GIS &GPS CoE -HCL Technologies Ltd
Associate Software Engineer
HCL Technologies Ltd
Software Engineer – HCL Technologies Ltd
Transportation organisations have historically defined their asset locations based on the linear referencing methods like mile posts, reference markers e.t.c. The advent of GPS has revolutionized the process of data collection. This has led us to a precarious situation where there are inconsistencies in the actual location of the asset or incident on the road. In addition, there is no platform to correlate and analyse asset data of a road obtained from multiple methods. This paper attempts to design an Enterprise Location Referencing System for Transportation (ELRS-T) which contains multiple location reference methods, each referring assets and incidents within their reference framework. The architecture and the components of ELRS-T has been identified and summarized to address two key important requirements viz. transformation of the location of the asset or incident data among methods of ELRS-T and the persistence of the asset data on the road, subjected to temporal attenuations. The ELRS-T would serve as an ideal tool for the transportation managers to study transportation asset or incident parameters under a common framework and achieve data flow among systems.
The road Infrastructure is a key accelerator for the growth of a nation. The transportation organizations rely on the discrete systems and applications to automate and facilitate operations associated with the roads. Majority of the road transportation systems are required to reference the asset or incident locations on the road. Traditionally, the road asset and the incident location are marked by linear reference methods like mile post, mileage, distance past reference markers and many others. GPS is the latest addition that has revolutionized the data collection process with its ease and accuracy. The location methods are independent and often do not give consistent view of the road network. Thus, it is hampers the swift flow of information from one system to another for combined analysis. The goal of the paper is to define an Enterprise Location Referencing System for Transportation (ELRS-T) which highlights the maintainability of the integrity of the asset or incident location when it is being transformed from one method to another. The paper also explains the persistence of the location component of the asset / incident data, adapting to the temporal changes of the road network.
2. GPS and Linear Referencing
The linear reference methods refer an asset with respect to a permanent feature and linear distance from it, measured usually using tedious distance measuring equipments. However, the advent of GPS technology has enabled the transportation managers to swiftly record the location of assets and incidents. Nevertheless, it is easy for the field inspectors who perform maintenance operations to identify the assets based on the linear distance from the milepost rather than GPS co-ordinates. For instance, describing the location of an accident as “1/2 km west of km-post 149 on National Highway-5,” is more easily understood by the rescue team driver than a set of coordinates. The situation can be addressed by the implementation of a system where asset and incident locations are available in linear methods (mile post, reference marker), Latitude and Longitude and Street Addresses.
3. Location Referencing System
According to NCHRP (1974), the Location Referencing Method (LRM) is a technique to identify a specific point or section of road either in the office or in the field. Location Referencing System (LRS) encompasses set of field and office procedures to retrieve records of specific point on a highway. In general, the available location reference methods can be classified as
- Geodetic Reference Method
- Linear Reference Method
- Geometric Referencing Method
In the Geodetic reference method, the asset and the event locations are marked based on the three dimensional orthogonal co-ordinate system with origin at the Earth’s center. GPS is a geodetic reference method and adopts WGS 84 datum.
Linear Reference Methods are one-dimensional methods as the road asset and the incident locations are indicated as the linear distance/offset from a known point. Linear Reference Methods were widely used by the transportation agencies before the advancement of GPS.
Alternatively referred as the co-ordinate reference method, this method would be used to display the road asset and event locations on a map. Besides viewing and editing asset locations on the road network, the method is imperative for the transformation of the location of the asset among the linear and the GPS method.
4. ELRS-T – Functional Requirements
The scope of ELRS-T is limited to address two important functional requirements  of any Location Referencing System:
- Transformation of asset data:
- Persistence of the asset data
The ELRS-T would support the transformation of asset location among the Linear and Geodetic methods.
The ELRS-T would support the sustainability of the location of the asset after any changes to the road network. Thus, the network can be regenerated to showcase the roads and incidents that were relevant to a particular time period.
5. Architecture of ELRS-T
I. ELRS-T Datum – GIS Data Model
The ELRS-T datum would be a combination of Linear and the Geometric datum of a region. Both the Linear and Geometric datum would be maintained in an RDBMS with spatial support. Besides location component, time and date of measurement are also collected to check persistence of the asset data and temporal analysis .
Linear datum comprises of anchor sections connected by two anchor points. The anchor section follows the logical centerline of the roadway, with anchor points being located along that centerline. Anchor points must have a unique identifier, location description/remarks, and real-earth location as their attributes. Anchor sections have mandatory attributes of a unique identifier, designation of terminal anchor points, and length. A Traversal in the linear datum can have many connected anchor sections, with their field measurements varying sequentially according to its arrangement. The density of anchor points determines the positional accuracy of the linear datum. The anchor section length serves as a quality check for the linear field measurements. The ability to detect and correct measurement errors varies directly with the number of anchor points and inversely with the average anchor section length .
