Measuring Britain
Shot taken for Saturday Self Challenge 02/04/2022 ----
Measurement and units of measurement . However , not used in the end .
My first thoughts were to work with the measurement of Albedo , but this form of measurement is on a scale and not with any units - so then back to a previous life for this week’s challenge .
Righto , back to the 1970s then when I was a land surveyor and that was all about measurement , there was linear albeit imperial or metric measurement and there was angular measurement ie Degrees-Minutes & Seconds .
For any large scale survey , it could be linked to the National Grid system of the Ordnance Survey by the network of Triangulation Pillars throughout the United Kingdom . Here we see one of the 6,500 or so pillars and this one is on Box Hill overlooking Dorking and southern Surrey - like many showing signs of wear and without the usual white paint . What you can see on the top is the 3 pronged brass inset plate in which a theodolite would sit and straight away be accurately centered . In the first comment box is another view of the trig pillar here with the elevation bracket and trig number - and for a laugh a terrible old shot of me working with a theodolite , this time on a tri-pod .
Of course it is all GPS now and fancy bits of kit that I would be lost on nowadays , at least doing the job old school you had to know the fundamentals and not rely on tec to do the work - no more computations with giant log, sine,cosine and tangent tables to 10 or so decimal places - just a push of the button to convert polar to rectangular co-ordinates !!
Triangulation Pillars --
Trigpoints are the common name for "triangulation pillars". These are concrete pillars, about 4' tall, which were used by the Ordnance Survey in order to determine the exact shape of the country. They are generally located on the highest bit of ground in the area, so that there is a direct line of sight from one to the next. By sitting a theodolite (an accurate protractor built into a telescope) on the top of the pillar, accurate angles between pairs of nearby trigpoints could be measured. This process is called "triangulation".
A major project to map out the shape of Great Britain began in 1936. The network of triangulation pillars, with accurately known positions, led to the excellent OS maps which we enjoy today. The coordinate system used on these maps is known as the "National Grid", and it is essential that you are familiar with this system if you are to get the most of OS maps. The OS provide an introduction to the National Grid on their website.
Baseline measurement - measuring size
The triangulation pillars determined the exact shape of the country, but what about distances? Triangulation itself only shows you the shape of the land, not the scale. The scale of the mapping was determined in 1784 by laying a series of glass rods along Hounslow Heath. Using this single measurement, plus the network of triangulation pillars, the size and shape of the country was determined to within about 20m!
Fundamental Benchmarks - measuring profile
In order to determine heights, a different type of surveying was required. Around 200 "fundamental bench marks" (or FBMs) were located across the country. These consist of an underground chamber topped by a small pillar. Between these 200 FBMs, around 750,000 "lower order benchmarks" were scratched into walls etc. Precise levelling built up a picture of the profile of the land, giving rise to the spot heights and contour lines you see on today's maps.
Newlyn Tide Gauge - measuring sea level
Just as the triangulation pillars determined the shape of the land, but not its size, the FBMs determine the profile, but not the absolute height. In order to define a zero height, the sea level, measured by a tide gauge in Newlyn, Cornwall was averaged over a period of 6 years from 1915 to 1921. This gave "Mean Sea Level" (MSL) and all heights on OS maps are quoted as height above mean sea level.
The modern era - GPS
Nowadays, most of the monuments described above have fallen into disuse. However a number of the old triangulation pillars and FBMs, along with various newly installed bolts and rivets, now form the "Passive Station" network. This is a set of around 1000 locations which have been accurately measured using GPS (Global Positioning System) technology. There are also about 30 "active stations" which transmit positional information, in much the same way as the GPS satellites, which can be received by survey-quality GPS receivers. Accurate locations of both active and passive stations are published on the OS's National GPS Network website.
( info - trigpointing uk )
For a sight & sound -AC/DC
Measuring Britain
Shot taken for Saturday Self Challenge 02/04/2022 ----
Measurement and units of measurement . However , not used in the end .
My first thoughts were to work with the measurement of Albedo , but this form of measurement is on a scale and not with any units - so then back to a previous life for this week’s challenge .
Righto , back to the 1970s then when I was a land surveyor and that was all about measurement , there was linear albeit imperial or metric measurement and there was angular measurement ie Degrees-Minutes & Seconds .
For any large scale survey , it could be linked to the National Grid system of the Ordnance Survey by the network of Triangulation Pillars throughout the United Kingdom . Here we see one of the 6,500 or so pillars and this one is on Box Hill overlooking Dorking and southern Surrey - like many showing signs of wear and without the usual white paint . What you can see on the top is the 3 pronged brass inset plate in which a theodolite would sit and straight away be accurately centered . In the first comment box is another view of the trig pillar here with the elevation bracket and trig number - and for a laugh a terrible old shot of me working with a theodolite , this time on a tri-pod .
Of course it is all GPS now and fancy bits of kit that I would be lost on nowadays , at least doing the job old school you had to know the fundamentals and not rely on tec to do the work - no more computations with giant log, sine,cosine and tangent tables to 10 or so decimal places - just a push of the button to convert polar to rectangular co-ordinates !!
Triangulation Pillars --
Trigpoints are the common name for "triangulation pillars". These are concrete pillars, about 4' tall, which were used by the Ordnance Survey in order to determine the exact shape of the country. They are generally located on the highest bit of ground in the area, so that there is a direct line of sight from one to the next. By sitting a theodolite (an accurate protractor built into a telescope) on the top of the pillar, accurate angles between pairs of nearby trigpoints could be measured. This process is called "triangulation".
A major project to map out the shape of Great Britain began in 1936. The network of triangulation pillars, with accurately known positions, led to the excellent OS maps which we enjoy today. The coordinate system used on these maps is known as the "National Grid", and it is essential that you are familiar with this system if you are to get the most of OS maps. The OS provide an introduction to the National Grid on their website.
Baseline measurement - measuring size
The triangulation pillars determined the exact shape of the country, but what about distances? Triangulation itself only shows you the shape of the land, not the scale. The scale of the mapping was determined in 1784 by laying a series of glass rods along Hounslow Heath. Using this single measurement, plus the network of triangulation pillars, the size and shape of the country was determined to within about 20m!
Fundamental Benchmarks - measuring profile
In order to determine heights, a different type of surveying was required. Around 200 "fundamental bench marks" (or FBMs) were located across the country. These consist of an underground chamber topped by a small pillar. Between these 200 FBMs, around 750,000 "lower order benchmarks" were scratched into walls etc. Precise levelling built up a picture of the profile of the land, giving rise to the spot heights and contour lines you see on today's maps.
Newlyn Tide Gauge - measuring sea level
Just as the triangulation pillars determined the shape of the land, but not its size, the FBMs determine the profile, but not the absolute height. In order to define a zero height, the sea level, measured by a tide gauge in Newlyn, Cornwall was averaged over a period of 6 years from 1915 to 1921. This gave "Mean Sea Level" (MSL) and all heights on OS maps are quoted as height above mean sea level.
The modern era - GPS
Nowadays, most of the monuments described above have fallen into disuse. However a number of the old triangulation pillars and FBMs, along with various newly installed bolts and rivets, now form the "Passive Station" network. This is a set of around 1000 locations which have been accurately measured using GPS (Global Positioning System) technology. There are also about 30 "active stations" which transmit positional information, in much the same way as the GPS satellites, which can be received by survey-quality GPS receivers. Accurate locations of both active and passive stations are published on the OS's National GPS Network website.
( info - trigpointing uk )
For a sight & sound -AC/DC