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LIDAR and QGIS : modern technology applied tothe Leominster Canal

David Slater

LIDAR

Members of the Railway & Canal Historical Society(RCHS) will be well acquainted with sonar and radarand their respective applications. As an example,with respect to the Leominster Canal, groundpenetrating radar (GPR) has played an indispensiblerole in researching its Southnet tunnel. It had alwaysbeen assumed that the initial section of the tunnel, atBroombank, still existed in some form under theA456. GPR undertaken in 2015, however, showedunequivocally that the ground was still undisturbedto a depth of 100 feet and that therefore the longheld assumption of a tunnel beneath the road wasincorrect. This implies that the land slippage in thedeep approach cutting, recorded by Rennie in 1795,prevented significant further construction at this end.1

Intriguingly, it remains unknown, although it is stronglyrumoured, whether bodies lie buried beyond theblockage or elsewhere due to its partial collapse.

It is possible, however, that RCHS members maybe less well informed about the closely related andmore modern technique of LIDAR (or LiDAR), thatuses a different part of the electromagnetic spectrum.LIDAR is often considered to be an acronym for‘light detection and ranging’, whereas it is actually aportmanteau of the two words ‘light’ and ‘radar’.The Environment Agency has been developing andusing this type of Geographic Information System(GIS ) technology for nearly two decades and,approximately three years ago, placed its LIDARtechnical database covering the United Kingdomwithin the public domain. As a consequence the datacan now be downloaded from its open portal.2

LIDAR is based on laser pulses from an aircraft beingreflected back from the earth’s surface, activelycollected by the scanner, detected by a photosensorand then recorded in relation to distance travelled bythe impulses, their x,y,z reference co-ordinates andtheir geographical position being ascertained by aGlobal Positioning System (GPS). LIDAR uses an‘eye-safe’ laser with a wavelength over 1000nm andimpulses are sent at well over 100,000 per second.The resulting digital elevation model may be of

surface and terrain types. The digital surface model(DSM) includes physical objects (such as buildings,vegetation and trees) on the earth’s surface, whereassurface objects are filtered out by software in thedigital terrain model (DTM). The latter is moreapplicable to canal research and, if the tree canopyis not overwhelmingly dense, totally blocking out alllight impulses, trees can be manipulated to disappearfrom the image. Over 70 per cent of the UK hasnow been ‘mapped’ at 25cm, 50cm, 1m or 2m spatialresolutions. The composite digital data are notpresented as a predefined map or image but asgeospatial data for a defined geographical location.Although this makes the methodology much morechallenging, it does permit the personalised creationof a multitude of different imaging and mappingpossibilities. Very conveniently, the databasecorresponds to the standard Ordnance Survey mapgrid references.

QGIS

To obtain an image that is meaningful to interpret,the data must be loaded onto visualising software.One package commonly used in is QGIS (QuantumGeographic Information Systems) and, as withLIDAR, it is a downloadable open public informationresource.3 Users of Apple computers, however, willfind the installation more searching than those usingMicrosoft. Upgraded versions of the software andplug-ins for additional applications (such as Python)are released regularly. Although it is sophisticated tomanage, an image/map can be built up by usingmultiple layers of data, including primary vector andsecondary raster elements. Images can be visualisedin any style but I have found singleband greyscalethe most useful. A hill shade rendering raster isessential and the most revealing image can be foundby trialling the direction and height of incident light,using the azimuth and altitude settings. In essence,the digital data for a defined location can be convertedinto a ‘bare-earth’ shaded aerial relief image. Also,if required, the system can undertake digitalmeasuring on the image and be supplemented byoverview maps.

