Use of Digital Sensors in the fields of Transportation, Power and Telecom



Reida Elwannas
IIC Technologies,Ellicott City

Abstract
Digital Cameras and LiDAR have become widely used technologies during the past few years. Selected-IIC Technologies have used them successfully to support industries such as Transportation, Energy and Telecommunication.

Selected-IIC used the Digital Module Camera (DMC camera) for a 1”=50’ scale Transportation Project where the assessed accuracy of the final product met the NMAS.

LiDAR along with the Digital Camera were both used to fulfill the needs of a Power project. Selected-IIC used the ADS40 and ALS50 sensors to develop databases of DEM, Wires, Vegetation, Orthos…etc.

The ADS40 and IKONOS sensors were good solutions for Telecom projects in Europe and Africa. The ADS40 was used in Europe whereas IKONOS was used in Africa where flying is restricted.

Introduction
After the migration of the entire Map Production Workflow to the digital world (Softcopy Photogrammetry), the data acquisition (Aerial Photography) was the last analog step that was left out in the map making process.

In the mid 90’s, the ALTM technology was introduced and is now widely used for many applications worldwide. Few years ago, the digital camera was introduced and has been successfully used.

This paper discusses the experience that Selected-IIC has gained by using digital sensors in several projects in the fields of Transportation, Power and Telecom.

1- Digital Sensor for Transportation
Transportation engineers rely on very accurate Geospatial data for all their planning and design activities. This type of activities require very accurate mapping as a base for the decision making process. The mapping scales can range from 1”=50’ to 1”=10’. Up until recently, these projects were executed using conventional cameras flown onboard fixed-wing aircrafts or helicopters (Depending on the type of application). When digital sensors were introduced to the market (ADS40 primarily in addition to other small format cameras), it was difficult to accommodate projects of this nature because:
  • The resolution was too coarse at that time of the ADS40.
  • Small format cameras (Frame Based Primarily) would require additional flight lines which will impact the cost and schedule.
Upon introduction of the DMC to the market, it became interesting to try the limitations of this camera in the field of transportation.

The DMC was used in a transportation corridor project with a length of 4 Miles and a width of 1800 Feet. The project consisted of mapping the entire corridor at the scale of 1”=50’.The choice of the DMC camera was made based on 3 main reasons:
  • This project was a high priority therefore quick turnaround was a must. The DMC allowed us to bypass the scanning process.
  • The DMC was (at that time) the only large swath camera capable of flying at 2100 feet AMT. The resulting pixel resolution was 12 microns.
  • The aircraft with the DMC onboard was being used for a different project in the vicinity of this project. This helped minimize the mobilization cost.
There were 5 phases in this project:

1.1. Aerial Data Acquisition:
The overlap was 60% while the sidelap was 30%. The total number of flight lines was 3. ABGPS was used in this project to minimize the number of GCP’s and expedite the project. The advantage of the DMC is that even if we had issues with the AGBPS, we could still collect extra GCP’s and proceed with the Aerial Triangulation. The data was acquired at a flying altitude of 2100 Feet. The pixel resolution of the final panchromatic imagery was 2 inches (5cm). 114 frames were collected during this project.

1.2. Ground Control Point Collection:
The GCP’s were a combination of panels and Photo-ID’s. There was a total of 12 control points used in this project and 3 checkpoints.

1.3. Aerial Triangulation:
The aerial triangulation task was performed using the SSK software. The pass and tie points were measured using a semi-automated approach. The bundle block adjustment was performed using Pat-B software and led to an absolute root mean square error (RMSE) of about 0.12 feet in XYZ (The relative RMSE was 2.5 microns). The checkpoints were measured in stereo as an independent step to ensure the accuracy of the AT. The differences in Z of the 3 checkpoints were -0.078, -0.094 and -0.094 feet.

