的面积上激光点的个数；
把共纵线的高度与基础站有目的的进行高度比较，以保证准确性；
所有的上述工作都做完之后才能开始下一步的测量工作，为了保证第二天的工作能够正常进行，每天都要工作到深夜把所有的数据都处理完，并且处理所有出现的问题，这样做很不容易。
数据的传送和图像的传输
因为南非铁路局对所有的南非铁路的结构都很清楚，所以有他们来做最后的数据处理工作。
按和约上所规定的，弗仪柯提供三套处理系统，包括FLI地图处理系统，和对人员的特别训练等。
根据地理参考书在FLI地图软件上画出所有的关于铁路财产的地物。所画的线离铁路的距离和相对高度来自于预先处理的数据。这样就为铁路资产的判定提供了双重的参考数据。资产的位置和属性数据被输入到系统内部的数据库。采用了一个新增加的FLI地图处理软件进行数据图像的修正。在捕获一个特定位置的图形数据的下一格和激光数据进行比较，然后这个被捕获的图像经过激光数据在距离高度方面的修正，成为一个参考图像。这样就为数据的操作人员提供了较好的图像参考，不但使得物体的属性容易被确认，而且使操作人员对物体的位置和大小的判定提供帮助，因为图像的象素数比激光点的密度高。软件不但可以进行图像的修订，而且可以提供对由于在飞行中产生的马赛克现象的处理，可以达到天衣无缝的程度。
这个程序可被用于MPEG1 和MPEG2 格式的图像和在硬盘上储存的数字图像，如CD和DVD。它的最大优点是不需要等待它们传输到放映机之前就可以对录像带的每一个栅格进行格式的转换，除了节省时间以外还可以避免由于过的使用而使录像带遭到损坏。
结论
由于FLI地图系统的出现，以前的关于测量路线地图的困难现在已经消失了，FLI地图可以通过收集影像数据测量一个走廊形的地带，精度更高，花费更少，数据更可靠，并且不需要直接接触。
利用从南非铁路使用FLI地图系统的这个工程得到的经验，现在测量铁路线路并且同时得到附近地物的信息只需要花过去时间的一半。另外一个好处就是可以得到一个精确的电子数据，花费少并且不阻断交通。
在一个维护管理系统上设定好点的密度并且进行对各种财产成分的分析使进行FLI地图调查的第一步。
测量得出的数据可被用于财产调查，资产折旧，资产管理和进行预算。除此之外，FLI地图的测量数据还可用于各种工程，例如，进行设计，计划，建造和进行火车的调度。
我也需要， 谢谢各位了。
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