Fig 1. Data Model
Geometric datum is the cartographic (x,y) representation of the centerline of the road network. The geometric datum supports the transformation of asset location data from GPS to Linear Methods and vice versa. Geometric datum would assist in map analysis and reporting. Traversals in the linear datum and the geometric datum are tightly coupled by defining unique identifiers.
In ArcGIS, the linear and geometric datum of a traversal are recorded in terms of a linear feature and a measurement along it. Instead of being composed of x,y coordinates, a feature’s geometry in ArcGIS can be composed of x,y,m (or x,y,z,m) values .
The ELRS-T datum is illustrated below:
Fig 2. ELRS-T Datum for a Traversal
II. Components of ELRS-T
The components of the ELRS-T are
- Projection Module
- Linear Referencing Module:
- To retrieve the measure (m), given the geometric co-ordinate (x,y) and the traversal .
- To retrieve the x,y for a given measure(m) and Traversal id
- To create point/line event tables for asset or incident location to enable map analysis and reporting (Dynamic Segmentation).
- To overlay two event tables to create an output table that represents the union or intersection.
- Address Geocoding / Reverse Address Geocoding
- Point Snapping
The function of projection module is to project the Geodetic co-ordinate (Latitude and Longitude) to the x,y of the underlying co-ordinate system of the geometric datum. Though the module would be irrelevant if the underlying geodetic datum is WGS 84, it is best to retain the native projection adhered by the country for map analysis.
The functions of this module are
Address Geocoding is the transformation of the street address to x, y and m along the linear feature. On the contrary, the reverse geocoding would convert the given x, y into a street address.
The function of this module is to snap a point location (x, y, traversal) say, a GPS Co-ordinate or a geocoded street address to the nearest location on the traversal in the ELRS-T datum.
III Process Work flow – GPS to Distance Past RM Method
This section exemplifies the transformation of an asset data measured using GPS (Geodetic method) to Distance past Reference marker method.
Fig 3. GPS to Reference Marker (Linear) method
- Projection of Latitude and Longitude of the asset data into x, y of the Geometric datum
- Snapping the projected co-ordinate to the traversal (x’,y’)
- Query and Retrieve the measure ‘m’ at x’,y’.
- Query and Retrieve the reference markers on the traversal. Delineate the Reference marker with the nearest measure m.
- Compose asset location in the schema of the reference marker method.
IV Persistence of asset data
The road networks in the transportation are dynamic. New roads are being added, existing roads are extended and realigned, and old roads are demolished or abandoned . Therefore, it is imperative to regenerate the network that existed at a particular time period. Hence, each traversal in the ELRS-T datum has to be marked with the time stamps indicating their service time period.
Each traversal is marked with a start date and the end date. Hence, if a traversal is altered, the altered traversal record is added in the table with its start date (date of operation). The original traversal record is closed with end date same as the begin date of the altered traversal. Each road asset or the incident data is collected along with time stamps for temporal match with appropriate network history.
Fig 4. Persistence of Asset data
5. Economic Benefits
The incorporation of GPS method in the legacy applications will prove to be more expensive as it involves reconfiguration and rework on the system. However, the available of ELRS-T would enable the legacy applications to convert to the GPS into a linear data and store in it for subsequent analysis and reporting. The ELRS-T would facilitate the definition of more reference methods such as navigation/dynamic objects for building dynamic transportation applications (ITS). Also, the ELRS-T can be extended to work for other federal utilities like waste and waste water pipelines, Gas Pipelines and many others.
The concept of the ELRS-T is the outcome of Enterprise GIS and ability to maintain conventional file based GIS formats in to RDBMS with spatial relationships. The ELRS-T is an enterprise GIS where multiple transportation applications can derive network information from it. The proposed ELRS-T has addressed only two concepts of a Location Referencing System. There are other issues like propagation of errors and definition of new reference methods. However, those concepts require new routines and additional procedures and triggers in the ELRS-T datum.
- Teresa M. Adams, Nicholas Koncz and Alan P. Vonderohe (2000) ‘Functional Requirements of a Comprehensive Transportation Location Referencing System. Proceedings of North American Travel Monitoring Exhibition and Conference.
- Al Butler’ Transportation Networks in ArcGIS: An Alternative to Geometric Networks.
- ESRI Technical Paper (2003) ‘Linear Referencing in ArcGIS®: Practical Considerations for the Development of an Enterprisewide GIS’
- Intergraph Mapping and Geospatial solutions (2004) ‘GeoTrans Transportation Data Model’.