BENCH MARKS & TRIG POINTS
The roles of bench marks and trig points / triangulation posts overlap; the one being used for level and the other for location.
Bench marks
You have most likely seen these marks set into the side of buildings or even kerb stones, usually those of stone, and wondered what they were for. Some surmise that they marked an upper level of flood water at some time. Well they could be used for that, but are not that directly connected. These are bench marks. To some they look like arrows pointing upwards to a horizontal line. To others they look vaguely like a crude skecth of a park bench or table. But the similarity of name is coincidental. They are datum points fixed reference points from which to measure heights and elevations. Modern day markers may simply be nails with washer gunned into the pavement during surveys.
Whether an area is surveyed using sophisticated equipment as is done these days, or comparatively simple triangulation as was used in the past, you always need to refer your measured levels to a fixed point. And you needed to be able to return to that spot again to check or record changes such as with property ownership and development. Being able to refer to a fixed known point is crucial.
For a localised survey eg for a group of buildings, measurements are simply compared to known fixed points on a site. For a wider survey as for maps, levels are recorded relative to sea level.
The term benchmark, bench mark, or survey benchmark originates from the chiseled horizontal marks that surveyors made in stone structures, into which an angle iron could be placed to form a "bench" for a leveling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in the future. These marks were usually indicated with a chiseled arrow – specifically a broad arrow – below the horizontal line. A benchmark is a type of survey marker.
The term is generally applied to any item used to mark a point as an elevation reference. Frequently, bronze or aluminum disks are set in stone or concrete, or on rods driven deeply into the earth to provide a stable elevation point. If an elevation is marked on a map, but there is no physical mark on the ground, it is a spot height. [Wiki]
If you are intrigued by bench marks, this website is for you. See the link below. The interactive benchmark archive.
It can be great fun to locate as many as possible near your area. Unfortunately though, not all have been maintained and may have been lost, but those that can be found will be carved into the sides of old buildings or on kerbstones. Bench Mark Bagging is a pastime that is described in the Mrs Physics blog. See the link below. So instead of just going for a walk you arm yourself with a map, put a grid reference on your smartphone and off you set with a description of the position of the benchmark. By exploring this way, you will most likely find places and features you would mornally have missed.
For abbreviations see the reference below.
This is the view for the area near Dumbarton Central Station. At this zoom level you can just make out the red bench mark symbols with labels coming into view as you look closer.
This is how it appears for Dumbarton Castle. Zoom in to see the small red bench mark symbols and location information. As you look closer, more information will appear as seen here in dark rectangles. Each one has a brief description of its location.
This is a closer view from the website and shows the sharp bend in Cardross Road above Renton. You can clearly see the obsolete settlement tanks for the water treatment plant. The bench mark to the left is on the surrounding wall right next to the gate as shown below. The label reads : "GT P ENTSEWAGE WKS (DIS) NE SIDE CARDROSS RD. 92.743m". GT P = gatepost. We surmise that "ENSWAGE WKS" = enstrance to sewerage works. "DIS" = disused. Heights are measured above sea level. That on the north east corner is no longer visible as it is very overgrown.
Even at a basic level, surveying needs some fundamental considerations. The surveyor behind the level aparatus has to know where that is and be able to clearly see the point being measured. An assistant will stand at that point with a measuring staff with levels on it. The assistant then moves to another point and that compared to those already taken. So when the surveyor comes to a corner that is obscured such as this one with a high wall, there needs to be a means of going around it. He will have stood at the outer corner with good visibility of those being measured. As the surveyor moves up a slope such as this one, the view of the measuring staff may be obscured by it being too low or too high. This is coped with by clearly recording the points at which the measuring staff was set and the survey aparatus is then moved on, back referencing and reading new height levels as he progresses.
Bench marks can be spaced out quite widely, if the surveyor has good visibility such as on flat clear land.
This is the bench mark on the wall alongside the gateway to the water settlement tanks as noted on the map above. It is naturally highlighted by the almost white fungi growing over it. Most will be less conspicuous.
Instructions on the maps. You can choose between satellite imagery or simple maps. Satellite views are generally easiest to use.
Trig points / triangulation pillars
If you have ever done even the most basic of surveys, be that simply with a tape measure across a room or perhaps outside as a scout or guide project, you will know that measuring diagonal dimensions and the plotting them as a combination of triangles can allow you to draw up almost any terrain to scale. This is known as triangulation. In experienced hands it can become quite complex.
This three pointed metal shape set in a concrete post on the edge of Carman Muir overlooks the Clyde. It provides a firm fixing for subsequent survey equipment at a well established datum point.
These days survey equipment is very sophisticated can measure levels and location simultaneously and the development of scanning surveying is impressive. There are other types of survey too - LIDAR being one. The Atlas of Hillforts of Britain and Ireland has an illustration of such a survey of the Carman Hillfort. See link below.
ATLAS OF HILLFORTS OF BRITAIN AND IRELAND : https://hillforts.arch.ox.ac.uk/records/SC1338.html
BENCH-MARKS.ORG website: Abbreviations : https://www.bench-marks.org.uk/help/abbreviations
INTERACTIVE MAPS - Bench marks : - https://interactivemaps.uk/os-benchmark-archive/#16.23/55.944826/-4.564839 This reference centres on the Glasgow Road / Church Street roundabout in Dumbarton. Re- centre it your your own needs, using the zoom.
MRS PHYSICS Bench Mark Bagging blog : https://www.mrsphysics.co.uk/blog/benchmark-bagging/
WIKIPEDIA : https://en.wikipedia.org/wiki/Benchmark_(surveying)