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Application to the Leominster Canal

There are innumerable well established diverse usesof LIDAR and QGIS. To name some, these includemapping, flood control, coastline management,surveying, engineering, forestry, archaeology, geology,space exploration, traffic enforcement and automaticvehicles. I am not aware, however, of a systematicapplication of LIDAR to waterways or railways asa means to facilitate observational interest orundertake research. Accordingly, after a steeppersonal learning curve, although a fascinatingjourney, I have now developed basic skills on LIDARand QGIS and have applied it to the whole length ofthe Leominster Canal. To assist any interestedmembers of the RCHS in their own fields of interest,I have put together the following observations. Inparticular, these are to describe my perceptions ofthe strengths and weaknesses of LIDAR and QGIS,as applied to one canal that has been disused forover 150 years.

LIDAR and QGIS certainly represent a new andinnovative way of visualising and researching awaterway, although it is not for the technically faint-hearted. As with all comparable methodologies (suchas 1940s aerial survey ), it has the capability to revealnew and valuable information, although it does haveits own specific limitations. Often the findings aresimply as to be expected but show features in adifferent manner. Not infrequently, however, apreviously unknown or unseen gem suddenly revealsitself. At the forefront of its limitations is theinconsistency of ground coverage by LIDAR andwhether data are available for a specific area. Then,even if data exist, a question arises whether suitableresolutions are available for the location. A level ofresolution at 2m is usually just about adequate butideally 1m or greater is to be preferred. There isalso the ‘sods law’ inconvenience that some featuresof interest lie predictably on the edge of tiles oroverlap. For canal sections that are still extant, withor without water, the technique shows the canal orits feeder exquisitely well. This may be as a grey orblack narrow linear band or more commonly as awhite line in the middle of a black linear band or viceversa. The clear visibility of extant canals seemsonly to be matched by that of railway lines. As withcanals, both used or disused railways are seen equallywell. Canals are often more apparent than riversand hedges and invariably much better seen thanroads. For canals and feeders that have been in-

filled or deeply ploughed, although nothing may benow apparent on the ground surface, their originalcourse can often be observed as a faint grey linearband. Unfortunately locks, bridges, and aqueductsare not displayed well, partly due to the poor resolution.LIDAR and QGIS excel in revealing the hiddenground terrain under a tree canopy or vegetation,ground construction earth works and the location oftunnels or mine shafts. The variable appearance ofcanals in the images is a consequence of the reflectivepercentage of the laser impulses and this is called theLIDAR intensity. For example, snow is approximately90 per cent, dry sand 57 per cent, wet sand 41 percent and water less than 5 per cent. Although notusually applicable to canals, beer foam is an interesting8 per cent! The figure is primarily based on thecomposition of the material reflecting the light buttakes into account other factors such as the degreeof light absorption by the material and the direction ofthe light. It is evident that much more work isnecessary on the nature of reflective percentages, tomore accurately explain the images containing canalsand railways. A mathematical LIDAR equation existsto assist in this understanding but is most sensibly leftin expert hands.

This survey on the Leominster Canal identified nolengths of canal other than those already known tohave been started, namely from Dumbleton in the eastto Kingsland in the west. To illustrate the observationsthat I made, there follows a short series of personallyconstructed images. They are intended to show thenature of LIDAR and QGIS images and thereby helpmembers of the RCHS to decide if the technique maybe useful in their own research area. They are all at1m or 2m resolution and the best available for thelocation. Unfortunately, Pensax, which includesdiverse features such as the site of a previous tunnelportal, shafts, mining activities and a tramway, is alocation disappointingly not yet covered by LIDAR.Understandably the Environment Agency hasconcentrated LIDAR mapping on the country’ssignificant water courses. OS grid references aresupplied for any member wishing to directly comparethe image with the OS map. There can be no doubtthat a good working knowledge of the canal prior tosuch an exercise is indispensable. Without suchknowledge, there could be a high risk of potentialmisinterpretation and errors. Reciprocally, a goodbackground knowledge facilitates new discoveries.