1.4. Mapping:
Planimetric and DTM features were compiled in a Microstation environment. The DTM was compiled to suit 1-foot contour generation according to NMAS. The features which were collected ranged from basic mapping features (Such as roads, buildings, trees…etc), to large scale mapping features such as manholes and mailboxes. Hard breaklines were imported from the planimetric features to the DTM files to accurately model the terrain and meet the specifications. The photo-interpretation of the planimetric and topographic features was excellent due to the quality of the DMC imagery.


Figure 1: Examples of planimetric features collected using the DMC camera.


1.5. Editing and Final Delivery:
1-foot Contours were generated using the DTM. Contours were then overlaid on top of the stereo models to ensure that the accuracy was maintained and that the DTM was accurate. The rest of the editing tasks took place and the final deliverable was a seamless DGN file.

2- Digital Sensors For Power:
GIS has been used as a support system in power transmission (Decision making, maintenance…etc). All these activities require Geospatial data to be collected and maintained. The main objective of the project described below is to feed the existing GIS system with suitable vector and a raster databases. The imagery was captured using the ADS40 whereas the vector data was captured using the ALS50 LiDAR system.

The ADS40 and the ALS50 technologies were selected for this project for the following reasons:
  1. Speed at which data can be acquired using the ADS40.
  2. LiDAR was a technology that suited the specifications and needs of this project.
  3. The ADS40 is more adequate for Ortho-Only projects.
This project was executed in 3 main phases:

2.1. Data Acquisition:
The project consisted of 4,500 Miles of Power Lines. The width of the corridor was 2,500 Feet. The total number of poles to map was 18,000. The data was acquired separately for the photography and the LiDAR. The total area mapped was approximately 2060 square miles.

The ALS50 LiDAR system was flown at an altitude of 300 Feet using a helicopter. The average distance between the points was 0.5 Feet. The ADS40 camera was flown at an altitude of 4800 Feet using a fixed-wing aircraft. The final GSD was 0.5 feet. The entire project was divided into segments. There were 310 segments in the entire project.

2.2. LiDAR Processing
Upon completion of the LiDAR data acquisition, files containing XYZ and Intensity values were derived for various segments of the project (After the ABGPS and IMU were processed). The classification of the LiDAR points took place using TerraScan and TerraModel software. The points were classified into the following categories:
  • TOP’s: These are points falling at the Top Of Poles.
  • POA’s: Points Of Attachments are the points that fall at the intersection of the wires and the poles.
  • GW’s: These are Guy Wires.
  • Poles: Poles are made out of all the points that have been intercepted by the poles on the ground and therefore made it back to the sensor.
  • DEM: The Digital Elevation Model is made out of all the LiDAR points that belong to the bare earth (Primarily the last LiDAR return). These points have been filtered out from any noise that could affect the quality of the DEM. These are also points that could have reached the ground through thick vegetation.
  • Vegetation: All vegetation falling in the corridor has been classified in a separate layer.
  • Substations: All substations have been classified separately into a separate layer.
The final deliverables to the end customer were EBN files. The EBN files were used for modeling purposes as well as other applications.

2.3. ADS40 Imagery Processing
The ADS40 data was processed using GPro, SocetSet and Orima. The Aerial Triangulation was performed using Automated Point Matching (APM) technique. ABGPS and IMU data was used within the AT process. The final Orthos were derived using the AT results and the LiDAR-Derived bare earth DEM. The final pixel resolution was 6 inches. There were about 8000 ortho tiles in total covering the entire AOI. The Mosaicking and Tiling was done using OrthoVista software while the final tone balancing was done using Photoshop.


Figure 2: LiDAR Points depicting wires and poles.


3. Digital Sensors for Telecom:
In the field of Telecom, RF Planning involves Cell Planning, Frequency Planning, Coverage interference prediction, Co-Channel Interference Prediction and Optimization. The use of accurate and up-to-date Geospatial data for these tasks is essential.

SELECTED-IIC participated in the development of GIS databases (to be used for RF Planning purposes) for several cities in Europe and Africa.

In Europe and because aerial photography acquisition is relatively easier than Africa, the ADS40 technology was used as a solution. In Africa IKONOS stereo imagery was easier to access and was the solution that was offered to develop the required databases. In both cases, the main objective of the project was to develop an up-to-date and accurate database that meets the specifications of the RF planning tasks.

