Every road alignment, every building foundation, every canal and every property boundary begins with the same question — where exactly is everything? Surveying is the discipline that answers it.
This post covers what surveying means, how the results are presented, and the distinction between a plan and a map that examiners return to again and again.
What Surveying Is
Surveying may be defined as the method of making measurements of the relative positions of natural and man-made features on the earth’s surface, and the presentation of this information either graphically or numerically.
Three parts of that definition carry weight.
“Relative positions”
Surveying does not measure absolute positions in space. It measures where things are in relation to one another — how far this corner is from that one, what angle this line makes with that one. Everything is referenced to something else, which is why control points and baselines matter so much.
“Natural and man-made features”
Both count. A river and a boundary wall are equally the subject of a survey.
“Graphically or numerically”
Measurement is only half the job. The results have to be presented in a usable form — as a drawing, or as a table of coordinates. A survey nobody can read is no survey at all.
Plans and Maps
The commonest methods of presentation are by means of a plan or a map.
- Both plans and maps are graphical representations of features on a horizontal plane.
Plan = large scale representation
Map = small scale representation
The Distinction That Trips Students Up
Read that box carefully, because it is the reverse of what intuition suggests.
A plan shows a small area — a building site, a plot of land — but at a large scale. A map shows a large area — a district, a country — but at a small scale.
| Plan | Map | |
|---|---|---|
| Scale | Large scale | Small scale |
| Area covered | Small | Large |
| Detail shown | More | Less |
| Typical use | Site plans, building work | Topographical maps of regions |
The confusion arises because “large scale” sounds like it should mean “large area”. It does not. Scale refers to the ratio of map distance to ground distance, so a large scale means the ground is drawn bigger on paper — which means less of it fits.
A useful way to hold it: a plan of your house is large scale; a map of India is small scale.
Showing Height Information
Plans and maps show position in the horizontal plane. Height must be added separately, and there are two ways to do it.
| Method | What It Gives |
|---|---|
| Spot heights | Individual heights of specific points |
| Contours | A less detailed but better visual representation of the area |
The Trade-off Between Them
Note the honest wording used for contours — less detailed but better visual. That is a genuine trade-off, and it is worth understanding.
A spot height is exact. It tells you that this precise point stands at 142.36 m. But a page covered in scattered numbers tells you nothing about the shape of the land.
A contour gives up that precision. It only says “everywhere along this line the ground is at 140 m” — you cannot read the height of a point between two contours exactly. In exchange, the eye immediately sees valleys, ridges, steep slopes and flat ground.
So the choice depends on the question being asked. Setting out a foundation needs spot heights. Planning a road alignment through hills needs contours.
What “Horizontal Plane” Actually Means
Since plans and maps represent features on a horizontal plane, it is worth being precise about what that is.
A horizontal plane is normal to the direction of gravity, as defined by a plumb bob at that point.
This is a physical definition, not a geometric one. Horizontal is whatever is perpendicular to the pull of gravity where you are standing — which is exactly what a plumb bob reveals.
The Consequence
Such a plane will in fact be tangential to the earth’s surface at that point. It touches the curved earth at one place and departs from it everywhere else.
Therefore, if a large area is considered, a discrepancy becomes apparent between the area of the horizontal plane and the actual curved area of the earth’s surface.
This single fact is the reason surveying divides into plane surveying and geodetic surveying, which is the subject of the next topic. Over a small area the discrepancy can be ignored; over a large one it cannot.
How the Topics Connect
The introduction sets up everything that follows in this chapter:
- The tangent plane problem leads directly to plane versus geodetic surveying.
- The idea of presenting results at a scale leads to scale of a map, shrinkage, and errors from using the wrong scale.
- The need for accurate measurement leads to verniers, which let a scale be read finer than its smallest division.
- The requirement to fix relative positions leads to the principles of surveying — work from whole to part, and locate a point by at least two measurements.
Quick Revision Notes
- Surveying measures the relative positions of natural and man-made features on the earth’s surface and presents them graphically or numerically.
- The commonest presentations are plans and maps, both being representations on a horizontal plane.
- Plan = large scale; Map = small scale.
- Height is shown by spot heights (individual point heights) or contours (less detailed but better visual representation).
- A horizontal plane is normal to the direction of gravity, as defined by a plumb bob.
- Such a plane is tangential to the earth’s surface at that point.
- Over a large area, a discrepancy appears between the horizontal plane and the actual curved earth surface.
Mistakes Students Commonly Make
- Reversing plan and map. A plan is large scale covering a small area; a map is small scale covering a large area.
- Assuming “large scale” means “large area”. It means the ground is drawn larger on paper.
- Saying contours are more detailed than spot heights. Contours are less detailed but give a better visual picture.
- Defining a horizontal plane geometrically. It is defined by being normal to gravity.
- Forgetting that surveying includes the presentation of information, not only the measurement.
Conclusion
Surveying is the business of finding where things are relative to one another and then presenting that knowledge so others can use it. Plans do this at large scale for small areas, maps at small scale for large ones, and height is added either as exact spot values or as contours that reveal the shape of the land. And because a horizontal plane is defined by gravity, it can only touch the curved earth at a single point — a limitation that leads directly into the division between plane and geodetic surveying.
Frequently Asked Questions
What is surveying?
The method of making measurements of the relative positions of natural and man-made features on the earth’s surface, and presenting this information either graphically or numerically.
What is the difference between a plan and a map?
A plan is a large scale representation, typically of a small area with much detail, while a map is a small scale representation covering a large area with less detail. Both represent features on a horizontal plane.
Why does large scale mean a small area?
Because scale is the ratio of map distance to ground distance. A large scale draws the ground bigger on paper, so less of it fits on the sheet.
How is height shown on a plan or map?
Either as spot heights, which give the individual heights of particular points, or as contours, which are less detailed but give a better visual representation of the area.
When would you use contours rather than spot heights?
When the shape of the land matters more than exact values at particular points — for example when planning a road alignment through hilly terrain.
What is a horizontal plane?
A plane normal to the direction of gravity, as defined by a plumb bob at that point. It is tangential to the earth’s surface at that point.
Why does a horizontal plane cause a problem over large areas?
Because it is tangential to the curved earth, so over a large area a discrepancy becomes apparent between the area of the flat horizontal plane and the actual curved area of the earth’s surface.