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Fig 1. Southnet tunnel (OS grid reference SO6770. 2m resolution)

Acknowledgements

I am grateful to both my late father (whoundertook radar research in World War II andsubsequently kept up to date with the subject) andmy nephew (who works for the EnvironmentAgency) for introducing me to LIDAR and itsmysteries. Both, however, expressed caution onnon-expert usage (that is, myself!) due to itscomplexity. Accordingly I am particularly gratefulto Professor Michael Rosenbaum (Ludlow) forsharing his expertise and demonstrating thatLIDAR/QGIS can be a workable practical tool, evento the inexperienced enthusiast, and can thereforebe used highly successfully for making newobservations and to undertake research on theLeominster Canal. Hopefully others will find thesame for other canals and railways.

References

1. John Rennie, ‘To the Committee of Management of theLeominster Canal … 1795’, pp 1–11 (East Riding ofYorkshire Archives DDCC 147/44)

2. <https://data.gov.uk/publisher/environment-agency>[accessed 24 March 2017]

3. <www.qgis.org> [accessed 24 March 2017]

4. Thomas Dadford Jr and J Waring, ‘To the Company ofProprietors of the Leominster Canal Navigation …1794’, pp 1–2 (East Riding of Yorkshire ArchivesDDCC 147/43)

5. Thomas Dadford Jr, ‘A Plan of the intended Canal fromKington in the County of Hereford to the River Severnnear Stour Port in the County of Worcester 1789’[Copies held in British Library and Hereford CityLibrary]

The road going southwest to northeast is the A456. To the east is Ash Coppice. The trees, however, havebeen filtered out in the DTM (digital terrain model) revealing a line of several shafts for Southnet tunnel,spaced at approximately 300 feet. It had not been previously fully appreciated that this number of shafts stillexisted, although they are briefly mentioned in the reports made by Dadford and Rennie in the 1790s.1,4 Thisnew finding is under further investigation on the ground. The deep cutting to the northern tunnel entrance/portal underwent considerable slippage in the 1790s; it lay originally to the west of the road and is neither nolonger apparent on the ground nor revealed in the image. Three different grids must be combined to coverthe whole length of the tunnel, but this image is the most revealing.

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Fig 2. Rea aqueduct towards Marlbrook (OS grid reference SO6570. 2m resolution)

The white line going southwest to northeast is the course of the disused Tenbury to Bewdley railway crossingthe Trapnell and Mill brooks. Below the railway is the irregular course of the River Rea. The horizontaleast–west white line below the river is the Leominster Canal from the Rea aqueduct towards Marlbrook.The DTM has filtered out the dense tree canopy to reveal the canal. This length had two locks but littleevidence now survives on the ground and the two sites are only unequivocally seen in the image. TheLeominster Canal crosses the River Rea by the historic and listed Rea aqueduct (which unfortunately had amajor partial collapse in 2013). Fine details of the aqueduct are not visible in the image, but it is uncertainwhether the LIDAR data were recorded before or after the partial collapse. Close to the aqueduct apresumed winding hole or basin can just be seen in the image and is also present on the ground.

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Fig 3. Corn Brook canal loop (OS grid reference SO6168. 1m resolution)

The River Teme is shown going northwest to southeast. Above this is the straighter A456. Above the roadis the course of the disused Tenbury to Bewdley railway which was largely built on the line of the canal.Above the railway, towards the east, is the surviving ‘horse-shoe’ shaped Corn Brook canal loop. The CornBrook is seen going north to south and entering the Teme. The tree canopy is usually very dense but in theDTM it has been filtered out. The Corn Brook passed beneath the canal in a large culvert which was openedout in the early 1900s due to a collapse and/or blockage. The adjacent original embankment workings areshown extremely well. The canal is seen to widen at both east and west apical bends to facilitate boatpassage.

There is an intriguing ground disturbance on the northwest bend which to date was thought to have been abasin and/or the start of Dadford’s Corn Brook canal branch, shown on his survey maps but not completed.5

LIDAR, however, strongly suggests a new alternative possibility, that this was also the point of entry of acanal feeder fed by the Corn Brook. The feeder can be seen as a short vertical black line crossing thenorthern edge of the canal bank, itself seen as a curved horizontal white line. This is shown further in Fig 4.

