Levelling compares heights — which means it needs somewhere to start. A bench mark is that starting point: a fixed spot whose height above the datum is already known.
Four kinds exist, and they differ in exactly two respects: how permanent they are, and how precisely their level is known.
What a Bench Mark Is
A bench mark is a fixed point of known elevation above the datum. Any point whose elevation is definitely known can be used as a bench mark.
The Word “Definitely” Matters
Read the second sentence carefully — it is remarkably permissive. A bench mark does not have to be an official brass plate set in concrete. A doorstep, a culvert, a plinth, anything at all can serve, provided its elevation is definitely known.
What makes something a bench mark is not its appearance but the certainty of its height. And that immediately explains why four types exist: they represent four different degrees of certainty and permanence.
The Basis of Classification
The following types of bench marks are established and used, depending upon the permanency and precision required.
1. G.T.S. Bench Marks
The Great Trigonometrical Survey (G.T.S.) bench marks are established by the Survey of India throughout the country. Their levels are determined very accurately with respect to the mean sea level at Bombay port.
The Highest Order
GTS bench marks sit at the top of the hierarchy. Three features mark them out:
- Established by the Survey of India — the same national body responsible for geodetic surveying.
- Spread throughout the country, forming a national framework.
- Determined very accurately against a single reference — mean sea level at Bombay port.
Why a Single Reference Port
This is the vertical equivalent of the geodetic control network. Every GTS bench mark in India traces back to one origin, which means heights measured in Kerala and in Kashmir are directly comparable.
Had different regions each adopted their own local sea level, the country would have had several incompatible height systems. Tying everything to one port creates a single national vertical datum.
Note the pattern repeating from earlier chapters — this is working from whole to part applied to heights. GTS bench marks are the “whole”; everything else hangs off them.
2. Permanent Bench Marks
Permanent bench marks are established at a closer interval between widely spaced G.T.S. bench marks. They are established either by the PWD or the Survey of India.
Filling the Gaps
The key phrase is closer interval between widely spaced GTS bench marks.
GTS bench marks are highly accurate but sparse — establishing them to that precision is slow and expensive, so they cannot be placed everywhere. A surveyor working on a local project might find the nearest one many kilometres away, which would mean a long levelling run just to get started.
Permanent bench marks solve this by densifying the network. They are less accurate than GTS marks but far more numerous, so one is usually within reach.
Again the same structural idea: a sparse, very accurate framework, then a denser, slightly less accurate one filling it in.
3. Temporary Bench Marks
These are bench marks established temporarily whenever required. They are generally the points at which a day’s work is closed and from which the next day’s work is started.
Their Practical Purpose
This is the most practical of the four, and its purpose is simple continuity.
A levelling run may take several days. At the end of each day the work must stop somewhere — and the next morning it has to resume from exactly the same known height, or the two days’ work will not join up.
So the surveyor establishes a temporary bench mark at the stopping point: a nail in a tree, a mark on a culvert, anything stable enough to last overnight and be found again. Its elevation is carried forward from the day’s levelling.
Note the vulnerability — a temporary bench mark is only as good as the levelling that reached it. Any error accumulated during the day is built into it and passed on to the next day’s work.
4. Arbitrary Bench Marks
These are bench marks whose elevations are arbitrarily assumed, for levelling of a small area.
When Assuming a Height Is Legitimate
An arbitrary bench mark makes no claim about mean sea level at all. The surveyor simply declares a convenient point to be, say, 100.000 m and works from there.
This is entirely acceptable for a self-contained survey of a small area, because levelling measures differences. If every height in the survey is 37.2 m too high, all the differences between them are still exactly right — and differences are what matter for a drainage fall, a floor level or an excavation depth.
The limitation is equally clear: such a survey cannot be connected to any other survey until its arbitrary datum is related to a real one.
This is precisely the same logic as the arbitrary meridian in compass surveying — an assumed reference that gives correct relative results but no absolute meaning.
