© 2014 Mouchel. All rights reserved www.mouchel.com

Client: TfL

Value: £14,500

Date: May 2013 to July 2013

Services: GIS, Digitisation, Geo-referencing

Production of digital spatial database of electrical ducts and

manholes of London’s dismantled tram network

The challenge

Transport for London (TfL) wished to procure geospatial consultancy services to geo-reference a set of scanned PDF plans for use in a GIS. The set of plans consisted of 718 individual PDFs of varying image quality and resolution.

The deliverables to TfL were:

1. 718 JPEG images of the PDF plans,

geo-referenced to British National

Grid, and associated world files;

2. a GIS dataset of the British National

Grid control points for each geo-

referenced image;

3. a GIS key plan showing the extent of

each plan;

4. a digitised GIS line dataset for the

cable route alignments;

5. a digitised GIS point dataset for

manholes; and,

6. a quality check report of 20% of all

geo-referenced images and digitised

GIS datasets.

The solution

TfL Geo-referencing & GIS Datasets

PDF files are not directly compatible with most GIS software, and therefore the PDF plans were converted into a format that would be recognised by GIS. The JPEG file format was agreed upon as it is recognised and used by most GIS and many readily available software applications.

In order to convert the large number of PDFfiles in the most efficient time, parameters relating to image quality were input into GhostScript software, so that the conversion process could be automated and the files converted into JPEGs in one

batch.

During this process, care was taken to strike a suitable balance between image clarity and acceptable file size, so that the resulting JPEGs were of a high quality without being prohibitively large.

Once the PDF plans had been converted

into JPEGs, it was necessary to geo-reference each image, (i.e. to define the location of each image in relation to British

National Grid) as at this stage, there was no spatial reference information (world files)

associated with these files in the GIS.

purposes, 150 (over 20%) of the geo-referenced images were checked at random by senior GIS consultants, for locational accuracy and potential warp or transformation errors. These images were also checked to ensure that every manhole and cable duct had been captured accurately and with no omissions.

Database integrity was maintained throughout the project through the use of versioned geodatabases, which allowed the database to be managed and maintained in multiple states. This permitted multiuser editing, undo/redo capability and an auditable record of amendments.

The control point coordinates were saved as separate text files and named after each sheet. These control point files were collated and a single GIS dataset was created of the points with the appropriate sheet attributes so that the client could easily identify the control points used for each transformation to validate each shift.

Using ESRI’s ArcGIS to transform the image (raster dataset) to the correct location, it was necessary to add control points to link known positions on the plans to known positions on a spatially referenced dataset. In this case we used two Ordnance Survey datasets with the correct British National Grid coordinate system. Firstly, we used the OpenData medium-scale Streetview dataset served from our ArcServer central mapping repository to get each JPEG to the approximate vicinity. We then used the large-scale MasterMap vector dataset for the detailed work of identifying suitable control point locations. Landmarks on each plan were identified, which corresponded to points on the same landmark in the MasterMap dataset, i.e. the control points selected were at locations that could be accurately identified on both the images and spatially referenced dataset.

Once the images had been correctly positioned, the resulting world files provided an accurate spatial reference for each of the original plans.

Once each image was geo-referenced to its spatially correct location, manholes and cable ducts were digitised. Digitising in GIS is the process of ‘tracing’ information from images/maps, in a geographically accurate way.

A geodatabase was created to contain the cable and manhole feature classes. Each cable duct and manhole was identified and carefully traced, while each cable was split into segments at manholes (according to topological rules) or at the edge of each image. The feature classes had attribute tables which were populated as each feature was created. Attributes included unique ID, XY co-ordinates, plan sheet number, and road name.

In addition to two comprehensive checks by the digitisers, for additional quality control

For more information contact Gary Simpson on +44 (0)7836 281593 Gary.Simpson@mouchel.com

Landscape/Portrait

Image (Optional)

Effective communication with our client allowed us to establish a collaborative environment, whereby we were able to identify potential challenges and establish a framework for the successful delivery of a high-quality service, early in the project. With this in mind, the client was provided with sample data shortly after project commencement, in order to ensure data interoperability. As a result of providing this sample data early on in the project, potential compatibility issues could be rectified immediately.

Harnessing open source data (e.g. Bing aerial photography and Open Street Map) and software (e.g. GhostScript), along with open data provided by the Ordnance Survey, enabled Mouchel to provide cost-effective solutions to our client.

While the project was concerned with the area of London, the use of ArcServer technology allowed Mouchel to harness the expertise of its GIS professionals in various offices throughout the UK (e.g. Belfast, Liverpool and London), for efficient and cost-effective collaboration.

The project was completed in-house by

Mouchel’s Geospatial Systems, a highly experienced team of GIS professionals, led by a Project Manager with 25 years of experience in this field, and supported by a Senior Consultant with over 20 years of experience in GIS.

The latest IT and GIS technology was employed in the completion of this project, including ArcGIS, ArcServer and GhostScript, along with the most up-to-date datasets from the Ordnance Survey and

the open source community to deliver the project on time and to budget.

The outcomes

.