3.1. Europe Telecom Experience
This project was conduced in two phases (2004 and 2005) covering 9 cities. A total of 3,481 Km2 was mapped for this purpose. Because of budget and schedule restrictions, a decision was made to avoid the costly and lengthy stereo mapping process. This project included mapping elevated structures and assigning height attributes, therefore True Orthos along with a DSM were a solution for this project.

SELECTED-IIC used True Orthos and DSM derived from the ADS40. The pixel resolution of the True Orthos was 0.25m whereas the vertical accuracy of the DSM was assessed to be around 0.5m. This was sufficient to reach to goal of this project.

Some of the Cities were mapped from scratch while others were simply updated. Upon overlaying the existing databases on top of the True Orthos, we discovered that they matched perfectly although the existing databases were developed using conventional photography in a 3D environment.


Figure 3: Image showing the mapping before and after


The ADS40 True Orthos were used to either update or develop the following features:



The heights were derived from the difference between the DSM value at the top of the features and on the ground. This process was semi-automated whereby routines were developed to fill the Height field automatically by simply clicking on the feature, the top of it and then the ground next to it. The names were derived from existing road maps.

The existing datasets were provided to SELECTED-IIC in a shapefile format. All the digitizing was performed in ArcGIS environment. Topological rules were an important piece of this project. The client had mandated several topological rules for the entire project dataset. Examples of these rules are:
  • No segment should be less than 1 meter.
  • No buildings should have less than 3 meters in height.
  • No polygons should have more than 10,000 vertices…etc.

These rules were mainly to accommodate the RF planning software. FME software was used to build routines that were used to detect the non-compliant features and flag them. The final vector deliverables were in MapInfo format and delivered as seamless files for each City. Raster deliverables consisted of the following:
  • 5-meter DTM grid in RAW format
  • 5-meter Grid of heights in RAW format
  • 5-meter Clutter in RAW format

3.2. Africa Telecom Projects:
The flying restrictions in Africa dictated the type of sensor to be used to develop the required databases for RF planning. The main objective of this project was to develop a database that includes the following:
  • Buildings (Height is an Attribute)
  • Digital Terrain Model (DTM)
  • Road Network
  • Hydrography Network
  • Vegetation (Height is an Attribute)
  • Railroad Network
In order to fulfill these requirements, IKONOS stereopairs was the solution adopted for this case because it offered the following advantages:
  • Good geometry
  • Ability to acquire stereopairs within acceptable schedule
  • Resolution and Accuracy levels were acceptable for this application
  • Price

For RF planning purposes, and using almost the same specifications, 29 cities in 5 countries were mapped using IKONOS stereopairs covering an area of 3,000 Km2.

The coordinates of the GCP’s were derived using GPS technology onsite. The number of GCP’s varied between 4 and 12 depending on the size of the city to be mapped and the number of models covering the AOI.

The Aerial triangulation was completely automated. The pass and tie points were correlated. The relative RMSE was around 3 microns. The final absolute RMSE value was always in the range of 0.10m to 1.5m in XYZ. These results met the expectations and the accuracy requirements of the RF planning tasks.

The mapping portion of the project was all performed in Microstation environment using the SSK tools. The final deliverables consisted of the following:
  • DTM File in DXF format
  • Vegetation File in DXF format
  • Transportation File in DXF format
  • Building File in DXF format
  • Hydrography File in DXF format

These files were later on converted to a format more suitable for the RF planning software they were intended for.


Figure 4: IKONOS Digital City Model


Conclusion
The choice of a particular digital sensor for mapping applications will primarily depend on the capabilities of that sensor to fulfill the requirements of a project in terms of specifications, accuracy, budget and schedule. Every project is unique; therefore a decision as to what sensor to use will need to be made on a project by project basis.

Though digital sensors are fast and accurate, their use is sometimes prohibited because of cost. It is anticipated that their use will increase due to the fast advancement in technology as well as competition among the industry.