All Four Compared
| Type | Established By | Referred To | Permanence | Precision |
|---|---|---|---|---|
| G.T.S. | Survey of India | MSL at Bombay port | Highest | Very accurate |
| Permanent | PWD or Survey of India | Nearby GTS bench marks | High | Good |
| Temporary | The surveyor | The current levelling run | Short — a day or so | Depends on the run |
| Arbitrary | The surveyor | An assumed value | Lowest | Relative only |
The Descending Hierarchy
Read the table downwards and you can see one continuous scale:
- GTS — national, very accurate, tied to one sea-level origin
- Permanent — regional, densifying the GTS network
- Temporary — local and short-lived, carrying a single survey forward
- Arbitrary — assumed, with no connection to the datum at all
Both permanence and precision fall together as you move down the list, which is exactly what the classification statement said would happen.
Quick Revision Notes
- A bench mark is a fixed point of known elevation above the datum.
- Any point whose elevation is definitely known can be used as a bench mark.
- The four types are classified by permanency and precision.
- G.T.S. bench marks are established by the Survey of India throughout the country, determined very accurately with respect to mean sea level at Bombay port.
- Permanent bench marks are established at closer intervals between widely spaced GTS marks, by the PWD or Survey of India.
- Temporary bench marks are established whenever required, generally where a day’s work closes and the next day’s work starts.
- Arbitrary bench marks have arbitrarily assumed elevations, used for levelling a small area.
Mistakes Students Commonly Make
- Forgetting that GTS bench marks are referred to mean sea level at Bombay port specifically.
- Saying permanent bench marks are established only by the Survey of India. They are established by the PWD or SOI.
- Confusing temporary and arbitrary bench marks. A temporary mark has a real elevation carried from the survey; an arbitrary one has an assumed elevation.
- Thinking a bench mark must be an official monument. Any point of definitely known elevation qualifies.
- Overlooking that the classification is based on permanency and precision.
- Assuming an arbitrary datum makes a survey useless. Differences in level remain exactly correct; only absolute heights are meaningless.
Conclusion
A bench mark is simply a point whose height is definitely known, and the four types form a clear hierarchy of permanence and precision. GTS bench marks provide the national framework, all referred to mean sea level at Bombay port so that heights anywhere in India are comparable. Permanent bench marks densify that framework so a starting point is always near. Temporary bench marks carry a single survey from one day to the next. And arbitrary bench marks abandon the datum entirely — perfectly adequate for a small self-contained site, where only differences in level matter.
Frequently Asked Questions
What is a bench mark?
A fixed point of known elevation above the datum. Any point whose elevation is definitely known can be used as a bench mark.
On what basis are bench marks classified?
Depending upon the permanency and precision required.
What are GTS bench marks?
Great Trigonometrical Survey bench marks, established by the Survey of India throughout the country, whose levels are determined very accurately with respect to mean sea level at Bombay port.
Why are all GTS bench marks referred to a single port?
So that every height in the country traces back to one origin, creating a single national vertical datum in which heights from different regions are directly comparable.
What are permanent bench marks?
Bench marks established at closer intervals between widely spaced GTS bench marks, either by the PWD or the Survey of India.
Why are permanent bench marks needed?
Because GTS bench marks are highly accurate but widely spaced, so a local surveyor might otherwise have to run a very long levelling line just to reach one.
What are temporary bench marks?
Bench marks established temporarily whenever required, generally at the points where a day’s work is closed and from which the next day’s work is started.
What are arbitrary bench marks?
Bench marks whose elevations are arbitrarily assumed, used for levelling of a small area.
Is a survey based on an arbitrary bench mark still valid?
Yes, for a self-contained small area. Since levelling measures differences in height, all the differences remain exactly correct even though the absolute heights are assumed. Such a survey cannot, however, be connected to another survey until its datum is related to a real one.