Choosing a contour interval is a genuine engineering decision. Too large and the map misses the shape of the ground. Too small and the contours crowd together illegibly — and the survey costs far more than it needed to. Five factors decide the answer, and they pull in different directions. What Contour Interval Means The vertical…

A map shows where things are. But a road, a canal or a dam depends just as much on how high the ground is — and a flat sheet of paper has no obvious way to show that. Contour lines solve the problem, and they do it so effectively that they have become the standard way of…

Two special cases of levelling deserve their own treatment. Differential levelling carries a height from one point to another regardless of what lies between. Reciprocal levelling solves the problem that arises when an obstacle — a river, a valley — makes it impossible to place the instrument midway. Both rest on the same underlying idea: arrange the observations so that…

Two things distort a long levelling sight, and conveniently they act in opposite directions. The earth curves away from the line of sight, making distant points read too low. The atmosphere bends the light downwards, partly cancelling that effect. This post covers both corrections, their combination, and the distance to the visible horizon that follows from…

Levelling produces a column of staff readings. Turning those into reduced levels — actual heights above the datum — requires a systematic method, and two exist. They give identical answers but differ in one important way: how thoroughly they check themselves. This post covers the terms first, then both methods. The Essential Terms Term Definition Height of instrument…

Levelling needs only two pieces of equipment — something to give a horizontal line of sight, and something to measure against it. Those are the level and the staff. This post covers both, along with the telescope’s optics and the sensitivity of the bubble tube that makes the whole thing work. The Two Instruments The Level and…

There are four ways to find the difference in height between two points, and they are strikingly different in principle. One uses a bubble, one uses trigonometry, one uses air pressure, and one uses the temperature at which water boils. The choice between them comes down to a single question — can you actually reach the…

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…

Every road gradient, every drainage scheme and every building foundation depends on knowing how high things are. Levelling is the operation that determines it — and it rests on a surprisingly subtle idea, because “height” has to be measured from somewhere, and the surface of the earth is curved. What Levelling Is Levelling is the operation…

A traverse produces a great many numbers — lengths, bearings, latitudes, departures, corrections and coordinates for every line. Without a system, it becomes very easy to lose track of one and never notice. Gale’s traverse table is that system. This post covers the table and the five-step computation it organises. What Gale’s Traverse Table Is Traverse…