Snake Projection

{{Short description|Map projection for linear projects}}

File:SnakeTrendExample.png

{{Geodesy}}

The Snake Projection is a continuous map projection typically used as the planar coordinate system for realizing low distortion throughout long linear engineering projects.{{cite journal|last1=Iliffe|first1=J. C.|last2=Arthur|first2=J. V.|last3=Preston|first3=C.|title=The Snake Projection: A Customised Grid for Rail Projects|journal=Survey Review|date=2007|volume=39|issue=304|pages=90–99|doi=10.1179/003962607X165041|s2cid=128833605 }}

== Details ==

The Snake Projection was originally developed by University College London and Network Rail to provide a continuous low distortion projection for the West Coast Mainline infrastructure works.{{cite web |last1=Gibbons |first1=Chris |title=Working with SnakeGrid |url=https://journals.cices.org/ces/ces-february-2023/features/working-with-snakegrid |website=CICES |access-date=24 February 2023 |date=2023 |archive-date=22 April 2023 |archive-url=https://web.archive.org/web/20230422093137/https://journals.cices.org/ces/ces-february-2023/features/working-with-snakegrid |url-status=live }} The parameters defining each Snake Projection are tailored for the specific project. The most typical use is with large-scale linear engineering projects such as rail infrastructure; however, the projection is equally applicable to any application requiring a low distortion grid along a linear route (for example pipelines and roads).{{cite book|last1=Reynolds|first1=John|title=An Introduction to Applied and Environmental Geophysics|date=2011|publisher=John Wiley & Sons|isbn=978-0-471-48535-3|edition=2|url=https://www.wiley.com/en-gb/An+Introduction+to+Applied+and+Environmental+Geophysics%2C+2nd+Edition-p-9780471485353|access-date=2018-02-06|archive-date=2018-02-07|archive-url=https://web.archive.org/web/20180207005342/https://www.wiley.com/en-gb/An+Introduction+to+Applied+and+Environmental+Geophysics%2C+2nd+Edition-p-9780471485353|url-status=live}} The name of the projection is derived from the sinuous snake-like nature of the projects it may be designed for. Typical map projection distance distortion characteristics of a Snake Projection are minimal over the whole route within approximately {{Convert|20|km|mi|abbr=on}} of the centre line.{{cite journal|last1=Iliffe|first1=J. C.|title=The development and analysis of quasi-linear map projections|journal=Cartography and Geographic Information Science|volume=45|issue=3|pages=270–283|date=2017|doi=10.1080/15230406.2017.1325332|s2cid=67328277|url=http://discovery.ucl.ac.uk/1556463/|access-date=2018-11-26|archive-date=2023-04-22|archive-url=https://web.archive.org/web/20230422093142/https://discovery.ucl.ac.uk/id/eprint/1556463/|url-status=live}}

The principal advantage of the projection is that, for the corridor defining the design space, distances measured on the ground have a nearly one-to-one relationship with distances in coordinate space (i.e. no scale factor need be applied to convert between distances in grid and distances on the ground).{{cite web|last1=SCSI|title=Map Projections and Scale Factor|url=https://scsi.ie/wp-content/uploads/2020/11/Map-Projections-and-Scale-Factor.pdf|website=SCSI|date=26 November 2020|publisher=Society of Chartered Surveyors Ireland|access-date=24 February 2023|archive-date=22 April 2023|archive-url=https://web.archive.org/web/20230422100845/https://scsi.ie/wp-content/uploads/2020/11/Map-Projections-and-Scale-Factor.pdf|url-status=live}} The length of the applicable corridor is variable on a project basis; however, when required, the projection can extend over several hundreds of kilometres to achieve grid distortion of less than 20 parts per million along the route.{{cite web |title=Use of Global Navigation Satellite Systems (GNSS) in land surveying and mapping, 3rd edition |url=https://www.rics.org/content/dam/ricsglobal/documents/standards/Use%20of%20GNSS%20in%20land%20surveying%20and%20mapping_3rd%20edition.pdf#page=34 |website=Royal Institution of Chartered Surveyors |access-date=5 June 2023 |page=34 |date=16 May 2023}} The main disadvantage is that away from the design corridor, the distortion of the projection is not controlled.

The Snake Projection is suited for engineering purposes due to its low distortion characteristics. An example of its differentiation from mapping grids is the {{Convert|60|m|ft|abbr=on}} increase in length of the London to Birmingham section of the HS2 rail line, purely due to the more accurate grid representation compared to the length when using the national mapping coordinate system British National Grid.

== Usage ==

The Snake Projection is the engineering coordinate system used for a significant proportion of primary rail routes in the UK,{{cite web|last1=XYHT|title=Corridor Projections Solved|url=http://www.xyht.com/professional-surveyor-archives/corridor-projections-solved/|website=XYHT|publisher=Flatdog Media|accessdate=5 February 2018|date=2015-01-15|archive-date=2018-02-05|archive-url=https://web.archive.org/web/20180205072722/http://www.xyht.com/professional-surveyor-archives/corridor-projections-solved/|url-status=live}} including that of the HS2 London to Birmingham high speed line.{{cite web|last1=UCL|title=HS2 becomes fifth project this year to adopt SnakeGrid system|url=https://www.cege.ucl.ac.uk/news-events/Lists/News%20%20Announcements/Announcement/displayifs.aspx?List=55c7dbd4-3027-423f-bfcf-ed525bb40245&ID=121&ContentTypeId=0x010400A41508A6A59B4D45AC85D44B776AC063|website=CEGE|publisher=UCL|accessdate=5 February 2018}}{{Dead link|date=May 2023 |bot=InternetArchiveBot |fix-attempted=yes }} For the London to Glasgow West Coast Main Line, the distortion in the Snake Projection used is no greater than 20 parts per million within {{Convert|5|km|mi|abbr=on}} of either side of the track.{{cite book|last1=Uren|first1=John|last2=Price|first2=Bill|title=Surveying for Engineers|date=2010|publisher=Palgrave Macmillan|location=Basingstoke|edition=5|url=https://www.macmillanihe.com/page/detail/Surveying-for-Engineers/?K=9780230221574|access-date=2018-02-06|archive-date=2018-02-07|archive-url=https://web.archive.org/web/20180207005249/https://www.macmillanihe.com/page/detail/Surveying-for-Engineers/?K=9780230221574|url-status=live}}