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Fig 4. Corn Brook feeder (OS grid reference SO6168/9. 1m resolution)

This is a montage of all of grid square SO6168 and part of SO6169 but with different altitude and azimuthsettings compared to Fig 3. The course of the Corn Brook feeder is marked as a black line with arrows,immediately adjacent to the intermittent feeder features present on LIDAR. The feeder ran from above aweir, still extant but not apparent on LIDAR, on the Corn Brook below Boraston ford to the clearly seennorthwest corner of the Corn Brook canal loop. The feeder ran approximately on the 66m contour and in thecentral part lay just west of Search Mill which existed on the Corn Brook at an earlier date but is no longerextant. As a consequence of the LIDAR discovery, the feeder has been the subject of intensive investigations.The results will form the basis of a separate detailed paper, including residual ground evidence for its existenceand historical aerial surveys, which both confirm the LIDAR observations and thereby support the existenceof a previously largely unrecognised feeder. It should be noted that the line of the feeder lies entirely onprivate land with multiple landowners and permission must be obtained to visit.

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Fig 5. Teme aqueduct area (OS grid reference SO5368. 2m resolution)

The River Teme tracks west to east towards the top (north ). The Brimfield Brook has a junction with theTeme and tracks south and then west. Crossing the brook, west–east, is very the faint A456 going throughGosford. The clear white line, also going west–east, is the course of the disused Tenbury Railway. On thewest side of the image, the railway was largely built on the canal, although the residuum of a minor deviation(containing the site of a Gosford lock ) can just be identified. In the centre of the image the line of the canaldiverts from the course of the railway to go north on a large embankment to the river, which it originallycrossed by the Teme aqueduct. The latter was partially blown up in World War II as a defence measure andis not well seen. The original construction workings for the embankment can be seen in the adjacent fieldson both sides. Just north of the Teme aqueduct, the canal turns 90 degrees to go east. Usually this area ishidden by a dense tree canopy but when filtered out in the DTM, significant widening of the canal is seen onthe bend to facilitate boat passage. This was also the intended site for the Leominster Canal to join anotherintended Dadford canal (John not Thomas) (the Ludlow Canal) to join the Montgomery Canal. It is notknown whether the sizeable widening was also to accommodate this future potential possibility.

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Fig 6. Berrington area (OS grid reference SO5063. 2m resolution)

The dark track going north–south is the old route from the current A49 to Orleton via Shuttocks Hill (all offimage). It is now part of the Marches Way. To the east is the boundary to the National Trust Berringtonestate. Just to the west, the line of the Leominster Canal appears as faint grey linear band, also going north–south and running parallel to the Marches Way. At the northern end is the extant Moreton wharf and at thesouthern end the extant canal enters a small wood. Significantly, although apparent in the image, the sectionof canal in the fields is no longer visible on the ground. Why the canal took a slightly wavy line rather thana straight course is unclear but the course seems to exactly mirror a difference in the geological strata and tospecifically avoid alluvial deposits. Dadford, however, will not have been armed with a geological map at thetime!

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Fig 7. Dumbleton area (OS grid reference SO7069. 2m resolution)

The Dumbleton Brook tracks approximately from the northwest to southwest corner as a ‘C’ curve. Fromthe northwest it goes into/around Brookhill Wood (seen as a dark area) to join a tributary above Menith Wood(seen as a white area). From near the ‘Y’ junction the Dumbleton feeder tracks southwest as a slightlyirregular white line. Again the DTM has removed the tree canopy of the dense wood. Its first part inBrookhill Wood can no longer be easily seen on the ground, although its lower portion in Dumbleton Farmfields is still extant. The gap in the middle constitutes part of the access road and the farm. The constructionof the canal/feeder stops abruptly at the same access road to Dumbleton Farm on another image.