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The following steps will explain the processes by which point transects and plots can be randomly or systematically laid for sampling.


Randomization of point transect and plot sampling -

Any method that can achieve a high level of randomness for the selection of a point transect or plot will be satisfactory for this.


1.Computerized Methods -


Many geographic information system (GIS) softwares (such as ArcGIS or a free alternative  DIVA GIShave functions that can be used to randomly lay point transects or plots satellite images of the study area (which can be obtained freely from  many sources (a list will be put up soon)). Once these are laid on the satellite image by the computer program their gps coordinates can be obtained and you can go to the site of those coordinates to lay the plot or point transect. This method does require a gps device, if that is not available the next method will have to suffice, though it is not as effective as this one in randomization.



2.Methods that can be used in the field -

Upon reaching the study site randomization of the selection of the location the point transect or plot can be achieved up to an extent within a small area of the locality by having a researcher close their eyes and turn around repeatedly to lose track of their bearing and cast a stone. The location the stone is dropped at can serve as the point at which a point transect or plot is laid.


3. Using print outs of Google Earth Images -

If the study area is not too large and is well known to the researcher, a dart or other similar object can be thrown at a print out of a google earth image of the area. The location the dart falls in becomes the point at which a point transect or plot is laid.


Systematization of Point Transect and Plot Sampling-

For systematization of the sampling process the location of the point transects and plots have to be decided upon in a systematic manner. As above there are computerized and field based methods -


1. Computerized Methods -


A grid is created to overlay the satellite image of the study site (which may be found in a  variety of sources (a list will be put up soon)using functions in a GIS software. The corners of the grid squares where the gird lines intersect can be used as the points where the point transects and plots are laid or they can be laid in the centre of each grid square. Remember that scale is an important issue when creating a plan for sampling, the length of the sides of the grid squares are the distance that will have to be travelled from one point transect/plot to the next. Never make unrealistically large grid squares which will be difficult or impossible to use for practical sampling. Sample size is also another consideration, so the size of the grid squares should be small enough to allow you to take the required number of samples within the boundaries of the habitat you are intending to study.


systematically laying plots and point transects



2. Methods that can be used in the field -

After reaching the field site a plot or point transect can be laid at regular intervals of a certain distance (like 1km for example) . Though a device would be required to accurately obtain distances traveled when the intervals are far apart.



About the Author of this Article


Southwood T. , and P. Lenderson. Ecological Methods - Published by Blackwell Science Ltd.