Point, line and quadrat sampling

Overview

Point sampling, line sampling and weaknesses of quadrat sampling.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is point sampling, and what is it used for?

   Model answer

   Point sampling takes measurements at individual, separate locations rather than along a line or within an area - a soil pit at each of twenty grid intersections, a noise reading at each of ten street corners, a pebble measurement at set spots across a beach. The points may be selected randomly, systematically or in strata. It is used when the variable can sensibly be recorded at a single spot, and when you want to build a picture of how that variable is distributed across an area, for example in order to draw an isoline map.

2. 2

   What is line sampling, and when is it the right choice?

   Model answer

   Line sampling collects data along a line, usually a transect running across the study area, with measurements taken at points along it or with everything the line crosses being recorded. It is the right choice when the hypothesis concerns change with distance in one direction: vegetation change from the strand line inland across a dune system, land use and building height change outwards from the central business district, or change in channel form downstream. The line makes the gradient of change visible in the data itself.

3. 3

   What is the weakness of line sampling?

   Model answer

   It records only what lies on one narrow line, so it can badly misrepresent the area on either side of it - a single transect through a town may run down one unusual street, and a single dune transect may cross an eroded blowout. It is also directional: choosing where to put the line, and at what angle, is a decision that can be made, consciously or not, to produce the expected result. Using several parallel transects and averaging, rather than relying on one, is the usual safeguard.

4. 4

   What is a quadrat, and how is it used?

   Model answer

   A quadrat is a frame of known area, commonly a square of half a metre or one metre a side, often subdivided into a grid of smaller squares. It is placed on the ground at a chosen sampling point and everything inside it is recorded, giving a measure that relates to a known area and can therefore be compared between sites. Its position may be fixed randomly using coordinates, or systematically at intervals along a transect.

5. 5

   What can be recorded within a quadrat?

   Model answer

   Species present, which gives a simple list; species frequency, meaning in how many of the small squares each species appears; density, the actual count of individuals of a species within the frame; and percentage cover, the estimated proportion of the quadrat area shaded by each species when viewed from above. Quadrats are also used on shingle to sample every pebble within the frame for size and roundness, and in urban work to record features within a defined ground area.

6. 6

   When is a quadrat the right technique, and what should it not be used for?

   Model answer

   It is right whenever the variable is spread continuously over the ground surface and can only sensibly be measured per unit area - vegetation cover across a dune succession or a salt marsh, pebble size on a shingle beach, litter density in a park. It is the wrong technique for anything occurring as discrete separated objects across a wide area, such as settlements or shops, and for anything measured at a single point such as slope angle or a noise reading, where point sampling is appropriate.

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